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In this section, open- and closed-loop control will be reviewed. Most of the information has been previously presented. However, the focus of this section will be on the external devices that connect to the drive. These external components allow the drive to interface to a system and provide the necessary speed and torque control to maximize efficiency.
External devices (peripherals) are a vital part of any drive system- whether operating as part of a remote operator station or part of an entire production system. There is such a wide variety of peripherals that are used in drive control that to include all possible combinations would be beyond the scope of this book. However, the major categories of devices and their use will be covered in this section.
Open-Loop Control
Open loop control was presented in Section 4, under the direct current (DC) and alternating current (AC) drive sections. A brief review is given here. FIG. 1 indicates the basics of open-loop control.
FIG. 1. Open-loop control
The open-loop control is the simplest form of drive control. The drive operates the motor, given the speed reference that is delivered from the hand speed pot. If FIG. 1 were of a DC drive, motor-speed regulation would be 5-7% higher. If the figure were of an AC drive, motor-speed regulation would be 3-5% higher, possibly higher depending on motor design.
Open-loop control is more typical in AC drives, where the inherent motor speed regulation is acceptable in the 3-5% range. DC drives are almost never operated open loop because of the higher regulation characteristics.
Some type of feedback is used, such as armature voltage feedback (EMF) or tach feedback. These methods will be discussed in greater detail later in this section. Before these methods are discussed, a review of closed-loop control is required. In addition, the types of devices involved will also be presented.
FIG. 2. Closed-loop control
As indicated by the name, this type of control method requires the use of some peripheral device that closes the loop back to the drive. The device that is commonly used is the tachometer, or tach generator, as it is some times called. FIG. 2 indicates a simple closed-loop control scheme.
Closed-loop control is a bit more complex, but yields a great improvement in motor-speed regulation. The key to the success of this scheme is the tachometer (T) or feedback device. A speed reference is given to the AC or DC drive. The actual speed, determined by the tach, is fed back to the drive. Both the speed reference and feedback are combined into the summing circuit. The summing circuit produces an error signal. That error signal is now the speed reference that is sent to the power and logic control, which translates the signal into faster or slower motor speed.
If this happened to be an AC drive configured in torque-control mode, the feedback would be a current feedback instead of speed. If this was a position control, such as a servo controller, the feedback would be a position and use an encoder instead of a tach. The idea of this entire scheme is to have zero error on the output of the summing circuit. If error occurs, it must be corrected. How fast it is corrected is a function of the logic control in the drive.
The use of proportional integral derivative (PID) is critical to the success of the entire system. The PID controller will be programmed to correct for the error immediately or over a period of time. It will also be programmed as to how much error should be corrected over a period of time. The dynamics and stability of the system is dictated by the responsiveness or sloppiness of the feedback loop and control system. As with any type of control system, it is only as good as the weakest link. The remainder of this section will be devoted to closed-loop components, their accuracy, and how the responsiveness of the system will be affected.
Tachometers
A tachometer generator or tach is an electromechanical device that translates the rotational speed of a shaft to an electrical signal. An analog tach generator is a small generator that produces an output voltage whose magnitude is linearly proportional to shaft speed. An AC tach produces an AC voltage and a DC tach produces a DC voltage. DC tachs produce a negative voltage to indicate reverse rotation as shown in FIG. 3.
FIG. 3. DC tach output voltage vs. speed
DC tachs are the most popular type of analog tach, since AC tachs do not indicate direction and are generally less accurate than DC tachs. The out put of a DC tach is typically specified in volts per 1000 rpm. The V/1000 rpm voltage constant is the slope of the output voltage versus speed line.
Digital tachs, or pulse tachs as they are sometimes called, produce a series of pulses. The frequency of those pulses is proportional to shaft speed. This can be seen in FIG. 4.
The output of a digital tach is typically specified in pulses per revolution (PPR). Most pulse tachs are magnetic devices in which pulses are generated by gear teeth passing by a magnetic pickup.
Encoders
An encoder is an electromechanical device that translates mechanical position and/or velocity into electrical signals. A shaft encoder or rotary encoder translates rotational or angular position and/or velocity into electrical signals.
An incremental encoder provides a signal that indicates position change or incremental position rather than absolute position. In principle, an incremental encoder is essentially the same type of device as a pulse tach, previously discussed. The term encoder usually implies a device with more advanced features and performance. Terms such as incremental encoder, encoder, pulse tach, and digital tachometer are basically used for the same type of device. In industry, they are often used interchangeably.
Optical encoders, on the other hand, are devices that use pulses for rotational signal generation. A light beam is transmitted through transparent stripes or slots in a rotating disk. The photo detector picks up those light beams and converts them into a series of pulses, sometimes called a pulse train. FIG. 5 shows this process.
FIG. 4. Pulse tach output
FIG. 5. Optical encoder characteristics
Optical encoders are the most widely used types of encoder. They are capable of providing very high resolution.
Magnetoresistive encoders have a wheel or disk with numerous small magnetic poles imprinted on the disk surface in alternating north/south pairs.
Incremental position is indicated by a sensing element consisting of a combination of resistors that change resistance on the basis of a magnetic field.
The sensing element provides input to electronic circuitry, which generate pulses corresponding to each individual imprinted magnetic pole.
An absolute encoder produces a number of binary outputs that can be read as a binary word, which indicate the absolute angular position of the shaft relative to some unique index position. This type of encoder would be used primarily in a system where position feedback would be required (e.g., cut-to-length servo system).
This guide is primarily concerned with motor speed or velocity in drive systems. Therefore, the focus will be on the incremental encoder or pulse tach, rather than on absolute encoders devices.
Encoder Characteristic
The following paragraphs explain several encoder features that provide functions and performance that go beyond simply indicating shaft speed.
An encoder with a single output cannot indicate direction of rotation.
Quadrature outputs are two outputs with a 90º phase displacement, as shown in FIG. 6.
As shown in FIG. 6, quadrature outputs can be used to determine direction of rotation by detecting which of the two output signals is leading. The low-to-high transitions of the leading signal occur before the low to-high transitions of the lagging signal.
Incremental encoders can be used to indicate when the shaft is in one particular "index" position. This is done by using an index pulse or marker pulse that occurs only once per revolution at the index position. FIG. 7 shows this procedure. The width and phase relationship of the index pulse may vary among encoder models.
FIG. 6. Quadrature outputs of a pulse tach (encoder)
FIG. 8 indicates complementary or differential outputs. Complementary outputs are pairs of outputs that are configured in such a way that one output (A) is high whenever its complement (A) is low.
FIG. 7. Encoder marker pulses of index positions
A and A represent the signal voltages with respect to circuit common. The voltage between the A output and the A output is the voltage difference, or differential, between the A and A voltages. An encoder output signal that does not include the complementary A and B signals is called a single ended output.
Differential signals have better noise immunity than single-ended signals.
FIG. 9 indicates a differential line driver and receiver connected by a twisted pair of shielded wires-a transmission line.
FIG. 8. Complementary or differential outputs
FIG. 9. Differential line driver and receiver diagram
The use of a differential line driver and receiver protects against common mode noise between the transmitting and receiving locations. The use of a shielded, twisted-pair transmission line protects the signal from electro magnetic fields that might interfere with signal transmission.
The resolution of an encoder is determined by the number of pulses per revolution of the encoder shaft. The resolution determines the smallest amount of shaft rotation that the encoder is able to indicate. If an encoder generates 1024 pulses per revolution, it can indicate a movement of 1/ 1024 of a revolution or 0.35 degrees of rotation. The effective resolution of an encoder can be improved by using pulse multiplication. This procedure is shown in FIG. 10.
FIG. 10. Encoder pulse multiplication
The A and B signals shown in FIG. 10 are the outputs of a quadrature encoder. The 1X signal is a series of narrow pulses corresponding to the low-to-high transitions of the A signal. These pulses are typically generated by the device that receives or uses the encoder signal.
The resolution of the 1X signal is equal to the encoder PPR. The 2X signal contains pulses corresponding to both the low-to-high transitions and the high-to-low transitions of the A signal. The resolution of the 2X signal is twice the encoder PPR. The 4X signal contains pulses corresponding to the low-to-high and high-to-low transitions of both the A and the B signals.
The effective resolution of the 4X signal is four times the encoder PPR.
To correctly apply encoders, the specifications of the encoder must be determined. Then the applications specifics can be matched to the encoder that would best meet the requirements. At this point, a review of typical encoder specifications is in order.
Encoder Specifications
The most basic encoder specification is the resolution or number of pulses per revolution of the encoder shaft. In general, using the highest possible pulses per revolution allows the best possible drive performance. High PPR allows speed to be measured more accurately and sampled more frequently. The pulse frequency is determined by the encoder-operating speed and the resolution according to the following formula:
The following example illustrates the pulse-frequency calculation for a 2048 PPR encoder operating at 1800 rpm. Note that the pulse frequency unit of measurement, hertz, is equivalent to pulses per second. The minute and revolution units cancel, leaving pulses-per-second or hertz as the units of measurement.
The encoder PPR must be selected so that the pulse frequency is high enough to allow the required performance at the minimum operating speed. At the maximum operating speed, the pulse frequency must not exceed the maximum output frequency capability of the encoder. It also must not exceed the maximum input frequency capability of the circuitry that receives the signal.
Some drives can be adjusted to accept any PPR value within a specified range. Other drives accept only specific PPR values, such as 1024 PPR or 2048 PPR. The accuracy of an encoder is the maximum percentage variation between the actual positions of the pulse edges or transitions and the theoretical correct positions. The encoder accuracy is not the only factor that determines the speed-measurement accuracy. Speed-measurement accuracy is influenced by the encoder timing and counting accuracy. It is also influenced by the mechanical backlash in the coupling to the machine.
The measurement method may improve upon the encoder accuracy by averaging a number of pulses in each measurement. When speed is averaged over an extended period of time, the measurement accuracy will be determined entirely by the timing and counting accuracy.
The basic specification for the output signals indicates whether the encoder has a single output or quadrature output and whether or not a zero marker is provided. If complementary output signals are provided, the encoder specification may identify the IC-type number for the line driver. The output voltage and current ratings should also be specified.
Single-ended outputs are often open collector, current sinking outputs.
Most encoders provide square-wave outputs (50% duty cycle), but some models may be available with a fixed-pulse width.
Output specifications must be checked for compatibility with the circuitry that will receive the signal. Encoders require an external power source.
The most common requirements are 12 and 24 VDC. Encoders are also available for operation at 5 VDC. However, these encoders are not recommended for installations requiring a long cable distance between the encoder and the drive.
Most encoders require a regulated input supply voltage and provide an output at the same voltage level as the input voltage. If the encoder has its own voltage regulator, its output signal level will be considerably lower than the input supply voltage. If the drive is designed to provide a regulated encoder supply voltage, it may not accept an encoder signal at a voltage level that is significantly below the supply voltage.
Before installation, the user should verify that the output voltage of the encoder is compatible with the input voltage range of the drive's encoder input. In addition, all encoders have minimum and maximum operating temperature ratings. If the encoder is subjected to temperatures outside the specified limits, it may not operate correctly and can ultimately be damaged. A possible source of encoder damage is heat. Heat is very likely to be conducted to the encoder housing through the mounting structure from motor or machine. An encoder should be selected with an operating temperature rating that is suitable for the location and mounting method.
A variety of mechanical specifications define the type of encoder housing available and the type of mounting arrangement possible. Encoders are available with various degrees of protection from moisture and environ mental contaminants. The degree of protection is determined by the encoder housing type and material, the shaft material, the type of shaft seal, etc. Explosion-proof models are available for use in areas where fire or explosion hazards may exist because of flammable gases or vapors. Various bearing ratings are available to accommodate a wide range of radial and axial loads that might be subjected to the shaft of the encoder.
There will also be a maximum operating speed determined by the mechanical construction of the encoder. For each PPR rating, there will be a maximum speed corresponding to the encoder's maximum output frequency capability as described in a previous paragraph. The encoder manufacturer's installation and alignment procedures must always be carefully followed to ensure reliable operation.
Once the encoder is properly installed, it must be wired to the specifications of both the encoder and drive manufacturer. Careful attention to wiring is essential to ensure that a drive with encoder feedback performs properly. The following recommendations cover the most common requirements.
Each encoder output channel and the power supply should be connected using an individually shielded twisted pair of wires. Encoder wiring and a connection scheme are shown in FIG. 11.
FIG. 11. Encoder wiring diagram
Three individual cables or a three-pair cable can be used. If a three-pair cable is used, be sure that each pair of wires is individually shielded. The shields are to be grounded as recommended by the drive instruction material. Shielded cable is grounded at only one point to prevent ground loops or circulating currents through the shield conductor. The best ground location is usually at the drive. The encoder cable must be run in a steel conduit. Cables from more than one encoder can be run in the same conduit, but the conduit cannot carry power wiring.
Encoder cable should be a low-capacitance type of cable, designed for high-speed digital signals such as RS-422/485 serial communications signals.
Since the relationship between encoder shaft rotation and quadrature signal phasing is not standardized, this relationship can vary among encoder models. The instruction material furnished with the encoder is the best source of determining the encoder phasing. Phasing corresponds to the selected motor direction. If the drive does not operate properly upon initial startup, encoder phasing could be a suspect. It may be necessary to determine proper phasing by trial and error.
Note: Today's microprocessor-based adjustable-speed drives are designed for use with incremental encoders rather than analog tachometer generators. Although some models may accept a feedback signal from a DC tach, the best performance is obtained by using an encoder. Analog tachometer generators have better EMI immunity compared with encoders. However, with the use of differential signals and careful attention to wiring, EMI immunity of an encoder can be equal or better than that of an analog tach.
If an analog tach is used, it is necessary to determine what range of signals are compatible with the drive circuitry that it will be connected to. The circuit must be designed for either a DC or AC tach. The maximum input voltage and tach-generating voltage at maximum speed are factors to consider before purchasing a tach. Using a tach with the maximum compatible volts per 1000 rpm will provide the best performance over a wide speed range.
Resolvers
A resolver is a "position" transducer, with characteristics that resemble a small motor. This type of feedback device would be used mainly with servo motor applications where precise feedback of rotor position is critical to system accuracy. For example, in cut-to-length applications, linear position of a sliding table or cutting arm would be a function of the position of the rotor. FIG. 12 shows a diagram of a resolver.
FIG. 12. Resolver diagram
A resolver is very similar to an AC induction motor. A resolver contains a single winding rotor that rotates inside fixed coils of wire, called stators. A reference voltage is typically applied to the rotor winding. This rotating winding has a magnetic field that induces a voltage in the stator windings, which produce an analog output. This analog output is proportional to shaft (rotor) rotation.
Basically, the resolver is a rotating transformer. The rotating primary (rotor) induces a voltage in the fixed winding secondaries (stator windings). The analog output is what is fed back to the drive as an actual speed signal, or what would be considered a position signal.
Because there are no electronics involved with resolvers, they are better suited for dirty environments than encoders. Also, the resolver is a device that is an absolute measuring instrument. It can retain its exact location during a power outage. Typically the resolver can transmit information over distances up to 1000 feet with little effect from electrical noise. The resolution of some resolvers can be rated as high as 16,384 counts (14 bit or 214 ).
Drive Control Methods: DC
As stated earlier, the commonly used methods of speed control are open and closed loop. If speed regulation is not a factor, then a DC motor can be operated in open-loop control. However, most applications require some type of regulation to gain the most efficient use of the mechanics of the system. Therefore, a means of sending the drive an actual speed signal is essential to speed regulation. In DC drives there are basically two forms of closed-loop control-tach feedback and armature voltage feedback (EMF control). Though armature voltage feedback does not use an external device, it is termed feedback and can be considered a form of closed-loop control.
Armature Voltage Feedback (EMF Control, Speed Regulation) FIG. 13 shows an armature voltage feedback control scheme.
FIG. 13. Armature voltage feedback (EMF control)
As shown in FIG. 13, the drive requires a speed reference signal and a feedback signal of opposite polarity. The feedback is used to balance the control when the desired output speed is reached. All of the drive control systems are within the dotted lines.
A speed reference is sent to the summing circuit. By sensing the armature voltage at the drive output, the drive can sense the CEMF (Counter Electromotive Force) of the motor. This CEMF signal (negative polarity) is sent as feedback to the summing circuit. When the error is at zero, the drive will stabilize at the desired speed.
Another summing circuit is located after the speed amplifier and before the current amplifier. This summing circuit would use a shunt or other device to sense the armature current. The negative current feedback is sent to the summing circuit, with the resulting signal used to limit the amount of current output. If the current level is within limits, then the speed signal will be in control. But if the current exceeds the limits, it will lower the speed control until the current is reduced to a safe level.
With armature voltage feedback, motor speed tends to droop between full load and no-load situations. To help compensate for this speed "droop," a feedback called IR compensation (an acronym for "current resistance" compensation due to a voltage drop across the armature due to load-- E=IxR, Ohm's Law) is included in the drive. This circuit senses the armature current and feeds a small additional signal back to the speed amplifier.
At the summing circuit, three signals exist: positive speed reference, negative armature voltage feedback (EMF), and positive IR compensation. The IR compensation signal adds to the speed reference signal to compensate for speed droop created by the load.
With three signals summing at the same point, there is a possibility for instability. To set up IR compensation properly, speed and armature voltage feedback adjustments should be made with the IR compensation off.
While observing the motor during speed step changes, IR compensation is gradually increased until oscillation occurs. Then IR compensation is decreased until the oscillation (instability) stops. Speed regulation of 2-3% is possible with this type of feedback control.
Armature Voltage Feedback (EMF Control, Torque Regulation)
The relationship between torque regulation and speed regulation in a standard DC drive configuration illustrates the importance of torque response. Since the armature current in a DC motor directly determines torque, the DC controller is configured as a closed-loop current regulator, using armature voltage feedback (EMF). The speed regulator then commands the current regulator to produce whatever torque is required to maintain the desired speed.
Torque-regulating drives are often used in load-sharing applications where a speed-regulating drive controls the speed of the driven machine, while a torque-regulated "helper" drive provides a controlled level of torque at some other location on the machine. If the load does not restrict the speed of a torque-regulated drive, the drive speed could exceed the safe operating limit. Therefore torque-regulating drives must have a speed-limiting mechanism that prevents the speed from exceeding a safe limit if the torque presented by the driven machine drops to zero.
With a DC drive, torque can be regulated directly by regulating armature current. In any motor, torque is the result of the force between two magnetic fields. In a motor (DC), torque is easily and directly regulated by regulating the currents that control the flux in the two magnetic fields. The field winding flux is the motor's magnetizing flux, which is held constant by providing a constant field current. The motor's torque-producing flux is the flux created by the armature current, which is controlled to regulate torque. The torque produced at any speed is given by:
Torque = K××IA
where:
K = a constant
Φ = the magnetic flux produced by the stator field
IA = the armature current
Tachometer Feedback
When DC motor speed is of primary concern, it can be measured with a transducer and regulated with a closed-loop regulator as shown if FIG. 14.
FIG. 14. Closed-loop speed regulation-tach feedback
The transducer in FIG. 14 is a tachometer generator. As previously reviewed, a tach is a small generator that produces an output voltage that is very accurately determined by its operating speed. There are also pulse tachs, which provide a train of voltage pulses at an average frequency that is exactly proportional to average speed.
The closed-loop speed regulator compensates for any changes in the characteristics of the drive caused by changes in load or by outside influences such as line voltage and ambient temperature. With a closed-loop speed regulator, the most important characteristic of the drive is its ability to rap idly respond to changes in requirements for torque.
The transducer devices already presented would be involved in generating the actual speed feedback signal. The accuracy of the system will be dictated by the regulation of the feedback device and the responsiveness of the drive control.
Drive Control Methods: AC
The characteristics of open- and closed-loop control have already been discussed. In this section, the focus will be on AC drive control, with commonly used peripheral devices such as transducers, sensors, and other control inputs. In addition, the performance (static and dynamic) and stability of the system will be explored.
Closed-loop regulation can be used with an adjustable-speed drive to regulate a variety of processes. FIG. 15 shows the regulation of air pres sure in the duct of a ventilation system.
FIG. 15. Armature voltage feedback (EMF control)
As air-outlet dampers are opened and closed, the speed of the fan must be increased or decreased to match the demand for air flow and maintain a constant static pressure in the duct. The closed-loop control system uses a pressure transducer in the duct to measure the static pressure. The air pressure feedback is sent to the drive, which adjusts the speed of the fan as required. The regulator control loop not only adjusts for changes in air flow requirements but also compensates the characteristics of all of the equipment such as the drive motor and fan that are "inside the loop." When an adjustable-speed drive is inside the loop in a closed-loop control system, the speed-regulating accuracy of the drive is not the critical element. The movement of air or water volume is not an "exacting science." An exact amount of flow (measured in cubic feet per minute [cfm]) is not required in this particular application. The drive may have to pump air for a few more seconds to meet the demand. In this type of a closed-loop control system, the drive needs only to provide the required torque and respond to speed-correcting commands from the regulator.
However, if this were an application where drive speed directly determines the accuracy of the process, then the drive would be a critical part of the system. Such an application could be a coating line that processes paper stock into 12-ounce coffee cups. Precision control is needed to ensure that the cups are manufactured to the exact dimensions for 12-ounce capacity.
Feedback and Performance
The feedback devices of tachometers, encoders, and resolvers have already been presented. This section will be devoted to the overall performance of the AC drive system. The terminology will be presented and the performance characteristics reviewed.
In earlier sections, only steady-state, or static, performance has been reviewed. Static operation is where there are no changes in conditions.
FIG. 16 shows a drive's static and dynamic speed-regulating performance.
FIG. 16. Static and dynamic performance of a system
The figure indicates static operating conditions before and after a load change with a transition period of dynamic performance immediately after the change. Static speed regulation is the change in steady-state speed that is caused by a load change. Static performance measures the difference between two operating points, without considering the performance during the transition from one point to the other. At each point, operation is measured only after the system has been operating at that point for some length of time. Sufficient time is allowed so no further change will occur in operation related to the transition from one point to another.
Dynamic performance describes the operation during the transition from one operating point to another. The dynamic performance capability of a system defines the system's ability to respond to a load change or a reference change.
FIG. 17 is an enlarged view of FIG. 16, showing the parameters that quantify both the static response and the dynamic response to a step change in load.
FIG. 17. Step changes of load and the response
The transient deviation is the maximum deviation from set point immediately following the load change. The transient-response time is time required for the output to return to the steady-state regulation band after going through a period of damped oscillation. The steady-state regulation band is a small "dead band" of output variation that is not recognized by the regulator as a change. The regulation dead band is caused by regulator and transducer resolution limitations. The steady-state regulation is the change in steady-state output resulting from a load change. Drift is the change in steady-state output because of temperature changes and other long-term influencing factors. Drift is usually specified for a 24-hour period.
It is difficult to use transient-response time to compare the performance of two types of drives because total-system response time is determined by load inertia. FIG. 18 shows another way of quantifying the dynamic change in output speed because of a step change in load torque.
FIG. 18. Dynamic speed accuracy
The dynamic speed accuracy is the area under the transient-response curve measured in percent-seconds.
For the low-inertia load, the figure shows that the maximum transient speed deviation is 15% and the response time is 40 ms. The dynamic speed accuracy is the area of the shaded triangle or 15% x 40 ms/2 = 0.3%-seconds. For the high-inertia load, the dynamic speed accuracy is 7.5% x 80 ms/2 = 0.3%-seconds. This example shows that the dynamic speed accuracy is about the same for a low-inertia load as it is for a high-inertia load.
FIG. 19 shows the dynamic response of a drive resulting from a step change in reference.
FIG. 19. Reference step change and response
The parameters that quantify the performance are rise time, peak over shoot, and settling time. The rise time is the time required for the output to rise from 10 to 90% of its final value. The peak overshoot is the maximum amount by which the output overshoots the final value. The settling time is the rise time plus the time required for the output to reach a steady value after going through a period of damped oscillation.
In many applications, it is important for a drive to accurately follow a speed reference during acceleration and deceleration. In a web processing machine for example, the speeds of the various machine sections must match each other as a continuous web of material travels from one section to another. When the master speed reference is increased or decreased, the drives for each machine section must accurately follow the change. FIG. 20 shows the dynamic deviation that occurs whenever the speed reference changes.
Bandwidth, or small-signal bandwidth, is another parameter that is some times used to quantify a drive's capability for accurately following a changing reference signal. Small-signal bandwidth is measured by applying a small sinusoidal variation to the regulator reference and observing the effect on the output. The bandwidth is the maximum frequency range of input signal that the output can follow. Another name for bandwidth is frequency response.
FIG. 20. Dynamic deviation
The bandwidth can be given in radians per second (?) or in hertz (Hz). The relationship between radians/sec and Hz is ?= 2pf. Bandwidth is inversely proportional to the system time constant (t) or "Response Time" (?= 1/t). The response time is similar to the rise time shown in FIG. 19. The response time is the time required for the output to rise from 0 to 63% of its final value.
The bandwidth of a drive defines the maximum capability of the controller/motor combination with nothing connected to the motor shaft. The bandwidth of a controller defines the maximum electrical output capability of the controller without a motor connected to the output terminals.
The bandwidth of a system is the actual operating performance of the drive and load when the drive is adjusted for optimum performance with that specific load.
High-performance control systems often have multiple control loops as shown in FIG. 21.
FIG. 21. Bandwidth relationships in control loops
The outermost control loop regulates the process variable. An example of this would be a position control loop in servo drive systems. A general-purpose drive system might have a dancer position regulator that ultimately controls the tension of a web or filament. The speed-regulator loop is inside the process-regulator loop, and the torque-regulator loop is the innermost loop.
To provide stable performance, each inner regulator loop must be 3-10 times faster than the next outer loop. That is, omega 2 = 3-10 times omega 1 and omega 3 = 3-10 times omega 2. If the process is subject to fast changes, omega 1 is large. A high performance drive is required, and omega 2 and omega 3 must be large. Conversely, if the driven machine has a high inertia or other characteristics that dictate that its final output can change only slowly, then the drive does not need to have wide speed- and torque-regulator bandwidths.
Stability of the System
The performance examples presented all have stable performance characteristics. When the performance of a system is stable, the output is at a steady value, except during periods of transition from one operating point to another. During transitions, the output may oscillate, but the oscillation is damped and rapidly decreases to a low value. In unstable systems, the output may oscillate continuously for an extended period of time. FIG. 22 illustrates responses to a step change in reference for a stable and unstable system.
FIG. 22. System responses-stable and unstable operation
Systems are usually adjusted for slightly under-damped or critically damped operation. A system is critically damped when it responds with the fastest rise time that is possible, without any overshoot.
System performance is determined by the interaction among all of the components of the system. The components of a control system include both the controlling system and the controlled system. An adjustable speed drive system includes the adjustable-speed controller, the motor, all feedback and accessory devices, and the driven machine. The characteristics of the driven machine, or load, are an essential factor in determining the dynamic performance of an adjustable-speed drive system. FIG. 23 shows the load torque versus speed for a driven machine compared with the torque capability of a drive.
FIG. 23. Accelerating and decelerating torque
The difference between the load torque and the intermittent torque capability of the drive is the torque available to accelerate or decelerate inertia.
The reflected load inertia, plus the motor inertia and the torque available for acceleration or deceleration, determines the time to accelerate and decelerate. The acceleration and deceleration time are the main components of the drive's response to a step change in speed reference.
Acceleration or deceleration time is given by:
If the load inertia is expressed as WK^2 (lb-ft ^2 ), speed change is given in rpm, and torque is given in lb-ft, then the constant, K, is 308 for calculating time in seconds.
The drive's torque response is a very important factor in determining the drive's dynamic performance. When the load torque increases suddenly, or when the speed set point is suddenly changed, the drive is asked to instantaneously change the level of torque that the motor is providing.
The drive's torque response is the response time (typically milliseconds) required for the drive to respond to a step increase of 0 to 100% torque demand.
Accurately predicting the complete performance of a drive system and driven machine requires detailed information. Drive and machine characteristics must be known. To achieve optimum performance, the drive must be adjusted (tuned) to the characteristics of the driven machine. One aspect of tuning is to set the speed regulator gain adjustments for the best regulation that can be achieved without getting too close to unstable operation. The system bandwidth must be tuned to a frequency that is below any mechanical resonance frequencies of the driven machine.
For existing machine designs, drives can be selected based on comparing the capabilities of available drives with the capabilities of drives that have been successfully used in the past. For new machines, the keys to success include experience with similar machines and close cooperation between the machine design engineers and the drive manufacturer.
Sensors and Controls
A variety of sensors and controls are used to interface with the drive unit, either AC or DC. In this section, attention will be given to the more common devices that control a drive or give information to the drive, in the form of feedback.
Transducers
A transducer is a device that senses the condition, state, or value of an item to be controlled and produces an output that reflects that condition, state, or value. The following is a listing of typical transducers and sensors used to feed information back to the drive. In some cases, the transducer itself is the controlling element (determining a speed reference or set point).
Temperature
Thermocouple and thermistor. This device changes resistance per the change in temperature. Some thermocouples are mounted directly on machines actually inside tanks to monitor temperature. This temperature is then fed back to the drive to change the controlled value (increase or decrease the flow of heat).
Flow
Electromagnetic flow meter. This device uses an electromagnet and fluid motion to register an output. The moving fluid provides movement. An EMF is detected by two electrodes embedded in the wall of the tube. The electrodes are insulated from the liquid being measured.
Differential pressure flow meter. The fluid flow causes motion within the measuring unit. A restriction in the orifice or pipeline causes an increase in velocity and a decrease in pressure. This decrease is a rate of flow, and is the controlling element.
Tension, Force, or Strain
Resistance gage. There are several forms of resistance gages. The ultimate result is that the output is a change of resistance, per the unit being measured. A tension gage changes resistance per the tension applied to the sensor. A strain gage changes resistance per the stretching of the wires inside the sensor. This sensor type needs a power source to feedback a voltage signal to the drive.
Displacement of Position
Potentiometers, proximity sensors, photo cells. These sensors change output resistance per the controlling element. Potentiometers change resistance per the location of the wiper arm or shaft. Linear pots are constructed in a linear fashion with a slider arm, as opposed to a rotary arm as in a standard pot. Proximity sensors consist of two plates that change capacitance per how close they are to the controlling element. Photo cells are available as photovoltaic, which sends out a voltage per the amount of the control ling element. There are also photoconductive and photoresistive devices.
Photoresistive devices change resistance per the amount of light sensed by a light-sensitive base plate.
Pressure
Diaphragm or bourdon tube. These are used to sense fluid pressures. The movement of the internal device is proportional to the pressure changes.
The output is a variable resistance, which requires a voltage source to pro vide feedback to the drive. Three to fifteen PSI or static pressure transducers are commonly used in the HVAC industry to monitor the static pressure in the heating or cooling ducts. These devices use the principle listed above, but include an actual transmitter that would deliver a 4- to 20-mA output. That output would be accepted directly into the drive feed back circuit, with no additional power source needed. Pressure transducers (4-20 mA) can effectively transmit signals several hundred feet back to the drive with little loss of signal.
Level
Level transducers. Level transducers are available in a variety of controlling element styles. A float device is nothing more than a variable resistor, with the wiper arm connected to a flotation device that sits directly on top of the fluid being monitored. Some photoelectric level transducers change the voltage output per the density of the fluid being monitored. Capacitive-type sensors change the amount of capacitance per the proximity of the fluid to the sensing plates. A certain amount of conductivity would be needed in the fluid being monitored.
Thickness
X-ray and ultrasonic. These devices change the output voltage or current per the amount of material being sensed. An X-ray sensor has to monitor X ray-sensitive materials for the sensing element to be effective. Ultrasonic sensors use an electronic oscillator to sense the thickness of the material. A receiver-transmitter is used for this purpose, along with a bridge circuit that does voltage comparisons.
Humidity
Hygrometer. This device uses a medium that changes dimensions according to the amount of humidity in the atmosphere. A hair hygrometer uses strands of human hair attached to a pivot arm. As the humidity changes, the pivot arm moves because of changes in hair length. The pivot arm is attached to a variable resistor, which can be connected to a power source to generate a voltage feedback signal.
Density Float devices. These devices change the amount of voltage output per the density of the fluid being monitored. A rod (moveable core) is attached to a float device that moves up or down, depending on material density. The moveable core fits inside a linear differential transformer. The transformer is energized with AC voltage, with the output changing value, depending on how high or low the moveable core slides into the differential transformer. Some sort of rectification is needed before this type of signal could be fed back to the drive.
There are a variety of other sensors and feedback devices not mentioned above. The point to remember is that the drive needs an electrical feed back or reference signal to change speed or torque (current) output. All drives accept a standard analog input value of 0-10 VDC or 4-20 mA DC. These values could be a set point (reference) or a feedback. The drive can respond to the analog input signals of the voltage or current values stated above. The drive does not care what the controlling element is, as long as the analog input is of the value and type it can respond to.
SUMMARY
The two types of drive control methods are open and closed loop. Open loop is operating a motor directly from the drive unit. Closed loop includes some type of feedback device that is connected directly to the motor shaft.
The speed regulation of the system can be improved by using a feedback device.
Typical drive feedback devices include tachometers (tachs), encoders (or pulse tachs), and resolvers. Tachometers are available in AC or DC versions. DC tachs are used in a variety of industrial applications, since they are able to indicate direction and are generally more accurate than AC tachs.
Encoders use a system of light beams directed toward a light sensor. The light beams are created by shining a light source through a rotating, slotted disk. The disk is connected to the encoder shaft, which is connected directly to the motor shaft through a coupling.
Resolvers are position-type devices. These devices are typically used on servo applications, where precise feedback of the rotor position is required.
Resolvers are similar to an AC induction motor, as well as acting like a revolving transformer.
DC drive control methods include closed-loop control using a tach feed back device. It also includes armature voltage feedback or EMF control.
With EMF control, a sampling of the output voltage is used as feedback to correct for output speed and current (torque).
AC drive control methods also include tach or encoder feedback, as well as operating the VFD by open-loop methods. A variety of transducers can be used as feedback devices. Pressure, flow, level, temperature, and humidity are just a few of the controlling elements sensed by transducers.
Dynamic and static speed regulation are part of the setup process of the performance drive application. The accuracy of the speed or torque is dependent on the feedback device, as well as the closed-loop control circuit within the drive. The amount of dampening will determine if the system will be stable or produce oscillations.
QUIZ
1. Describe open-loop vs. closed-loop control.
2. What is a pulse tach and how does it work?
3. What is the difference between an AC and a DC tach?
4. How is the resolution of an encoder indicated?
5. Describe the EMF, current, and IR compensation feedback loops in a DC drive system.
6. How is a pressure transducer used in a closed-loop AC drive system?
7. Describe the difference between static and dynamic performance of a closed-loop system.
8. What is bandwidth?
9. How is stable operation obtained with an AC drive system?
10. What is the operating principle of temperature and humidity sensors?
ANSWERS--Section 5
1. Open-loop control means the motor is operated directly from the drive unit. In closed-loop control, the motor is connected to the drive, and a feedback device is connected to the motor shaft. The feedback device sends a 0-10 VDC or 4-20 mA signal back to the drive as a representation of actual motor shaft speed.
2. A pulse tach is basically a digital encoder. It uses a light source that shines a light beam through a slotted disk. A photo-detector receives the light beam in the form of pulses. The longer the light beam creates pulses, the slower the disk is rotating (the disk is connected directly to the motor shaft).
3. An AC tach produces an analog output per the revolutions of a rotating device within a magnetic field. AC tachs are devices that indicate strictly speed. No directional indication is available. DC tachs produce a DC voltage output in terms of volts per 1000 RPM. They are generally more accurate than AC tachs and also have the ability of directional indication.
4. Resolution is indicated in PPR (pulses per revolution). An encoder that generates 1024 PPR indicates 1/1024 of a revolution or the equivalent of 0.35 degrees of rotation.
5. The armature voltage feedback (EMF) is a representation of the output voltage, which is a direct indication of speed. The current feedback is a representation of current output from the drive. The IR compensation feedback is used to increase the speed reference circuit to increase speed due to loading of the motor. All three signals are brought back to the summing circuit. If the current feedback signal indicates the drive is exceeding the current capability, the drive will automatically reduce the speed reference. The speed reference will remain at a lower value until the current drops to an acceptable level.
6. The pressure transducer supplies a direct reading signal back to the drive.
The signal would be 0-10 VDC or 4-20 mA. The feedback signal is fed back to the regular circuit (which contains a summing circuit). The regulator compares the feedback with the pressure set point and generates an error signal. That error signal is used as a reference to the drive power section, which increases or decreases the voltage and frequency output to the motor.
7. Static performance (static speed regulation) is a change in steady-state speed that is caused by a change in load. Dynamic performance describes the operation during the transition from one operating point to another.
8. Bandwidth is used to describe a drive's ability to accurately follow a changing reference signal. Bandwidth units of measure are given in radians per second.
9. The system is typically adjusted so that motor oscillations are at a mini mum or low value after a change in speed or load. Oscillations (instability) may occur after a change, but they rapidly decrease to a low value in a properly tuned system. The ideal adjustment result would be to have a slightly under damped or critically damped system.
10. Both devices use the principle of variable resistance. A temperature sensor uses a material that changes resistance per the ambient temperature.
A humidity sensor uses a material that changes length per the amount of humidity in the atmosphere. A hair hygrometer is a device that uses the changing length of human hair to operate a pivot connected to a variable resistor. In both cases, the output is a variable resistance, which if fed to a power supply, generates a variable voltage, which can be fed to the drive (as a reference or feedback signal).