DC Motors, RC Servos, and Stepper Motors

Perhaps one of the most entertaining things to do with electronics is make some mechanical device move. Three very popular devices used to “make things move” include dc motors, RC servos, and stepper motors.

-- 1. DC Continuous Motors

FGR. 1: RC servos; Stepper motors; Teeth Rotor

DC motors are simple two-lead, electrically controlled devices that come with a rotary shaft on which wheels, gears, propellers, etc., can be mounted. DC motors generate a considerable amount of revolutions per minute (rpm’s) for their size and can be made to rotate clockwise or counterclockwise by reversing the polarity applied to the leads. At low speeds, dc motors provide little torque and minimal position control, making them obsolete for pointlike position-control applications.

DC motors are available in many different shapes and sizes. Most dc motors pro vide rotational speeds anywhere between 3000 and 8000 rpm at a specific operating voltage typically set between 1.5 and 24 V. The operating voltage provided by the manufacturer tells you at what voltage the motor runs most efficiently. Now, the actual voltage applied to a motor can be made slightly lower to make the motor slower or can be elevated to make the motor faster. However, when the applied volt age drops to below around 50 percent of the specified operating voltage, the motor usually will cease to rotate. Conversely, if the applied voltage exceeds the operating voltage by around 30 percent, there is a chance that the motor will overheat and become damaged. In practice, as you will see in a second, the speed of a dc motor is most efficiently controlled by means of pulse-width modulation, whereby the motor is rapidly turned on and off. The width of the applied pulse, as well as the period between pulses, controls the speed of the motor. Also, it’s worth noting that a freely running motor (no load) may draw little current (power). However, if a load is applied, the amount of current drawn by the motor’s inner coils goes up immensely (up to 1000 percent or more). Manufacturers usually will provide what is called a stall current rating for their motors. This rating specifies the amount of current drawn at the moment the motor stalls. If your motor’s stall current rating is not listed, it’s possible to determine it by using an ammeter; slowly apply a force to the motor’s shaft, and note the current level at the point when the motor stalls. Another specification given to dc motors is a torque rating. This rating represents the amount of force the motor can exert on a load. A motor with a high torque rating will exert a larger force on a load placed at a tangent to its rotational arm than a motor with a lower torque rating. The torque rating of a motor is usually given in lb/ft. g/cm, or oz/in.

-- 2. Speed Control of DC Motors

FGR. 2 --- Bad Designs

FGR. 3 --- Better Designs. UJT/SCR Control Circuit

A seemingly obvious approach to control the speed of a dc motor would be simply to limit the current flow by using a potentiometer, as shown in the circuit to the left in the figure. According to Ohm’s law, as the resistance of de the pot increases, the current decreases, and the motor will slow down. However, using a pot to control the current flow is inefficient. As the pot’s resistance increases, the amount of current energy that must be converted into heat increases. Producing heat in order to slow a motor down is not good—it consumes supply power and may lead to potentiometer melt down. Another seemingly good but inefficient approach to control the speed of a motor is to use a transistor amplifier arrangement like the one shown to the right in the figure. However, again, there is a problem. As the collector-to-emitter resistance increases with varying base voltage/current, the transistor must dissipate a considerable amount of heat. This can lead to transistor meltdown.

In order to conserve energy and prevent component meltdown, an approach similar to what was used in switching power supplied is used to control the speed of the motor. This approach involves sending the motor short pulses of current. By varying the width and frequency of the applied pulses, the speed of the motor can be controlled. Controlling a motor’s dc speed in this manner prevents any components from experiencing continuous current stress. FGR. 3 shows three simple circuits used to provide the desired motor-control pulses.

In the first circuit, a UJT relaxation oscillator generates a series of pulses that drives an SCR on and off. To vary the speed of the motor, the UJT’s oscillatory frequency is adjusted by changing the RC time constant.

In the second circuit, a pair of NAND gates make up the relaxation oscillator section, while an enhancement-type power MOSFET is used to drive the motor. Like the preceding circuit, the speed of the motor is controlled by the oscillator’s RC time constant. Notice that if one of the input leads of the left NAND gate is pulled out, it’s possible to create an extra terminal that can be used power to provide on/off control that can be interfaced MOSFET with CMOS logic, circuits.

The third circuit is a 555 timer that is used to generate pulses that drive a power MOSFET. By inserting a diode between pins 7 and 6, as shown, the 555 is placed into low-duty cycle operation. R1, R2 and C set the frequency and on/off duration of the output pulses. The formulas accompanying the diagram provide the details.

FGR. 3 (Continued) --- CMOS/MOSFET Control Circuit; free one NAND input for ON/OFF control; 555 Timer/MOSFET Control Circuit; power enhancement mode; dc motor.

-- 3. Directional Control of DC Motors

To control the direction of a motor, the polarity applied to the motor’s leads must be reversed. A simple manual-control approach is to use a DPDT switch (see leftmost circuit in FGR. 4). Alternately, a transistor-driven DPDT relay can be used (see middle circuit). If you don’t like relays, you can use a push-pull transistor circuit (see leftmost circuit). This circuit uses a complementary pair of transistors (similar betas and power rating)—one is an npn power Darlington, and the other is a pup power Darlington. When a high voltage (e.g., +5 V) is applied to the input, the upper transistor (NPN) conducts, allowing current to pass from the positive supply through the motor and into ground. If a low voltage (0 V) is applied to the input, the lower transistor (PnP) conducts, allowing current to pass through the motor from ground into the negative supply terminal.

Another very popular circuit used to control the direction of a motor (as well as the speed) is the H-bridge. FGR. 5 shows two simple versions of the H-bridge circuit. The left H-bridge circuit is constructed with bipolar transistors, whereas the right H-bridge circuit is constructed from MOSFETs. To make the motor rotate in the forward direction, a high (+5-V) signal is applied to the forward input, while no signal is applied to the reverse input (applying a voltage to both inputs at the same time is not allowed). The speed of the motor is controlled by pulse-width modulating the input signal. Here is a description of how the bipolar H-bridge works: When a high voltage is applied to Q3’s base, Q conducts, which in turn allows the PnP transistor Q2 to conduct. Current then flows from the positive supply terminal through the motor in the right-to-left direction (call it the forward direction if you like). To reverse the motor’s direction, the high voltage signal is removed from Q base and placed on Q4’s base. This sets Q. and Q1 into conduction, allowing current to pass through the motor in the opposite direction. The MOSFET H-bridge works in a similar manner. The diodes within the H-bridge circuits help dampen transient spikes that are generated by the motor’s coils so that they don’t damage the other components within the circuit. All transistors (except the bipolar within the MOSFET circuit) should have high power ratings.

FGR. 5 --- MOSFET H-Bridge; MOSFET H-Bridge + 1 2V. A signal must not be applied here when a signal is being applied to the forward lead.

Now, it’s possible to construct these H-bridge circuits from scratch, but it’s far easier and usually cheaper to buy a motor-driven IC. For example, National Semiconductor’s LMD18200 motor-driver IC is a high-current, easy-to-use H-bridge chip that has a rating of 3 A and 12 to 55 V. This chip is TTL and CMOS compatible and includes clamping diodes, shorted load protection, and a thermal warning interrupt output lead. The L293D (Unitrode) is another popular motor-driver IC. This chip is very easy to use and is cheaper than the LMDI8200, but it cannot handle as much current and does not pro vide as many additional features. There are many other motor-driver ICs out there, as well as a number of prefab motor-diver boards that are capable of driving a number of motors. Check the electronics catalogs and Internet to see what is available.

-- 4. RC Servos

Remote control (RC) servos, unlike dc motors, are motorlike devices designed specifically for pointerlike position-control applications. An RC servo uses an external pulse-width-modulated (PWM) signal to control the position of its shaft to within a small fraction of its maximum range of rotation. To alter the position of the shaft, the pulse width of the modulated signal is varied. The amount of angular rotation of an RC servo’s shaft is limited to around 180 or 210° depending on the specific brand of servo. These devices can provide a significant amount of low-speed torque (due to an internal gearing system) and provide moderate full-swing displacement switching speeds. RC servos frequently are used to control steering in model cars, boats, and airplanes. They are also used commonly in robotics as well as in many sensor- positioning applications.

The standard RC servo looks like a simple box with a drive shaft and three wires coming out of it. The three wires consist of a power supply wire (usually black), a ground wire (usually red), and the shaft-positioning control wire (color varies based on manufacturer). Within the box there is a dc motor, a feedback device, and a control circuit. The feedback device usually consists of a potentiometer whose control dial is mechanically linked to the motor through a series of gears. When the motor is rotated, the potentiometer’s control dial is rotated. The shaft of the motor is usually limited to a rotation of 180° (or 210°)—a result of the pot not being able to rotate indefinitely. The potentiometer acts as a position-monitoring device that tells the control circuit (by means of its resistance) exactly how far the shaft has been rotated. The control circuit uses this resistance, along with a pulse-width-modulated input control signal, to drive the motor a specific number of degrees and then hold. (The amount of holding torque varies from servo to servo.) The width of the input signal determines how far the servo’s shaft will be rotated.

FGR. 6: +12V --- A typical servo-control signal and shaft-position response; Simple Servo Driver

By convention, when the pulse width is set to 1.5 ms, the servo rotates its shaft to neutral position (e.g., 90° if the servo is constrained within a 0 to 180° range). Now, to rotate the shaft a certain number of degrees from neutral position, the pulse width of the control signal is varied. In order to make the shaft go counterclockwise from neutral, a pulse wider than 1.5 ms is applied to the control input. Conversely, to make the shaft go clockwise from neutral, a pulse narrower than 1.5 ms is applied (see figure). Knowing exactly how much wider or how much narrower to make the pulse to achieve exact angular displacements depends largely on what brand of servo you are using. For example, one brand of servo may provide maximum counterclockwise rotation at I ms and maximum clockwise rotation at 2 ms, whereas another brand of servo may provide maximum counterclockwise rotation at 1.25 ms and maximum clockwise rotation at 1.75 ms. The supply voltage used to power servos is commonly 4.8 V but may be 6.0 V or so depending on the specific brand of servo. Unlike the sup ply voltage, the supply current drawn by a servo varies greatly, depending of the servo’s power output.

A simple 555 timer circuit like the one shown in FGR. 6 can be used to generate the servo control signal. In this circuit, R acts as the pulse-width control. Servos also can be controlled by a microprocessor or microcontroller.

Now, when controlling servos within model airplanes, an initial control signal (generated by varying position-control potentiometers) is first sent to a radiowave modulator circuit that encodes the control signal within a carrier wave. This carrier wave is then radiated off as a radiowave by an antenna. The radiowave, in turn, is then transmitted to the model’s receiver circuit. The receiver circuit recovers the initial control signal by demodulating the carrier. After that, the control signal is sent to the designated servo within the model. If there is more than one servo per model, more channels are required. For example, most RC airplanes require a four-channel radio set; one channel is used to control the ailerons, another channel controls the elevator, another controls the rudder, and another controls the throttle. More complex models may use five or six channels to control additional features such as flaps and retractable landing gear. The FCC sets aside 50 frequencies in the 72-MHz band (channels 11—60) dedicated to aircraft use only. No license is needed to operate these radios. However, with an amateur (ham) radio operator’s license, it’s possible to use a radio within the 50-MHz band. Also, there are frequencies set aside within the 27-MHz band that are legal for any kind of model use (surface or air). If you are interested in radio-controlled RC servos, a good starting point is to check out an RC model hobby shop. These shops carry a number of transmitter and receiver sets, along with the servos.

As a final note, with a bit of rewiring, a servo can be converted into a drive motor with unconstrained rotation. A simple way to modify the servo is to break the feed back loop. This involves removing the three-lead potentiometer (and unlinking the gear system so that it can rotate 360°) and replacing it with a pair of voltage-divider resistors (the output of the voltage divider replaces the variable terminal of the potentiometer). The voltage divider is used to convince the servo control circuit that the servo is in neutral position. The exact values of the resistors needed to set the servo in neutral position can be determined by using the old potentiometer and an ohmmeter. Now, to turn the motor clockwise, a pulse wider than 1.5 ms is applied to the control input. As long as the control signal is in place, the motor will keep turning and not stop— you have removed to feedback system. To turn the motor counter clockwise, a pulse narrower than 1.5 ms is applied to the control input.

-- 5. Stepper Motors

Stepper motors, or steppers, are digitally controlled brushless motors that rotate a specific number of degrees (a step) every time a clock pulse is applied to a special translator circuit that is used to control the stepper. The number of degrees per step (resolution) for a given stepper motor can be as small as 0.72° per step or as large as 90° per step. Common general-purpose stepper resolutions are 15 and 30° per step. Unlike RC servos, steppers can rotate a full 360° and can be made to rotate in a continuous manner like a dc motor (but with a lower maximum speed) with the help of proper digital control circuitry. Unlike dc motors, steppers provide a large amount of torque at low speeds, making them suitable in applications where low-speed and high-precision position control is needed. For example, they are used in printers to control paper feed and are used to help a telescope track stars. Steppers are also found in plotter- and sensor-positioning applications. The list goes on. To give you a basic idea of how a stepper works, take a look at the FGR. 7.

FGR. 7 Four-coil stepper motor (15 degrees/step)

Here is a simple model depicting a 15° per step variable-reluctance step per. The stationary section of the motor, called the suitor, has eight poles that are spaced 45° apart. The moving section of the motor, called the rotor, is made from a ferromagnetic material (a material that is attracted to magnetic fields) that has six teeth spaced 60° apart. To make the rotor turn one step, current is applied, at the same time, through two opposing pole pairs, or coil pairs. The applied current causes the opposing pair of poles to become magnetized. This in turn causes the rotor’s teeth to align with the poles, as shown in the figure. To make the rotor rotate 15° clockwise from this position, the current through coil pair 1 is removed and sent through coil pair 2. To make the rotor rotate another 15° clock wise from this position, the current is removed from coil pair 2 and sent through coil pair 3. The process continues in this way. To make the rotor spin counterclockwise, the coil-pair firing sequence is reversed.

-- 6. Kinds of Stepper Motors

The model used in the last example was based on a variable-reluctance stepper. As it turns out, this model is incomplete—it does not show how a real variable-reluctance stepper is wired internally. Also, the model does not apply to a class of steppers referred to as permanent-magnet steppers. To make things more realistic, let’s take a look at some real-life steppers.

FGR. 8

• Firing sequence
• Physical Model
• Unipolar Motors
• Schematic Diagram
• Variable Reluctance Motor
• Schematic Diagram

FGR. 8 (Continued) --- Bipolar Motor; Schematic Diagram Physical Model

Variable-Reluctance Steppers:

FGR. 8 shows a physical model and schematic diagram of a 300 per step variable- reluctance stepper. This stepper consists of a six-pole (or three-coil pair) stator and a four-toothed ferromagnetic rotor. Variable-reluctance steppers with higher angular resolutions are constructed with more coil pairs and/or more rotor teeth. Notice that in both the physical model and the schematic, the ends of all the coil pairs are joined together at a common point. (This joining of the coil ends occurs internally within the motor’s case.) The common and the coil pair free ends are brought out as wires from the motor’s case. These wires are referred to as the phase wires. The common wire is connected to the supply voltage, whereas the phase wires are grounded in sequence according to the table shown in FGR. 8.

Permanent-Magnet Steppers (Unipolar, Bipolar, Universal):

UNIPOLAR STEPPERS:

These steppers have a similar stator arrangement as the variable-reluctance steppers, but they use a permanent-magnet rotor and different internal wiring arrangements. FGR. 8 shows a 30° per step unipolar stepper. Its consists of a four-pole (or two- coil pair) stator with center taps between coil pairs and a six-toothed permanent- magnetic rotor. The center taps may be wired internally and brought out as one wire or may be brought out separately as two wires. The center taps typically are wired to the positive supply voltage, whereas the two free ends of a coil pair are alternately grounded to reverse the direction of the field provided by that winding. As shown in the figure, when current flows from the center tap of winding 1 out terminal Ia, the top stator pole “goes north,” while the bottom stator pole “goes south.” This causes the rotor to snap into position. If the current through winding I is removed, sent through winding 2, and out terminal 2a, the horizontal poles will become energized, causing the rotor will turn 30°, or one step. In FGR. 8, three firing sequences are shown. The first sequence provides full stepping action (what I just discussed). The second sequence, referred to as the power stepping sequence, provides full stepping action with 1.4 times the torque but twice the power consumption. The third sequence provides half stepping (e.g., 15° instead of the rated 30°). Half stepping is made possible by energizing adjacent poles at the same time. This pulls the rotor in- between the poles, thus resulting in one-half the stepping angle. As a final note, unipolar steppers with higher angular resolutions are constructed with more rotor teeth. Also, unipolars come in either five- or six-wire types. The five-wire type has the center taps joined internally, while the six-wire type does not.

BIPOLAR STEPPERS

These steppers resemble unipolar steppers, but their coil pairs don’t have center taps. This means that instead of simply supplying a fixed supply voltage to a lead, as was the case in unipolar steppers (supply voltage was fixed to center taps), the supply volt age must be alternately applied to different coil ends. At the same time, the opposite end of a coil pair must be set to the opposite polarity (ground). For example, in FGR. 8, a 30° per step bipolar stepper is made to rotate by applying the polarities shown in the firing sequence table to the leads of the stepper. Notice that the firing sequence uses the same basic drive pattern as the unipolar stepper, but the “0” and “1” signals are replaced with “+“ and “—“ symbols to show that the polarity matters. As you will see in the next section, the circuitry used to drive a bipolar stepper requires an H-bridge network for every coil pair. Bipolar steppers are more difficult to control than both unipolar steppers and variable-reluctance steppers, but their unique polarity-shifting feature gives them a better size-to-torque ratio. As a final note, bi polar steppers with higher angular resolutions are constructed with more rotor teeth.

UNIVERSAL STEPPERS

These steppers represent a type of unipolar-bipolar hybrid. A universal stepper comes with four independent windings and eight leads. By connecting the coil windings in parallel, as shown in FGR. 8, the universal stepper can be converted into a unipolar stepper. If the coil windings are connected in series, the stepper can be converted into a bipolar stepper.

-- 7. Driving Stepper Motors

Every stepper motor needs a driver circuit that can control the current flow sent through the coils within the stepper’s stator. The driver, in turn, must be controlled by a logic circuit referred to as a translator. I will discuss translator circuits after I have covered the driver circuits.

FGR. 9 shows driver networks for a variable-reluctance stepper and for a unipolar stepper. Both drivers use transistors to control current flow through the motor’s individual windings. In both driver networks, input buffer stages are added to protect the translator circuit from the motor’s supply voltage in the event of transistor collector-to-base breakdown. Diodes are added to both drivers to protect the transistors and power supply from inductive kickback generated by the motor’s coils. (Notice that the unipolar driver uses extra diodes because inductive kickback can leak out on either side of the center tap. As you will see in a moment, a pair of diodes within this driver can be replaced with a single diode, keeping the diode count to four.) The single driver section shown in FGR. 9 provides a general idea of what kinds of components can be used within the driver networks. This circuit uses a high-power Darlington transistor, a TTL buffer, and a reasonably fast protection diode (the extra diode should be included in the unipolar circuit). If you don’t want to bother with discrete components, transistor-array ICs, such as the ULN200x series by Allegro Microsystems or the DS200x series by National Semiconductor, can be used to construct the driver section. The ULN2003, shown in FGR. 9, is a TTL compatible chip that contains seven Darlington transistors with protection diodes included. The 7407 buffer IC can be used with the ULN2003 to construct a full- stepper driver. Other ICs, such as Motorola’s MC1413 Darlington array IC, can drive multiple motor winding directly from logic inputs.

The circuitry used to drive a bipolar stepper requires the use of an H-bridge circuit. The H-bridge circuit acts to reverse the polarity applied across a given coil pair within the stepper. (Refer back to the section on dc direction control for details on how H-bridges work.) For each coil pair within a stepper, a separate H-bridge is needed. The H-bridge circuit shown in FGR. 10 uses four power Darlington transistors that are protected from the coil’s inductive kickback by diodes. An XOR logic circuit is added to the input to prevent two high (l signals from being applied to the inputs at the same time. [ two high signals are placed at both inputs (assuming that there is no logic circuit present), the supply will short to ground. This is not good for the supply.] The table in FGR. 10 provides the proper firing sequence needed to create the desired polarities.

=== ==

• Single Driver Section
• Variable-Reluctance Driver Network
• Unipolar Driver Network
• Transistor and Buffer Arrays
• (from 5 to 24v) control; input; power transistor (e.g. TIPI2O Darlington)

FGR. 9

=== ===

FGR. 10: H-bridge used with Bipolar Stepper; Digital input; Polarity

As mentioned in the dc motor section of this section, H-bridges can be purchased in IC form. SGS Thompson’s L293 dual H-bridge IC is a popular choice for driving small bipolar steppers drawing up to I A per motor winding at up to 36 V. The L298 dual H-bridge is similar to the L293 but can handle up to 2 A per winding. National Semiconductor’s LMDI8200 H-bridge IC can handle up to 3 A, and unlike the L293 and L298, it has protection diodes built in. More H-bridge ICs are available, so check the catalogs.

-- 8. Controlling the Driver with a Translator

A translator is a circuit that enacts the sequencing pulses used to drive a driver. In some instances, the transistor may simply be a computer or programmable interface controller, with software directly generating the outputs needed to control the driver leads. In most cases, the translator is a special IC that is designed to provide the proper firing sequences from its output leads when a clock signal is applied to one of its input leads; another input signal may control the direction of the firing sequence (the direction of the motor). There are a number of steppers translator ICs available that are easy to use and fairly inexpensive. Let’s take a look at one of these devices in a second. First, let’s take a look at some simple translator circuits that can be built from simple digital components.

A simple way to generate a four-phase drive pattern is to use a CMOS 4017 decade counter/divider IC (or a 74194 TTL version). This device sequentially makes 1 of 10 possible output high (others stay low) in response to clock pulses. Tying the fifth output (Q to ground makes the decade counter into a quad counter. To enact the drive sequence, a clock signal is applied to the clock input (see FGR. 11). Another four-phase translator circuit that provides power stepping control as well as direction control can be constructed with a CMOS 4027 dual JK flip-flop IC (or a 7476 TTL version). The CMOS 4070 XOR logic (or 7486 TTL XOR logic) is used to set up directional control.

Perhaps the best translator circuits you can hope for come in integrated packages. A number of manufacturers produce stepper motor controller ICs that house both the translator and driver sections. These chips are fairly simple to use and inexpensive. A classic stepper controller chip is the Philips SAA The SAA1027 is a bipolar IC that is designed to drive four-phase steppers. It consists of a bidirectional four-state counter and a code converter that are used to drive four outputs in sequence. This chip has high-noise-immunity inputs, clockwise and counterclockwise capability, a reset control input, high output current, and output voltage protection. Its supply voltage runs from 9.5 to 18 V, and it accepts input voltages of 7.5 V minimum for high (1) and 4.5 V maximum for low (0). It has a maximum output current of 500 mA. FGR. 13 will paint the rest of the picture.

Above: Fig. 31: power-stepping sequence; single-stepping.

FGR. 12 shows a circuit that contains the translator, driver, and stepper all in one. The motor, in this case, is a unipolar stepper, while the translator is a TTL 74194 shift counter. The 555 timer provides clock signals to the 74194, while the DPDT switch acts to control the direction of the motor. The speed of the motor is dependent on the frequency of the clock, which in turn is dependent on R resistance. The translator in this circuit also can be used to control a variable reluctance stepper. Simply use the variable-reluctance diver from FGR. 9 and the firing sequence shown in FGR. 8 as your guides.

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FGR. 12

FGR. 13: Count input C (pin 15)—A low-to-high transition at this pin causes the outputs to change states.

Mode input M (pin 3)—Controls the direction of the motor. See table to the left.

Reset input R (pin 2)—A low (0) at the R input resets the counter to zero. The outputs take on the levels shown in the upper and lower line of the table to the left.

External resistor RX (pin 4)—An external resistor connected to the RX terminal sets the base current of the transistor drivers. Its value is based on the required output current.

Outputs Q through Q (pins 6, 8, 9, 11)—Output terminals that are connected to the stepper motor.

=== ==

As mentioned, the SAA1027 is a classic chip (old chip). Newer and better stepper control ICs are available from a number of different manufacturers. If you are interested in learning more about these chips, try searching the Internet. You will find a number of useful Web sites that discuss stepper controller ICs in detail. Also, these Web sites often will provide links to manufacturers and distributors of stepper motors and controller ICs.

-- 9. A Final Word on Identifying Stepper Motors

When it comes to identifying the characteristics of an unknown stepper, the following suggestions should help. The vast majority of the steppers on the market today are unipolar, bipolar, or universal types. Based on this, you can guess that if your stepper has four leads, it’s most likely a bipolar stepper. If the stepper has five leads, then the motor is most likely a unipolar with common center taps. If the stepper has six leads, it’s probably a unipolar with separate center taps. A motor with eight leads would most likely be a universal stepper. (If you think your motor might be a variable-reluctance stepper, try spinning the shaft. If the shaft spins freely, the motor is most likely a variable-reluctance stepper. A coglike resistance indicates that the stepper is a permanent-magnet type.)

Once you have determined what kind of stepper you have, the next step is to determine which leads are which. A simple way to figure this out is to test the resistance between various leads with an ohmmeter.

FGR. 14 --- Bipolar-- Unipolar (6-wire)-- Universal

Decoding the leads of a bipolar stepper is easy. Simply use an ohmmeter to deter mine which wire pair yields a low resistance value. A low resistance indicates that the two wires are ends of the same winding. If the two wires are not part of the same winding, the resistance will be infinite. A universal stepper can be decoded using a similar approach. Decoding a six-wire unipolar stepper requires isolating two three- wire pairs. From there, you figure out which wire is the common by noticing which measured pair among the isolated three wires gives a unit R worth of resistance and which pair gives a unit of 2R worth of resistance (see FGR. 14). Now, decoding a five-wire unipolar (with common center tap) is a bit more tricky than the others because of the common, but hidden, center tap. To help decode this stepper, you can use the diagram and table shown in FGR. 14. (The dots within the table represent where the ohmmeter’s two probes are placed within the diagram.) With the table you isolate e (common tap wire) by noting when the ohmmeter gives a resistance of R units. Next, you determine which of the two wires in your hand is actually e by testing one of the two with the rest of the wires. If you always get R, then you are holding e, but if you get 2R, you are not holding e. Once the e wire is determined, any more ohmmeter deducing does not work—at least in theory—because you will always get 2R. The best bet now is to connect the motor to the driver circuitry and see if the step per steps. If it does not step, fiddle around with the wires until it does.