Drives and Controls: Measurement Systems Terminology

1.1 Measurement Units

The linear unit of length is the meter (m). It is the distance light travels in approximately 1/300,000,000 (1/299,792,458) s. Linear measurement systems commonly define design parameters in units of the micron u ), or 0.000001 m. 1 u is equivalent to approximately 0.000040 in. 0.0001 in = 2.54 μ . The angular unit's the radian (rad), which is the angle subtended by an arc whose length is equal to the radius of a circle.

This unit of measurement is most commonly used in military applications. The degree (°) is used mostly in commercial applications. Fine angles are represented as both fractions of degrees and as minutes (') and seconds ("). 1 ' = 1/6 0 deg; 1 " = 1/6 0’.

1.2 Accuracy and Resolution Defined

Accuracy is the ability to repeatably indicate an exact location, while resolution is the ability to detect motion in finer and finer increments. For a rotary encoder, this is in cycles per revolution (cpr) or pulses per revolution (ppr). For a linear system, this is counts per inch, or it's defined in terms of the graduation pitch in microns.

Accuracy and resolution are not directly related. Although it's generally true that high accuracy systems usually resolve smaller increments, a measuring device could in principle have very coarse resolution and still be very accurate.

1.3 Quadrature

In ill. 10.1, the 9 0 deg electrical separation (one-quarter period) between the two signals is referred to as quadrature. Quadrature signals allow the user to know what direction the system is turning, and provide additional resolution by allowing edge counting.

1.4 Edge Counting

Again referring to ill. 10.1, it can be seen that within one cycle, there are four edge transitions between the two output signals. This can effectively be used to provide a resolution of 4 times the base resolution.

1.5 Direction Sensing

Referring to ill. 10.2, one can see that when B makes a low transition, the value of A is locked into Q. When the system is moving clockwise (CW), Q will be low.

When the system is moving counterclockwise (CCW), Q will be high. This scheme can be used within the sensor to provide a pulse output with a high/low direction indicator.

Ill. 10.1 Output waveform definitions.

1.6 Interpolation or Multiplication

Interpolation is the process of dividing an analog signal into phase-shifted copies, which are then recombined to give a higher effective resolution. When the output of a sensor is sinusoidal and there are two outputs in quadrature, the signals can be interpolated. Transistor-transistor logic (TTL) signals can't be interpolated. As a result, interpolation can be used to improve overall accuracy by reducing the error component due to quantization.

ill. 10.2 Quadrature direction encoding.

1.7 Contacting Systems

There are various interpretations of what this term means. Linear encoders that use bearings to control the gap between the read head and the scale are called noncontacting.

Linear encoders that use low-friction coatings on the glass surfaces to float the read head over the scale are contacting. A more explicit definition of contacting sensors includes potentiometers and pin-contact encoders. Although contact methods are still used, and some companies have developed very robust examples, long-term reliability is favoring noncontacting designs. Some applications still find uses for contacting sensors, especially pin-contact encoders. One major example is in the nuclear industry, where pin-contact encoders generally last as long as the measured system itself. Magnetic systems loose magnetization, and optical systems using plastic are fogged due to the radiation in these environments, so pin-contact encoders work very well.

1.8 Non-contacting Systems

These systems generally offer higher reliability, and are typified by the following Optical, capacitive, and magnetic encoders Brushless resolvers Most modular or kit encoders Open-frame linear scales.

Note that the use of incorporation of bearings into a feedback device does not exclude it from being described as a noncontacting sensor. Make sure you are fully aware of the manufacturing principles when specifying a noncontacting sensor. Truly noncontacting sensors, like modular rotary encoders or brushless resolvers, can still become partially contacting devices if seals are incorporated in the final installation to the application.