Pat Douglas
Kirby Risk-Mechanical Solutions & Service

An encoder is a type of feedback device that is often installed on a motor to monitor the motion (simple optical). A quadrature encoder indicates both motion and direction of the motor output shaft. 

Figure 1 indicates a good digital waveform.  Figure 2 indicates a noisy digital waveform. Both are scans from an oscilloscope used in a service center.

There are many different mounting options. Some are integral to the motor; others are coupled to the rear of the motor shaft. Some models have a wheel that fits on the motor shaft front or rear and the encoder stator mounts directly to a c-face on the motor. 

Most optical single-ended and quadrature encoders can be tested with just a few basic tools.

• Two-channel oscilloscope
• 0-24 volts DC (VDC) clean power source
• Battery or air driven drill motor or turn by hand (standard 110 volt drill may cause signal interference)
• Female pins to fit the encoder connector pins
• To make testing easier if a mating connector is available, solder a wire to each pin and label accordingly 
• The first check is to make sure the shaft turns free and smooth. If not, you may want to consider a replacement unit
• Acquire a wiring diagram and operating voltage specification for the encoder. Many times this information will be on the encoder label or is available on the Original Equipment Manufacturer (OEM) website. 

Follow these steps for the test procedure:

1. Locate and connect power supply leads to the DC+ pin and the common or DC- pin. Do not apply voltage to any of the signal pins as this may damage the encoder. Locate the pins for channel A and A', connect one each to the scope leads.
2. Turn on your power supply to the voltage rating on the encoder, usually 5 VDC or older units 7 VDC
3. Drive the shaft with a drill motor or turn by hand at any speed and you should be able to adjust your scope and see a square wave signal.  A' should have the direct inverse of A. See Figure 3.
4. Move one scope lead from A' to B channel and the waveform should be shifted 90 degrees from A.
5. Move the second scope leads from A to B' and the waveforms should be inverted from each other as shown in Figure 3. 
6. Move the scope lead to Z and Z',  this one is sometimes difficult to see as there will only be one pulse per revolution. 

There are numerous different variations of encoders so it is always best to acquire the wiring diagram and a waveform sample from the OEM.

This simple test will show if the encoder is functional but to insure it is working as designed you must have all of manufacturers’ data, proper test connector and be able to load the encoder and count pulses.

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