Using an optical encoder to measure angles

I have interfaced a 200 LC QD145 to a DL06 PLC to show how to convert from a line count to mechanical degrees. This type of conversion may be useful for any application needing to know an angular measurement.

To calculate a degree measurement we divide 360 by the line count to get the number of degrees per pulse.

(360 Degrees /200 Pulses per revolution) = 1.8 Degrees per pulse.

The High Speed counter we have set up will automatically add one to it’s running total any time the encoder is rotated counter-clockwise, and subtract one from the running total any time the optical encoder is rotated clockwise.

This value is loaded into the PLC’s accumulator and multiplied by 1.8  (K18) to convert to degrees. The number is then outputted to an address(V2500)that we can display on the screen.

When the index (Z) pulse occurs, we reset the counter to let it know we are back at zero.

Below is the PLC code for the pulses to degree conversion.

It is good to note that the PLC is set up to retain the count value when powered off, but if the optical encoder is rotated during this time, the count will not change and the value at power up will be different than the encoders real world position.

It is good practice to rotate an incremental encoder/optical encoder on power up until an index pulse is seen and start counting from there. This is technique is known as “homing”.

By default the quadrature counting mode within the PLC keeps track of negative numbers, so we are able to accumulate a negative degree value depending on the direction of rotation after zero. While this may seem a bit confusing, it is really just a matter of your point of reference. –90 Degrees is the same exact point as a positive 270 Degrees.  If we wanted to convert to where we stayed within a positive degree range, you could change the PLC code to add 360 to the measured value any time it went negative.

For more information on encoders go to

For technical questions Jim can be reached at:


About Quantum Devices Inc.
Quantum Devices, Inc. (QDI) Barneveld, WI, was established as a Wisconsin corporation in October 1989, as an outgrowth of activities in the general area of optoelectronics. The main goal of QDI is to provide customers with a complete source for all stages of product design and development, starting from concept to a marketable commodity. These products include application specific Silicon Photodiodes, Optical Encoders and Light Emitting Diodes (LED) for commercial, industrial and medical applications. QDI's photodiodes and LED's are used in instrumentation for photosynthesis and photobiological research, biomedical and medical instrumentation for measuring blood sugar levels (glucometer), oxyhemoglobin and pulse rate (pulse oxymeter). One of the most dramatic products developed at QDI was the application of LED lighting systems for use as the light source for Photodynamic Therapy (PDT). Sales of QDI products include both domestic and international markets. Quality and reliability are very important concepts in maintaining our company wide commitment to overall product performance. Quality simply means continuous process improvement. We are committed to continually increasing our product excellence through increased quality and reliability. Quantum Devices, Inc. is ISO 9001 certified and the ISO Standard will provide the guidance for the vital function of maintaining our commitment to constantly improve our product quality. Quantum Devices is now proud to introduce it's new line of rotary encoders. Incorporating QDI's patented sensor technology, our QPhase™ Family of Encoders feature high resolution, increased frequency response and superior reliability over temperature (0 to 120 degrees C). QDI incremental and absolute encoders set a new standard and are backed by a 2 year factory warranty. We invite you to request an evaluation unit, additional information or price quotations. Mission Statement Quantum Devices is dedicated to the principle that light provides the power for all life on earth. We believe the quality, delivery and control of light is essential to the wellness of man and his advancement into the future.

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