Skewed Encoder Waveform

I received this e-mail from a potential customer who is trying to determine why his Encoder waveform doesn’t look right.  His name has been changed to protect his identity.

Hi Jim

I have just come across your Web page on RPM calculation using an optical encoder and oscilloscope. I was keen to test out this method of RPM calculation so rigged up my little encoder and oscilloscope without hesitation. I don’t seem to be getting a nice wave wave form across my display, its rather skewed. Could you just point out where I’m going wrong?

Really enjoyed reading your articles. Look forward to hearing back from you soon.

Eddie

Eddie’s photos are below:

Hi Eddie,

I would love to say the problem is that you aren’t using a Quantum Encoder….

But instead it looks like you are just missing a ground reference for the scope.   There is usually a little black alligator clip hanging off the side of the scope probe. That clip needs to be attached to the signal common on the encoder (black or negative on the power supply)

The red arrow below indicates where the ground clip should connect to the scope probe.

The reason your waveform looks  skewed is because the absence of a ground reference causes the scope to pick up ambient 60 Hz noise (it is everywhere, outlets, lights etc.) and couple it with your encoder signal.

Connecting the scope ground to the incremental encoder signal common will clear that right up.

Below is a picture of a scope probe with the ground clip.

Take care,

Jim

Jim Miller is a Design/Application engineer working for Quantum Devices Inc.

He can be reached at (608) 924-3000, or via e-mail at jmiller@quantumdev.com.

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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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