Better Edge Determination for Rotary Encoders.

qd200

A standard edge sensing scheme for optical rotary encoders is to square up the raw analog sine wave by comparing it against a given voltage.   If your analog signal has a 2Vpp amplitude, the value at which you switch off of, creating the edges of your square waves, could be around one Volt.   This would put the determination point right in the middle of the analog signal and provide  symmetrical high and low pulse times.

An example of this can be seen below.  The Blue sin wave is the raw analog signal from the sensor, the red line is the DC voltage which it is compared against. You can see at that the crossing points of the red and blue lines are the points at which the edges of the square waves are determined. The  Magenta waveform is the digital output of the optical encoder.
Standard-low-amp

But what happens if the analog signal amplitude changes?   Raw signal amplitude does vary  over time, temperature, and rotational speed.  Below is an example of the raw analog amplitude of a signal changing to twice the normal value.

Note that the frequency of the signal hasn’t changed.  Frequency is determined by the optical encoders rotation speed.

Standard-high-amp

We see that the Red line is at the same voltage level, which it would be, as a resistor divider, or Zener diode often sets this value,  but the edge determination points have changed as the red line is now cutting though the sine wave at a position that is lower relative to the rest of the waveform.  As you can see by the magenta digital output, this creates asymmetric high and low pulse times.

This is real positional error that will vary with time, temperature, and speed.

Quantum Devices Inc. patented sensing technology combats this by using two sets of analog signals for each channel and comparing them against each other.

QDI-low-amp

Instead of using a fixed Voltage as the comparing signal, a complementary sine wave is used. The QDI interlaced sensor provides another sine wave that is a perfect match and exactly 180 degrees out of phase with the original.  This complementary signal is shown in Red. The standard analog signal as shown in Blue.

QDI-high-amp

When we look at a doubling in amplitude under this scenario, the advantage of making edge-determining decisions at the crossing points of the signal becomes clear.  Not matter how much the amplitude changes, the complementary signal changes to match, maintaining perfect symmetry of the encoder signal.

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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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