Temperature Effects on Optical Encoders

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In Optical Encoders the item most affected by temperature is the output of the Light source. In most cases this is an LED.

Below is a graph of the relative light output of the light source measured as photocell current on an optical encoder over temperature.

led-vs-temp2

You can see that the light output declines as temperature increases and increases as the temperature declines.

While colder looks better, it should be kept in mind that in a rapidly changing temperature environment or in one of high humidity there is the possibility of condensation on the optical disk. Condensation can occlude the disk, limiting light output.

The ultimate effect of high temperature on the optical encoder light source is that reduced light output means reduced signal amplitude.  Many encoders are susceptible to amplitude changes particularly when it comes to symmetry.  In order to square the signal to generate quadrature output, there is typically a comparator that determines “high” or “low” outputs by comparing the analog sensor output voltage to a given voltage.  A simplified version of this is displayed below:

se-50-dutyThe Red line is a typical analog sine wave style output from the encoder sensor.

The blue line represents the fixed voltage that determines the  decision or switching points to square off the analog signals into usable digital signals.

The black waveform at the bottom is representative of the digital output resulting from the crossing of the analog signal against the fixed voltage.

As the analog voltage from the sensor changes over temperature, the tripping points of the comparator will change as well. This can result in symmetry (duty cycle) swings, A to B phasing variation or loss of signal altogether.

Below is simple representation of a reduction in overall signal amplitude for this style of sensing.  Notice how the symmetry or duty cycle in the black digital waveform has been adversely affected by the change in amplitude.

se-off-duty1The amplitude has been reduced by about 40% using our light source graph from above, we see that this  indicates a temperature change equivalent to going from about room temperature to 100 Deg C.

Whether or not the change in symmetry will have an affect on the system this encoder is employed in will depend on the system itself and its ability to withstand phasing and duty cycle errors.

It is however, obvious from this graph that the amplitude doesn’t have to drop much further before it is below the blue voltage level line and the digital output signal is lost altogether.

It is easy to see why some manufactures using this scheme will limit their encoders to only 85 Deg C.

In these examples I have not shown a change in the amplitude of AC component, but only the DC offset.  Keep in mind that the peak to peak amplitude of the signal would be affected as well and further the effects of temperature on signal reduction.

The QDI sensing technology uses a set of complementary signals on each channel and compares the crossing points to determine digital signal switch points.  Since each signal rides along the same DC offset, amplitude variations have no effect.

Below is a representation of how the complementary signals are used to create decision points.  The fixed blue line is replaced by a sensor signal that is 180 electrical Degrees out of phase with the original sensor signal.

cm-50-duty1

This technology allows for large variations in amplitude due to temperature without affecting the signal integrity.

In the representation below the amplitude is again reduced by40%, as might happen with an increase in temperature.

This time there is no adverse affect on the digital signal symmetry or phasing.

cm-50-duty-reduced-amp

Once again the AC Peak to Peak amplitude was not changed in this example, but would be in real world application.

The end result would ultimately be the same (no signal change) as the interlaced sensor technology amplitude changes are symmetrical in nature and always at exactly the same DC offset level.

Because there are no changes in the relative crossing points of the sine waves this allows QDI Encoders to maintain excellent symmetry and signal integrity over temperature.


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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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