The HR-12 HEDS replacement optical encoder

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HEDS style encoders have long been a popular choice for an enclosed low line count positional feedback device.  The packaging allows for added enclosure protection in a lighter duty encoder. This type of product is often seen in applications such as vending machines, printers, plotters, positioning tables, etc.

The Quantum Devices Inc. HR-12 optical encoder picks up where traditional HEDS replacements leave off, offering line counts of up to 20,000 PPR, (80,000 Post Quad), high temperature range, and commutation (Hall) options. Besides providing all of these extended options, the HR-12 optical encoder also boasts a 500kHz frequency response.

This device can be mounted on multiple bolt circles, and accepts varying shaft sizes up to 10mm.

The HR-12 includes an internal bearing set that eliminates the need for the mounting shaft to hold tight TIR and Axial tolerances.

Installation of the HR-12 encoder can be seen below.

hr12_mounting

For more information on the advantages of the HR-12 encoder check the Quantum Devices Inc. main web site at http://www.quantumdev.com/products/optical_encoders/hr12.html


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What is Hysteresis and how is it used in optical encoders?

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Here are a few definitions of the word “hysteresis” I found online:

From Sensorland.com

Hysteresis – Non-uniqueness in the relationship between two variables as a parameter increases or decreases. Also called deadband, or that portion of a system’s response where a change in input does not produce a change in output.

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From Wikipedia

A system with hysteresis can be summarized as a system that may be in any number of states, independent of the inputs to the system. To be exact, a system with hysteresis exhibits path-dependence, or “rate-independent memory”[citation needed]. By contrast, consider a deterministic system with classical dynamics but no Hysteresis. In that case, one can predict the output of the system at some instant in time, given only the input to the system at that instant. If the system has hysteresis, then this is not the case; one cannot predict the output without looking at the history of the input, i.e., the state of the system for a given input. In order to predict the output, one must look at the path that the output followed before it reached its current value.


Here is my spin on hysteresis:

The place where we can most relate to hysteresis is in our home.  Furnaces and air conditioning systems use hysteresis to help buffer the set point at which they turn on and off.  If you can imagine having a temperature set point of exactly 70 degrees, the moment the temperature dropped to 69.9 degrees the furnace would come on. If it rose to 70.1 degrees, the AC would come on. You can see how this could result in frequent cycling of the equipment in our home.

In order to reduce this, hysteresis is used to buffer the area around our set point. In HVAC equipment it is often called “temperature swing”.  For example at my house I have a 3 degree swing set up in my programmable thermostat. This means that at a setpoint of  70 degrees, my furnace doesn’t come back on until it reaches 67 degrees.  I could tighten this up to say a one degree swing, but it costs a bit more to heat that way.

For me, it is a careful balance between saving money and keeping my girlfriend from complaining about how cold it is.

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How it relates to Optical Encoders:

In optical encoders hysteresis is used to buffer the analog signal coming from the sensor before a switching decision is made.  This is particularly needed when rotary encoders are turning very slowly. As the decision point slowly approaches any variation in signal amplitude due to noise could cause the digital output to quickly switch on and off this would be seen as several quick pulses at the leading and trailing edges of the digital square waves often referred to as “chattering”.

Below is a picture of how too little hysteresis may affect encoder signals. Particularly  in a system with electrical noise.

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Below is a representation of the analog signal (shown in red) of an encoder as it crosses the decision point (blue line). The output is the black digital signal.

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This is what we would expect, and get,  in a perfect electrical world.  In the real world we have to deal with issues such as electrical noise.

The image below shows how electrical noise introduced in a system can cause extra pulse or “chattering. The noise is shown as a voltage anomaly on the red analog line causing multiple crossing points along the blue decision line, resulting in extraneous pulses at the edge of our digital signal.

single-pulse-chatter

In the next image we see how the addition of hysteresis affects the digital output signal. The cyan line represents the delay is decision point or “deadband” that is created by the addition of hysterysis.  The digital signal switches high with the original red analog signal, but doesn’t switch off again until the cyan hysteresis line crosses the blue decision point.

The new hysteresis filtered digital output is shown in magenta with the original unfiltered digital output shown in black.

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One would tend to think that the more Hysteresis you add, the better as it adds more noise immunity to the system. This is true, but the other side of adding Hysteresis is that it results in positional error.

Since we are essentially delaying the point at which the digital decision is made, we are delaying the point in rotation before the signal is switched on.

In the image above when comparing the black digital signal to the magenta one we notice how the switch off point has been delayed in time. This is real system error that is the cost of hysteresis.

We can compare this to a HVAC system in that additional hysteresis around the temperature set point makes it easier for the system to withstand thermal “noise”, such as a quick temperature variation from opening a door, but ultimately keeps the system from tightly regulating to a temperature set point.

Encoder manufacturers always have to carefully balance the amount of hysteresis they add against the error introduced in the system from it.

The way that Quantum Devices fights this is through the use of their interlaced sensor, and use of differential signals to determine switching points. The interlaced sensor provides a large amplitude signal when compared to typical noise introduced into a system, so that minimum hysteresis is needed when making the switching decisions.

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Hysteresis in Magnetism
Here are some links to some information on the origin of the word hysteresis and how it relates to magnetism.

Magnetic hysterysis in the iron core or “stack” of electric motors is what is responsible for a significant portion of heat build up.
The last link is particularly interesting as it talks about how hysterysis curves in magnetic materials may not be smooth.
http://www.lassp.cornell.edu/sethna/hysteresis/WhatIsHysteresis.html

http://hyperphysics.phy-astr.gsu.edu/Hbase/solids/hyst.html

http://www.lassp.cornell.edu/sethna/hysteresis/hysteresis.html

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Quantum Devices Inc. is a leading manufacturer of optical rotary encoders their main website is at www.quantumdev.com They can be contacted via e-mail at info@quantumdev.com.

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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.

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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.

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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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The Versatile QD787 Optical Encoder

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This is a picture of the QDI Model 787 Hollowshaft encoder and a loose flex-mount showing the tabs bent 90 degrees for mounting to a surface larger than the diameter of the encoder.  The motor shaft would be inserted into the brass hub and tightened by two setscrews located 120 degrees apart.

Notice the groove in the housing of the encoder.  It is designed to allow infinite adjustability; allowing the technician to align the index pulse to a mechanical location on the machine used for homing or reference.  This is called an encoder “Servo Mount” and is intended to have “D” shaped washers used where after the alignment is complete, the larger diameter of the washer is inserted into the groove and screws tighten the encoder into place.  When mounting the encoder like this, the flex-mount is completely removed.

The housing of the encoder is made from Nickel coated, conductive carbon fiber that eliminates magnetic interference (EMI) generated by electric motors.  It is available in resolutions up to 2048 PPR or 8,192 quadrature counts per revolution (direct read) with index pulse being a standard feature.

Electrical output can be a Differential Line Driver for noise immunity via a ribbon cable with connector.

The QDI Model 787 encoder is about the diameter of a penny.

There are extended options that are not listed in the QDI online documentation for custom shaft lengths and diameters, making this encoder perfect for potentiometer replacement or other human interface applications.

QD787

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For more information on the QD787, check out the Spec sheets for both incremental encoder and absolute encoder varieties.

Download the printable versions of the spec sheets for the Absolute Encoder (PDF),  and the Incremental Encoder (PDF)

The Quantum Devices INC. main home page can be reached here.

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