Air gap in high resolution optical encoders

As the resolution of optical encoders increases, the distance from sensor to disk decreases. In the incremental encoder industry, this distance is called the “Air gap”.  In the side view photo of an optical encoder above, the two red lines indicate the air gap between the sensor and disk  in a QD145 incremental rotary encoder.

In the photo below I have added a human hair to show perspective.

For more information on optical encoders, contact Quantum Devices  at (608) 924-3000.

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Interlaced Sensor Technology in Rotary Incremental Encoders.

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If you search the web for a rotary encoder tutorial on incremental encoder construction, the simplest two channel A & B style encoders talk about a light source and a single photo receptive element for each channel.  The light beam of this optical pair is then interrupted by the rotating optical encoder disk.  The light shuttering from the disk occurs over rotation, alternately illuminating and darkening the single element sensor.

The photo sensor turns the intermittent light beam into low level electrical signals which are strengthened and squared up with some type of comparator circuit. This creates the rotary encoder output pulses.

There are a few weaknesses in this basic design for example, what if the light source strength changes? This can happen over time and with increased or decreased temperature.  A varying light source means that the signal out of the sensor will vary and possibly effect the decision point at which the rotary encoder channel is driven high or low. This variation in decision point results in real positional error and needs to be avoided. In order to combat this a differential sensor circuit is used.

In a differential circuit two sensors are placed so they are 180 electrical degrees out of phase. What this means in simple terms is that when one is “on” or illuminated by the light source, the other sensor is in the shadow or “off”. By maintaining the relationship between these two sensing elements, the ambiguity of a varying light source can be common mode errored out.  The light level is no longer important, but instead the relative relationship between these two equal and opposite sensors is now what matters.

There is, however, still a weakness with this set up. As we increase the line count of the encoder, the geometry of the disk and sensor elements becomes very small.  This opens up concerns for anything that may occlude the disk, such as debris or other contamination.

A way to handle this is by breaking up the individual differential sensor elements into several interlaced sensor elements over an array.  Instead of a single element and it’s complement, there are many elements and again as many complementary elements that are alternately positioned on the sensor.  As the optical encoder disk rotates, multiple window openings align with the elements illuminating them and darkening multiple complementary elements at the same time.  The signals from all of these elements are summed together. The complements are compared against the fundamental elements to determine a switching point.

The advantage to this set up is that the sensor becomes spread out over a wider area along the disk and needs multiple disk segments to activate it. This makes the reliance on any one individual disk or sensor segment less crucial to the overall signal integrity.  If there is any debris that makes its way onto the disk, or even creates total blockage of one or two sensor segments the output signal will not be effected.

The shadow technology sensing scheme patented by Quantum Devices Incorporated also  corrects for disk centering problems such as disk eccentricity and disk wobble, where the movement and alignment of the disk varies in three dimensions, and normally creates variations in signal amplitude and ultimately output signal.

For more information on interlaced sensing technology and how it is used to improve optical rotary encoder performance, check out the document titled Optical error components and their effect on signal quality.

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Quantum Devices Inc. is a leading manufacturer of Rotary Incremental Encoders. They can assist in encoder selection and can be reached at (608) 924-3000 or via E-mail

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.

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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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Interfacing to 26C31 Line Drivers in QDI Rotary Encoders

Encoder Electrical Structure
The internal electrical structure of the QDI series of encoders consists of a photo interrupted light source that is picked up by a common mode photo detector array.

“Common mode” means that there are two channels that are at exact opposite states in any position.  The advantage of common mode sensing is that light and noise interference does not create false triggering, but is instead filtered out.

Besides having common mode signals the incremental channels are also set up in an interlaced fashion. High-resolution channels need to be interlaced to provide a greater amount of signal strength and noise immunity at analog levels.

At this point the signals from the sensor are at small analog voltage levels. Depending upon the line count of the encoder these analog voltage levels may or may not be amplified before being digitized by a comparator.

The purpose of the comparator is to “square off” the signals to create digital signals. The comparator converts these Digital signals into single ended (Referenced to ground) signals.  Besides being single ended, these digital signals are still not robust enough to be used directly by most applications.

After the comparator, the signals sent to a Line Driver.  The line driver splits each signal back into two complementary signals, and also provides the ability to deliver much more current.

The output of the line driver is then electrically connected to flying leads or an interface connector of the encoder, making it the final interface device inside the encoder that is electrically connected to the outside world

Damage to the line driver can occur if the amount of power the line driver is delivering is greater than its specification. The amount of power is directly related to the resistive, inductive, or capacitive loading of the line driver.

DS26C31 Output Schematic

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Areas of concern that may contribute to Line Driver Failures

If the flying leads of the encoder are shorted to other leads or if the leads are shorted to the supply voltage, or ground. Excessive current may be allowed to flow through the output channel of the line driver. This current generates heat that the IC is not able to dissipate and damages the internal junction. This junction may fail open or closed effectively creating a locked Hi or Lo Electrically.  Proper termination of both used and unused leads before power is applied to the encoder is essential to prevent unintentional shorting.

If the output channels of the encoder are loaded too heavily (greater than 100 ohms) then the Line Driver IC may not be able to dissipate the heat from the internal junction and can damage the line driver.

If the encoder is operated in ambient temperature exceeding specifications there is a possibility for damage to the encoder outputs as the difference between the encoder temperature and the ambient temperature plays a key role in the ability for the Line driver to dissipate heat.  If there are any uncertainties as to the environment that the encoder is to be used in QDI engineering should be consulted for recommendations.

External Voltage that is applied to the outputs of the encoder may cause damage. This can come into play if a line driver is set up to be used in an open collector style application and pulled up to a voltage higher than VCC.  You can recognize this situation if you are using a pull up resistor with a power supply that is something other than the power supply for the encoder.  A pull up resistor is typically more effectively used with an open collector type output, where as line driver outputs should be terminated to signal ground or to complementary signals in a differential fashion.

Interfacing to a line driver output with a power supply different than the encoders. Using a separate isolated supply to pull up an output or to interface to the encoder can cause signal amplitude irregularities, noise problems and line driver failure.  The signal ground for the encoder should be electrically tied to the power supply common of any systems that are interfacing to the encoder.