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

The robust QD-H20 IP-66 Industrial Rotary Encoder

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The QD-H20 is an idea replacement encoder for size 20 and size 25 industrial applications.  It is an IP-66 sealed rotary incremental encoder.  This high IP rating provides outstanding environmental protection with all of the line count and output options of the QD-145 incremental encoder.

The QDH20 style Industrial Rotary Encoder is typically used in applications such as Machine Control, Process Control, Elevator Controls, Agricultural Machinery, Textile Equipment, Robotics, Food processing, Conveyors, Material Handling, as well as, any application where water/contaminant ingress or durability is a concern.

The QD-H20 Incremental Encoder has three styles of MS connectors, 6,7, and 10 pin, along with a flying lead option, and the ability to come with custom cable lengths.  Each connector option is possible in radial and axial configurations.

The QD-H20 Rotary Encoder boasts a wide –20 to 100 Deg. C temperature range and a 500kHz frequency response.  Multiple shaft sizes ranging from .250” to .650” in both hollow and solid shaft are available.

The rugged dual bearing set allows the QD-H20 industrial rotary encoder to handle overhung loads, such as direct mount pulleys, with side load forces as high as 40 pounds nominally and 80 pounds maximum.

More information can be found at the Quantum Devices main site at http://www.quantumdev.com/products/optical_encoders/qdh20.html

A PDF of the QD-H20 encoder specifications can be downloaded directly at http://www.quantumdev.com/pdf/qdh20.pdf

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

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Choosing the resolution of a rotary incremental encoder

grandma.

“What resolution of rotary encoder do I need?” is a question designers often ask themselves.  A common mistake is that a higher resolution is always better.  This may or may not be the case.  Ultimately it is the nature of the application that decides.

Here are some things to take into consideration.

Cost
Higher line count encoders typically cost more.  Higher line counts require tighter disk to sensor alignment and air gapping, as well as, improvements in the light source and the associated electronics.  All of these factor in to an increase in cost as resolution increases.

Frequency Limit & RPM
It is important to note that interface electronics have a maximum input frequency that will cap the speed of the encoder.  You would like to use a 5000 line count encoder in a 6000 RPM application, but if the encoder or the interface electronics is limited to 200 kHz, you can’t as this combination of RPM and resolution is putting out 500kHz.
Quantum Devices has a standard frequency response of 500kHz over most of their rotary encoder line.

How does the math work out?
Sometimes it is more important for a programmer to be able to make their math work cleanly than possibly introducing rounding errors that could compound over time.  A 360 line count works nicely as one pulse equals one degree. A 3600 line count rotary encoder is popular for this same reason.

Does the performance of my application actually benefit from a higher line count?
Higher line count rotary encoders allow a drive or controller to make faster and more accurate decisions with regard to speed regulation, but much of that is application dependent. For applications that are running with a large inertial load, the added information from a higher line count may not improve the systems performance as there is only so much any motor can do to overcome the mass of the load.  Higher resolution may add information that can aid in the electronic decision making, but the system may not be able to respond any better than with a lower line count device.

A crude analogy and the reason for the photo at the top:

A race car driver can certainly out-drive my elderly grandmother when it comes to auto racing, but I bet they are both on even ground with a lawn tractor.  The race car driver has a much faster reaction time than my grandmother, but the lawn tractor cannot benefit from this, so the race car driver would be overkill for this application.

You can also see how my grandmother behind the wheel of a race car would limit the performance of the vehicle, much in the way that too low of resolution on a rotary encoder can limit speed regulation, acceleration/deceleration profiles and the precision to which a system can position.

Choosing the resolution that best suits your application is a careful balance between Cost, Frequency limitations, Math, and application limitations.

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