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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Incremental Encoder Lathe Automation

I have been working on a project to automate a manual lathing operation for our incremental encoder/optical encoder line.

To keep things simple, thumb switches allow the set point, along with some offsets for fine-tuning, to be entered.

I am not completely finished with project, but in the video below you can get a feel for how the machine will mill down the incremental encoder shaft.  We have control of the tool position to within .0001”

QD145 Incremental Encoder IP-66 Sealing Option

Quantum Devices Inc. has a mounting option that allows for IP-66 sealing of the QD145 Incremental Encoder.  There are two o-rings in the clam shell design; One o-ring seals the encoder to the mounting surface (usually a motor), and the other o-ring seals the end bell housing that covers the encoder.  This is a popular, inexpensive, option for customers who may not need the IP-66 sealing,  but want some sort of end bell protection over the encoder.

Cross-referencing RENCO Incremental Encoders

We have always been able to come up with a suitable replacement for Renco encoders, but our customers are making us aware that we need to point that out.

For those who don’t know, Renco, after being absorbed by Heidenhain, made the decision to  eliminate much of their product line.  This move has left quite a few of their customers in the lurch. To help fill this need, we recently started promoting our ability to cross-reference Renco encoder lines on our web site with the rather obvious image you see above.  Clicking on this image will take you to a request form where you can let us know which Renco Encoder you are trying to cross.

Keep in mind that we can cross other encoder manufacturers as well.

As a  general reference, the following Renco incremental encoder cross-reference table can show you which style of Quantum Devices Incremental Encoder will likely work best.

Renco Encoder Quantum Devices Encoder
RHS15 QD145 or QR12
RCM15 QD145 or QR12
RM15 QD145 or QR12
RCH20 QD145 or QR12 or QD200
RHS20 QD145 or QR12 or QD200
RM21 QD145 or QR12 or QD200
RCM21 QD145 or QR12 or QD200
RCH50 QD145 or QD200
R50i QD145 or QR12 or QD200
R35i QR12 or LP12
R22i QR12 or LP12
RA25 QDH20
RDHI QDH20
RS25 QDH20

The use of potentiometers in Incremental Encoder Design

Recently I decided to catalog the competitive Incremental Encoders that have populated the shelves surrounding my desk.  In doing so, I was surprised to find that many of our competitors use potentiometers in their designs.

I can understand why they need to use potentiometers.  In most designs the potentiometers are used to balance the raw analog signals produced by the Incremental Encoder sensor.   A potentiometer is the perfect component for this, you fire up the Incremental Encoder during test, lay a scope probe on it and dial the value specified by Engineering.  For most encoder designs, the only other option is to guess at some resistor values hoping that you don’t have to solder and unsolder resistors too many times until you hone in the correct signal, as that would be a very time consuming process.

While I can understand the use of Potentiometers, the reason that I am a bit shocked by their ubiquity in competitor’s designs is that potentiometers are inherently a much less reliable component.  A resistor is all one solid piece, but a potentiometer (which is a variable resistor) has a resistive track and a movable wiper that slides along to vary the resistance value. Moving parts are inherently less reliable than a non-moveable part.

Here are a few of the PCBs from various manufacturers ,  the potentiometers are circled in Red:

 

This next one is my favorite.  Thirteen potentiometers!

I am proud to say that Quantum Devices Encoders do not use potentiometers in their Incremental Encoder designs.  The reason we can avoid potentiometers is because of our patented phased array sensor that provides perfectly balanced complementary signals right from the sensor.

Other incremental encoder sensors suffer from having their active areas in different locations along the length of the sensor. Since the light source spreads light unevenly over the sensor, some active areas receive more light than others creating signal imbalances.

In Quantum Devices Incremental Encoders the photosensitive areas of each channel are interlaced with each another,  so all active areas receive the same amount of light.  This eliminates the need to balance any signals, which in turn eliminates the need for potentiometers in the design.

Below is an drawing of the phased array sensor Red indicates Channel A active areas, Blue indicates channel B active areas.

The lost Optical Encoder Demo Box

Optical ENcoder Demo Box Main Menu

We recently teamed up with one of our distributors to create a Demo box that showcased our QD145 optical encoder with a Delta Tau PLC and touchscreen panel.  It was sent off to a trade show where potential customers would get to spin the encoder and watch on the screen as counts were incremented and decremented and needles on dials spun.

I thought the Optical Encoder Demo box would make for a fantastic topic to write a post on, so that was my plan as soon as the demo box returned from the show.

Well… it never did come back.  I would love to tell you that I did such a great job on it that our distributor insisted on keeping it but the truth of the matter is that it was lost in shipping.

What I do have is the code and screenshot of the Optical Encoder Demo box, which should be more than enough to explain the functionality. What I don’t have are pictures, or video of the Optical Encoder Demo box in action, so you will have to use a little imagination on your part.
I  mounted the PLC, HMI and encoder to an enclosure that can set on a table.  The default screen (shown above) tells a little about the encoder. From this screen you can select a few different screens that allow you to interact with the encoder.

Count Screen:
This screen shows Pulse Count, Angle, RPM and direction.

Optical Encoder Data Screen

Degree Screen:

This screen shows mechanical degrees. The needle rotates in conjunction with encoder rotation:

Optical Encoder Mechanical Degree Screen

Pulse Count:
This screen shows the direct read count of the encoder, the needle rotates in conjunction with  encoder rotation:

Optical Encoder Pulse Count Screen

Tank Screen:
This is sort of a fun screen where rotation the encoder fills tanks in sequence, tank one fills, when tank one is full it “empties” into tank 2, when tank 2 is full it “empties” into tank 3.  Tank 3 continues to accumulate until 2 billion counts or so. The Drain button clears the levels on all of the tanks.

Optical Encoder Tank Filling Screen

ABZ screen:
This screen indicates status of inputs coming from the encoder.  Since I used a 5000 LC encoder, the screen was not be able to keep up real time when the encoder was rotated really fast, and it was nearly impossible to land on Z(Index) and have it light.

Optical Encoder Incremental Signals Screen

Programming

The Delta Tau was pretty easy to program, with only a couple hiccups.  The manual was a little vague in its explanation of the way two registers were used for some of the counter functions, but a little troubleshooting showed me which bits were activated when the counter set point was hit.

The Ladder Logic

This first rung of code is needed to do some basic housekeeping to ensure that D1022 is properly configured with a “1” on the first program scan. It is set by  M1002 and forever latched by M110.  The value of 1 tells the high speed input that we want a double frequency selection A/B phase counter.

The second rung sets up our high speed counter and checks the count to see if we have gone negative in value. If so, bit M120 is set high.

Rung three turns on a physical output Y11 if the counter set point has been hit.

Rung four moves the set point of 5000 back into the counter if we have gone negative in value. This allows the needles on our display screens to rotate continuously and not peg out to a high or low value.

Rung five updates Register D300 with our counter value every 10mS via the handy M1011 10 mS clock pulse.

Rung six divides the counter value by 16 and moves the answer into Register D302. This is where we start our math for the degree conversion. The rest is scaled by the configuring the screen register in the screen editor software.

Run seven uses the trailing edge of our 10 mS clock pulse to move the counter value into register D310.  This value is held for comparison in time to get RPM.

Rung eight uses the leading edge  of our 10mS clock pulse to find the difference in our stored value and put it out to register D312.  D312 is then doubled and sent out to register D350.

For any of you interested in repeating the project, I have included the BOM below.

Of course, I also used some miscellaneous wire and hardware to construct the Optical Encoder Demo box, but the list below includes all the big ticket items.

From Cymatix:

QD145-5/26-5000-0-02-T1-01-02 Optical Encoder

DOP-AS35THTD 3.5” Color touch screen

DVP12C11T Micro programmable Logic controller 8 inputs 4 High Speed outputs

DRP024V060W1AZ CliQ 24 Vdc, 60 Watt power supply

DVPACAB2A30 3 Meter connection cable

 

From Automation Direct:

DN-R35S1 Din Rail 35mm X7.5mm

WC12C12  N12 Desktop enclosure 12”X12”X9”

DN-T12A Terminal Blocks

DN-EB35MN End Brackets

DN-LAB terminal block labels

Why can’t I see my Optical Encoder Output Signals?

A simplified schematic of the externalized (op...

Image via Wikipedia

Why do my incremental encoder signals disappear?

Today I was called to the production floor after one of the test Technicians noticed that the encoder signals disappeared from the oscilloscope when they ran the encoders “unloaded”.

I asked one of the techs if the encoders being tested were of the open collector variety. The answer was  “No, they’re line drivers”

I immediately pictured our standard 26C31 line drivers, which are push-pull devices.

When I checked the label on the line driver IC on the encoder. I saw that it was an ET7273. This meant that the right answer was that they were both a line driver and an open collector device. The ET7273 is used in our optical encoder when a customer wants complementary open collector outputs, or incremental side open collector outputs

The problem was that I, as well as the Tech I talked to, am guilty of using the term “line driver” to synonymously mean “push pull” outputs.

Open collector style outputs are used when you want to be able to pull up to a different voltage level from the rest of the system. For example most of the optical encoders we sell are 5V devices, but the open collector output can be pulled up to many times that level. The ET7273 has a max spec of 30Vdc.

So why was she not seeing any outputs when the encoder wasn’t loaded?

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