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.

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”

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
RS25 QDH20

Optical Encoder Calculator

Quantum Devices optical encoder Resolution/RPM/Frequency Calculator.

We just added a new page to the main Quantum Devices Inc. web site that provides a way to calculate RPM, Frequency, or line count (optical encoder resolution) by entering in the other two variables.

Encoder RPM-Frequency-Resolution Calculator

When would you want to use such a tool?  You might find it useful under the following circumstances:

Read more of this post

Finding the Index Pulse of an Incremental Encoder

The index pulse, often also called “Z” or “Marker pulse”, of an optical incremental encoder is a once per revolution digital pulse that is used for homing or count verification of incremental signals.

In the QD145 and QD200 series of encoders the index pulse fires when the mark on the top cover of the optical incremental encoder and the mark on the encoders shaft are aligned.

This mark also indicates the rising edge of the U channel for commutated optical incremental encoders.  Knowing the location of this edge is useful for the initial rough timing of Brushless DC motors.

Quantum Devices is a leading manufacturer of Optical Incremental Encoders.

Jim can be reached at 608.924.3000 or by e-mail at

Quantum Devices Optical Encoder Black/White Wire

What’s with this black-white wire?

Quantum devices QD145 and QD200 Optical Encoders feature an extra black & white-stripped wire that serves as an alternate grounding connection for the encoder.

This wire is intended to be used in situations where the encoder flex mount is not case grounded.  The Black/white wire is at the same potential as the Optical Encoder’s conductive polymer housing and  flex mount.

Internally the electrical path of the black-white wire is tied to the optical encoder housing and flex mount through the circuit board.  Our optical encoders tend to live on the back of hardware like Brushless DC motors, where the motor housing is at ground potential.  In cases like this it is usually best to leave the black-white wire tied off and floating.

Arrow showing flex mount grounding Optical Encoder to Motor.

Jim is an application engineer for Quantum Devices INC, a leading manufacturer of optical encoders.

Jim reached at

Inverted Flex Mount for QD145 Optical Encoder


.Another way to mount an Optical Encoder

Instead of using an end bell,  a lot of our motor manufacturer customers recess the QD145 optical encoder into an extended motor housing and then seal the motor with an end plate.

This has the advantage of being a lower cost item to manufacture, as the  motor housing is often extruded  or cast.  Just making the housing longer and providing an end plate usually costs less than casting a separate end bell.

Conventional Motor End Bell

But the downside to a recessed motor housing is that mounting an optical encoder in a recess like this creates a problem when tightening the set screws to the shaft. It often has to be done through an MS connector hole, or blind, by reaching the Allen wrench around and under the encoder.

Cast recess in motor housing

Because of this, we occasionally get asked for a a version of our QD145 optical encoder that allows the assembler to  access the set screws above the body of the encoder.  The inverted flex mount turns the encoder upside down allowing easy access to the set screws that are normally underneath the encoder.

The QD145 inverted flex mounti9ng option is not currently listed on our web site, but can be ordered using the following part numbers under the mounting options:

Use an 06 Mounting option for the inverted 1.157″ Bolt Circle flex mount.

Use an 07 Mounting option for the inverted 1.812″ Bolt circle flex mount.

The wiring is changed internal to the encoder so that the QD145 maintains correct phasing for all channels.

Another option for a drop in recess mounted encoder is the JR12 Jam Nut style encoder which does not use set screws to secure the encoder to the shaft, but a compression nut instead.

Dimensions for the 07 version of the QD145 Inverted flex mount.