Skewed Encoder Waveform

I received this e-mail from a potential customer who is trying to determine why his Encoder waveform doesn’t look right.  His name has been changed to protect his identity.

Hi Jim

I have just come across your Web page on RPM calculation using an optical encoder and oscilloscope. I was keen to test out this method of RPM calculation so rigged up my little encoder and oscilloscope without hesitation. I don’t seem to be getting a nice wave wave form across my display, its rather skewed. Could you just point out where I’m going wrong?

Really enjoyed reading your articles. Look forward to hearing back from you soon.

Eddie

Eddie’s photos are below:

Hi Eddie,

I would love to say the problem is that you aren’t using a Quantum Encoder….

But instead it looks like you are just missing a ground reference for the scope.   There is usually a little black alligator clip hanging off the side of the scope probe. That clip needs to be attached to the signal common on the encoder (black or negative on the power supply)

The red arrow below indicates where the ground clip should connect to the scope probe.

The reason your waveform looks  skewed is because the absence of a ground reference causes the scope to pick up ambient 60 Hz noise (it is everywhere, outlets, lights etc.) and couple it with your encoder signal.

Connecting the scope ground to the incremental encoder signal common will clear that right up.

Below is a picture of a scope probe with the ground clip.

Take care,

Jim

Jim Miller is a Design/Application engineer working for Quantum Devices Inc.

He can be reached at (608) 924-3000, or via e-mail at jmiller@quantumdev.com.

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 Engineer interview

I was featured in an interview with EEweb.

Image

Jim Miller – Application and Design Engineer at Quantum Devices

How did you get into electronics/ engineering and when did you start?

I started when I was pretty young, like nine or ten, taking apart radios and using an old wood-burning tool to desolder components from the circuit boards I scavanged. I would pick up anything that was broken or being thrown out and tear it apart. I had no idea what I was doing but eventually stumbled onto some of the “Engineers Notebooks” that Forrest M. Mims III wrote for Radio Shack, those gave me the knowledge I was missing. Before long I was able to blow fuses out in the house on a regular basis. I have come a long way since then – we now have circuit breakers.

Can you tell us about your work experience/ history before becoming an Applications Design Engineer at Quantum Devices?

I have mainly worked with Industrial controls for Food & Beverage, and Pharmaceutical companies, which always includes quite a bit of PLC Programming. That is why you will see a bit of Ladder Logic in with some of the Quantum Devices Blog posts that I do. While I predominantly work with discrete electronics today, the industrial controls experience dovetails nicely with our Optical Encoder lines. Encoders are used on the back of motor and often in industrial applications, so I am able to better understand the way an end user might be trying to implement a design, and to some extend the way they might think.

Read more…

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”

Cable length considerations with Incremental Encoders

The QD145, QD200 and QR12 series of optical encoders have  28 AWG conductors in the standard flying lead cable.

This gauge of cable is excellent for tight bends and fitting in applications where space is a premium.  The conductors can easily handle the 250 mA max current requirement of the encoder.

A smaller gauge conductor means that there will be a limit to the length of the cable. This is due to the DC resistive loss in the conductor that causes a slight voltage drop.  The longer the cable, the greater the voltage drop.

This voltage drop reduces the voltage seen at the encoder.

For an incremental encoder with a 28 AWG cable operating at 5VDC,  this limitation occurs at 17.85 feet.

There are a few ways around this incremental encoder cable length limitation:

1)       Splice the cable and go with a larger wire gauge for longer cable runs.

2)       Increase the power supply voltage to compensate for the voltage drop in the cable.

3)       For Incremental only (Non-commutated Encoders) Quantum Devices offers a 26 AWG cable.  26 AWG conductors bring the cable length limitation to 28.1 feet.

For other options, or help in determining the right wire gauge or incremental encoder for your application, you can reach Jim at (608) 924-300.

QD145 Incremental Encoder Shaft Tolerance

I received a phone call the other day that caught me a bit off guard. A customer was asking what tolerance they need to have on their motor shaft for it to fit with one of our incremental encoders.   I reflexively told the customer that we machine our shaft I.D. to  a –0.0000” +.0005” tolerance. He told me he already knew that, but what did we recommend for his shaft, the motor shaft that the incremental encoder was to be mounted on?

Since I am an Electrical Engineer, I wanted to make sure I had all of my ducks in a row before I took my second shot at answering a mechanical related question. I let him know that I would look up the information he needed and call him back.  After I got off the phone I immediately knew what I should have told him.  This answer may surprise you, but we don’t specify tolerances for the fit of our customer’s motor shafts. – not in the way one would expect.  Instead of a fit tolerance, we have TIR and Endplay tolerances on total encoder movement after it is mounted.

The reason for this is because unlike modular incremental encoders, which rely on the mounting shaft to hold the disk and sensor air gap, the QD145 series of incremental encoders has an internal bearing set that maintains the air gap. This takes the need for an exacting precision shaft to shaft fit out of the list of problems motor manufacturers face when designing a new motor.

I called the customer back and let him know the good news; that instead of some tight machining numbers, he only needed to keep his QD145 Incremental encoder within .007” of radial shaft runout and within +/- .030” for axial shaft runout.

I also pointed him to our QD145 Incremental Encoder Mounting Instructions, which contain other incremental encoder mounting, and wiring suggestions.

How to calculate pulses per degree for an incremental encoder

When using an Incremental Encoder, you often have to know how many pulses there are per degree of rotation. This is a very straightforward math problem.

Pulses per Degree = Number of encoder pulses per rotation/Number of degrees in a circle

For a 5000 Line count incremental rotary encoder we divide 5000/360 to get 13.89 pulses per degree of rotation.

Calculating Degrees of rotation per pulse for an incremental encoder

If you want to find out how many mechanical degrees of rotation there are for one pulse, you would do the problem the other way:

  Pulses per Degree = Number of degrees in a circle/Number of encoder pulses per rotation

For a 5000 rotary incremental encoder we divide 360 by 5000 to get 0.072.

This means that there are  0.072 Mechanical Degrees of rotation for every incremental encoder pulse.

This calculation is often useful if you have a counter totalizing incremental encoder pulses over a given distance or rotation.