How to use encoders / high speed counters

Hi guys, I'm new to encoders and high speed counters. We are going to be using a Avtron Model HS25A Encoder. It has 100 PPR instead of 1024 PPR that we were using before.

Basically this Encoder is going in to all of our sites for our drives and motors. We have some sites that are going to use a MicroLogix 1500 and some sites that use a ControlLogix 5555.

We already have the controllogix site configured using the 1024 PPR encoder. Is there anything special that needs to be done to change the distance that would get counted because we are moving to a 100PPR instead of 1024PPR? If so where is this changed? or is it simply a logical change. Currently our machine uses "42000 steps per foot" so I'm guessing that's where I'd need to change?

Next, we've never used an encoder with a MicroLogix 1500. Is it simply buying a 1769-HSC module? Then how is it configured.

Thanks much!
-Tom

Big mistake. You went the wrong way

You should have gone from 1024 line/rev to a 10240 or higher line per rev encoder. Especially if you are going to use this encode with drives.

Was not my decision. How much will 100PPR affect encoding vs. 1024PPR?

I guess it depends on how close you want your position to be. You are reducing the resolution X 10

Since you asked

ftp://ftp.deltamotion.com/public/PDF/Mathcad%20-%20t1p1%20pid%20mrplc.pdf

I did this simulation a while back for the same question on a different forum. Ignore the numbers and look at the graphs. The first graph is showing how well one can control a motor with 40000 ( 10000 PPR ) counts per revolution and the second with 300 counts ( 75 PPR ). Both are controlling exactly the same system with the same gains and making the same motion. The only difference is the encoder resolution and the difference in response because of the different resolutions. Encoder resolution is very important to good control. These numbers are a lot more extreme that yours but you can see that using the same gains the 40000 counts/rev will control MUCH better and 300 counts or even 400 counts per rev ( 100 PPR ) is not enough.

Think about this. Digitized feedback is non-linear. The feed back is used to represent the actual position but it is a stair step approximation to a smooth slope. You want to make the stairs as smooth ( small steps ) as possible.

I wouldn't recommend using the same encoders for the drives as for the ML1500. The drives should be able to handle fine resolution encoders whereas these same encoders will generate counts faster than what the ML1500 can count them.

I just save you a whole lot of trouble and hair pulling. The extra cost of the encoders will be more than offset by ease of set up and performance.

USE THE HIGHEST RESOLUTION YOU CAN!

I think this is applicaton specific, related to how you use the encoder value.

If you are using the encoder to perform closed loop positon or speed control then the resolution needs to be much higher than 100 PPR. However, if your application simply uses the encoder as a state trigger (when past XXX counts, do this) then the encoder resolution isn't as important. Also, the lower your dynamic response requirements the less resolution you can get away with.

However, there is obiously no technical advantage to what you are doing. Additionally I see no economic advantage to what you are doing. Within a pretty broad range, encoder PPR doesn't influence cost. So why are you doing this. While more may not necessarily be better, more certainly won't be worse.

Keith

kamenges said:

I think this is applicaton specific, related to how you use the encoder value.

If you are using the encoder to perform closed loop positon or speed control then the resolution needs to be much higher than 100 PPR. However, if your application simply uses the encoder as a state trigger (when past XXX counts, do this) then the encoder resolution isn't as important. Also, the lower your dynamic response requirements the less resolution you can get away with.

However, there is obiously no technical advantage to what you are doing. Additionally I see no economic advantage to what you are doing. Within a pretty broad range, encoder PPR doesn't influence cost. So why are you doing this. While more may not necessarily be better, more certainly won't be worse.

Keith

Click to expand...

we are simply using the value to determine a distance. Right now we are accurate up to 1/1000 of an inch. We don't need that much accuracy. With 100PPR we will get approximately 1/64in accuracy at the speed we are moving our device.

There are a few more reasons for the downgrade in PPR. The pros outweigh the con of a lower resolution:

-It reads 100 PPR (instead of the 1024 PPR)
-It has an unbreakable optical disk
-It has wide gap technology (basically this means that we don’t have to hold such a precise distance between the shaft and the sensor). These allow up to 8X the distance that we currently use.
- It is a direct retrofit of the current models in the field (minus programming), but there is no additional wiring.
- 2 year no hassle warranty