Encoder losing position

[GaryS]

A couple of things
I see the encoder is drawn as a differently line driver output but if you look at the drawing it shows a pull up resister on the output ( resistor connected to the V supply )
That’s typically and open collector driver configuration. Open collector drivers as prone to noise particularly at high frequencies.
I noticed you said that if you turn it by hang the counts are dead on but if driven the counts are off.
Is motor driven from a VFD if so it could be generating noise in the encoder lines.
Even with shielded cable it can happen, try to change the carrier frequency of the VFD a small change can make a difference
Check the grounding on the system
Check the grounding on the encoder cable shielding
We need to have more information
The encoder used
As wired drawings not the speck drawings

 

[keithkyll]

Noise isn't the issue. The problem is with return paths, floating ground.
Those voltages are a problem. When the transistor turns on, the low should be ~.8V.
5.5 V indicates a floating ground.

From the encoder. Black lead of meter on Blue wire. You get +24 on Red, and 5.5V to 12V on the Black and White wires? One resistor is Red to White, other is Red to Black?

 

[Doug-P]

You also have to be sure to keep enough resistance in the circuit to keep from swamping the open collector in the encoder. Had that situation once and the result was erratic counts because the OC didn't have time to come out of saturation - especially if it had sat awhile conducting. Got quite hot!

 

[keithkyll]

GaryS. Was typing during your post. Encoder is known. Definitely open Collector. VFD noise could be an issue. It's not going to work until we see low signals below 1 Volt.

Image 5274 shows + for encoder as Red. Should be Brown. Encoder is TRD-S1024BD from AD. Output max is 30 mA. Low should be .4 Volts.

 

[jonrya91]

All of these motors are on vfds. Should I assume I could have noise?

One resistor is red to black. No resistor on red to white. My drawing didn't show a resistor to the white wire.

Ok so here are the measurements keithkyll asked for:

blu to red: 24v
Blu to blk: .125v to 12v (I had a helper last time and he was turning the encoder too fast)
blu to wht: 0v to 5v
blk to wht: .05v to 12v

 

[PLC Pie Guy]

All of these motors are on vfds. Should I assume I could have noise?

There will always be noise... The kicker is if you get rid of it or not.. Shielded cable, grounding and wiring methods will determine if you can successfully get rid of it. Sometimes its not a problem, depends on the environment and the equipment within the environment.

 

[keithkyll]

Your drawing is wrong. The one in the HSC manual is correct. Add a resistor from Red to White

 

[keithkyll]

12 Volts across 2700 Ohms is too low. Allen Bradley math is a bit off too. I like the two 2.2K in parallel. Do that twice, for a total of 4 resistors

 

[GaryS]

I wouldn't be concerned about the 5.5V on the output when it's on
most open collector drivers have a resistor in the collector circuit to limit the current in the case the output is connected directly to the 24V source to protect the output.
5.5v should be low enough to trigger the HSC
check the grounding I think it's noise.
when I have trouble like, I like to know the circuit of both the output and the input
it helps to determine the problem.
different companies use a different circuits for open collector outputs they work differently
just because it wired as an open collector don't mean it is.

 

[keithkyll]

if it is really wires as shown on the print then there may be a possible problem
I see 2 load resistors on channel A to common and none on channel B
I think it's just drawn wrong but worth checking

Gary, you hit the nail on the head right here.
There must be a pullup resistor on channels A and B.
When correct, both will read same voltages.

Figure 15 in the HSC manual is correct, but resistor value calculation is wrong - doesn't take input impedance of 1950 Ohms into account (value from table 26 in HSC manual). Take the minimum resistance of 3147 - 1950 = 1197 Ohms. 1.2K is the real minimum value for the pullups at 24 Volts.

OP's schematic is drawn wrong. No resistors on Channel B, but total resistance of 1.1K sounds accurate.
I was wrong about the resistor. 2700 Ohms isn't giving enough pullup. You can see this in post 35. Black to Blue is 12 volts. 12 Volts across 2700 Ohms is only 4.4 mA. Module needs a minimum 6.8 mA.

1K resistors would give 7.6 mA, and .15 Watts.
Math: 22.4V / 2950 Ohms. 22.4V * .0076A.

Solution: A couple of 1K, 1/4 or 1/2 Watt connected using figure 15 in the HSC manual. I would go 1/2 or 1 Watt. 1/4 Watt will work, but they'll run a bit warm to the touch.

 

[keithkyll]

Solution: A couple of 1K, 1/4 or 1/2 Watt connected using figure 15 in the HSC manual. I would go 1/2 or 1 Watt. 1/4 Watt will work, but they'll run a bit warm to the touch.

Edit. Forgot to calculate wattage when output is low.
With a 1K, it's 24 mA, .58 Watts. Encoder limit is 30 mA.
Use 1 or 2 Watt.

That's why Line Driver models are better. Need to add a 5 Volt supply, but you don't have to fool around with resistors, better noise immunity, etc.
Mandatory for high speed.

 

[daba]

That's why Line Driver models are better. Need to add a 5 Volt supply, but you don't have to fool around with resistors, better noise immunity, etc.
Mandatory for high speed.

I've recently had issues with Line Driver encoders in a "noisy" (VFD) environment.

Wired exactly as per recommendation by encoder manufacturer, but we were blowing up output stages at an alarming rate !

Manufacturer kept telling us that we were shorting out the outputs, but then I read an article about "totem-pole" or "push-pull" outputs "locking up" with both FETs ON.... That causes them to burn out.

It seems that noise injected into the encoders can propagate backwards through the FETs and turn them both on, effectively shorting the supply through both push-pull FETs.

We ended up having all six encoders converted to Open-Collector outputs, as well a re-routing encoder cables away from VFD cables.

 

[jonrya91]

Just wanted to update everyone: we had a Rockwell engineer in here to try and figure this encoder situation out. He couldn't explain why it wasn't working.

We ended up installing a prox switch with a trigger and now we're all good.