Encoder losing home distance

Leukic

Member
Join Date
Oct 2018
Location
Leesville
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93
I have a 842A AB absolute encoder that tells a setter(it holds brick) where it is at so It can set the brick in the proper location.

Basically It would travel one way and it would lose 2inches negative back at the home spot.
I was using an offset to limp it through till a new encoder arrived.

I installed the new encoder Tuesday. Reset eveything bam they were running like champs for 10hrs!!
Wednesday---I get a call saying the setter is not stacking in the right location. Sure enough the home position was off by 3.75inches. So I adjusted it in the offset while I look things over so they can run. Bam they ran all day long with no hiccups.

The ONLY thing that happened from Tuesday night when they stopped to Wednesday morning was the mechanic cut power off to the plc panel when he replaced i few hydraulic cylinders.

Now is there any way possible that scale factors in the plc can get screwed up or glitch or some **** when power is cycled. Because that is the only thing I can think of that cause this to happen. Because there is no other way for it to lose counts or slip.

Here is the
2rnu1j6.png
 
Not in my experience.

Are the scaling factors (or any tags used as constants) different then what you set them to? that would imply some piece of logic or outside source has changed them.

A band-aid would be to add a preceding rung with MOV 909.5 Carriage_Carriage_Scaling Factor.
 
The set home position is 7.25ish.. Which is the carriage scaled position
After they started up it was at 3.5. So I added the 3.75 offset to make it 7.25
I rechecked everything and nothing mechanically is slipping or broken no teeth missing on the encoder belt.
It just seems odd that the only difference was the power being shut off tuesday night. The machine is running just fine right now with no change from yesterday. The power was NOT shut off last night.

jstolaruk--No none of those tags are used as a constant i just checked.
This is an online and running SS of the program the previous one was offline

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Mechanical slippage.
Bad connection between encoder and thing that is turning it.
Loosing pulses due to electrical interference.
Loosing pulses due to encoder moving too fast for the input setting.
5V encoder on a 24V system.

Have seen all of the above lead to problems with machines not going back to zero. I used an old smart relay to program a box that could be piggybacked on to an encoder signal so that I could have independent verification of the count, it goes to help in diagnostics. After that you are needing an oscilloscope to watch the quality of the encoder output.
 
Leukic, sorry, when I say constants I mean tag whose values aren't not meant to change in your program. Such as your gain. But you're indicating you have to change the offset to get the number you desire. What is the electrical connection? Analog I suspect, single ended? differential? Any chance you've got a bad common?
 
Leukic, sorry, when I say constants I mean tag whose values aren't not meant to change in your program. Such as your gain. But you're indicating you have to change the offset to get the number you desire. What is the electrical connection? Analog I suspect, single ended? differential? Any chance you've got a bad common?

The Carriage Scaling factor is constant. It doesnt change unless I change it.
This is the type of encoder I am using. And it uses a single cable. When they get done running I will check for any bad connections in the panel above the setter which has the wiring for the encoder.
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How is the 842A interfaced with the PLC? I suspect BryanG is spot on, it would explain the need to change the offset.

Running at 22VDC from the psu to the encoder and other proximity switches and various others.

The encoder for up and down on this machine is the same but have not had an issue.

When the operators take their lunch ill go up top and check the electrical box out for connections.
 
The Allen-Bradley 842A is a mechanical absolute encoder with an SSI digital serial interface to the ControlLogix. There is no way for the encoder to "miss pulses" in a conventional sense.

The rung comments in that program even remind us that the encoder has separate digital inputs that will set the reported value to zero, and that will switch between CW/CCW positive polarity. A brief unintended signal on either of those pins (9 = Reset, 12 = Direction)) could affect the reported position of the encoder.

The rung comments also suggest that the Reset input is triggered at some point during the machine setup. Is that done by a physical switch or pushbutton, or by a relay ? Is the Reset input otherwise tied to DC Common so it can't "float" and pick up an induced voltage ?

The Encoder Reset feature can also be triggered by physically removing the back of the encoder and pressing a button inside. While these switches are well protected and built to be simple and reliable, it's not impossible for one to be damaged or for debris, defects, or impact to cause it to mis-trigger.
 
Ken, aren't there different manufacturers of SSI interface modules? AMCI? RA? I was going to suggest that he review the design and verify/inspect the wiring to the interface module he was using.
 
The Allen-Bradley 842A is a mechanical absolute encoder with an SSI digital serial interface to the ControlLogix. There is no way for the encoder to "miss pulses" in a conventional sense.

The rung comments in that program even remind us that the encoder has separate digital inputs that will set the reported value to zero, and that will switch between CW/CCW positive polarity. A brief unintended signal on either of those pins (9 = Reset, 12 = Direction)) could affect the reported position of the encoder.

The rung comments also suggest that the Reset input is triggered at some point during the machine setup. Is that done by a physical switch or pushbutton, or by a relay ? Is the Reset input otherwise tied to DC Common so it can't "float" and pick up an induced voltage ?

The Encoder Reset feature can also be triggered by physically removing the back of the encoder and pressing a button inside. While these switches are well protected and built to be simple and reliable, it's not impossible for one to be damaged or for debris, defects, or impact to cause it to mis-trigger.

The encoder goes through 3 different boxes with terminal strips then directly into the AMCI SSI card.

Yes when you wish to ZERO the encoder ei(put a new one on) you have to remove bump stops and move the setter so that its frame to frame and then you can either push the button on the encoder or take the resest pin and touch it to +VDC.

I will check on the connections when I get a chance. I didn't really think about the reset and induced voltage
 
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There are a handful of ways to get an SSI encoder input into a ControlLogix. The most popular modern one for an application like this would be the 1734-SSI POINT I/O module.

There might also be a networked module like a Turck block, or a motion module like 1756-SSI, or a servo with an SSI feedback connection.

In general, even a malfunction of the SSI input module would only result in a "stale" value in the input tag, not an incorrect one. Certainly putting logic traps/latches on any error bits or communication status bits would be part of my troubleshooting.
 
3 different boxes with terminal strips

take the reset pin and touch it to +VDC.

That description of the physical wiring makes it very likely that the wiring is the source of the trouble. Maybe there's a loose power wire and you're looking at a "stale" value when the machine comes to a stop. Maybe that Reset wire is coming into contact with something or getting an induced voltage.
 
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