Encoder Issue

Hello! Wonder if you gents can establish some light on this intermittent fault we are having.

Basically we have a hoist, this hoist starts at the bottom in which it will trigger a proxy, this then inputs a 0 value in turn ''homing'' it. We then move a -65mm value in as a offset due to it lifting from a conveyor. So the hoist is sat at a value of -65mm before any movement.

Now the drive motor is fitted with two proxy cells (BI6U-M18) triggered via 6 plates on the motor fan, this is then wired back into a 1756-HSC/A card. So are readings are -65mm to 1414mm for its motion of travel.

Now here is the issue! Every so often, when the hoist is sat in it's down position, it'll be sat reading -65mm due to the above, but suddenly the value will just drop, in the example provided down to -240mm with no activity at all. Now what do you gents think would cause this? There is no physical movement in the hoist at all, so not suspecting the brake failing on the motor or hoist dropping. We thought maybe the card itself? We are using another hoist machine off the same counter card however and that's reading fine so suspecting maybe its a faulty channel for this hoist in particular if at all possible? Also we have only a 1756-HSC/B card, many issues with swapping this from a series A?


Thanks in advance, any input is much appreciated

How are you determining count direction? One possibility is that the counter is set to count down and one prox is turning on and off.

We have the card set to Encoder X4 Mode on both channels

Can we see the PLC code that generates the Position (Stacker_xxx_Act_Poss_hoist)

It is possible that you are capturing the position value for the trend while in the middle of a calculation

What is the change in position (mm) per HSC count?

http://www.filehosting.org/file/details/509302/Stacker.xps

Attached is the code for the above function.

At the moment of the trend there is no calculation, there should be once it hits the down position switch, to zero it then move a value of -65 in. In effect it will sit there at this value as it has a value it has to travel to i.e. 1414 which is it's up position. If the value changes i.e. the fault value of -240mm then our up value is incorrect effecting our production. This only happens very randomly sometimes 4 times a hour, sometimes twice a day! Nothing out of the ordinary when it happens too

Can you do a history on the prox inputs? A recording o'scope maybe? I had a somewhat similar problem once that showed up when the conveyor was stopped for awhile - it ended up being the load resistance on an open collector encoder output.

Unfortunately cant stick a recording scope on it as not got one to hand. We're using a SEW-NV26 type encoder if you've had any dealings?

Now the drive motor is fitted with two proxy cells (BI6U-M18) triggered via 6 plates on the motor fan, this is then wired back into a 1756-HSC/A card. So are readings are -65mm to 1414mm for its motion of travel.

Click to expand...

We're using a SEW-NV26 type encoder if you've had any dealings?

Click to expand...

The above two statements seem contradictory. The first implies a "roll your own" encoder, the second a purchased unit.

Your right. Will edit

*edit* Cant edit that post due to old post..

Originally it was thought it was via two normal proxys. But I have inspected it and found it to be a SEW-NV26 fitted on. Still 2 x proxy mind

halla26 said:

We're using a SEW-NV26 type encoder if you've had any dealings?

Click to expand...

No, I haven't

Page 18 of this manual (PDF) shows the construction and the specifications for this encoder.

Cheers Bernie, been reading through that earlier. The plan is to switch the HSC card, and NV26 encoder to eliminate both. From what Doug-p said earlier sounds like an avenue to chase

Quick question

Just doing more reading. We have a instance when a switch is activitate i.e. rung 14 of the attached document earlier. This then triggers LoadPreset.0. Which I have read that it will move the value in the preset into the Presentvalue. Where is this preset value set? Can it change or is it fixed? Could this be a cause i.e. moving a presetvalue in. I.e. like I said earlier, it only happens in the down position when stopped and this rung can happen when in that position.

*edit* nevermind just found it now

halla26 said:

From what Doug-p said earlier sounds like an avenue to chase

Click to expand...

In our case the external resistance had been drastically reduced (contractor installed resistance in parallel) causing the OC output transistor to overheat fifty percent of the time, in a random, coin-flip sort of way, when stopped. After running a few minutes the encoder output transistor cooled enough to operate normally.

Look at the code you posted

-65 + 1477 does not equal -240

Please cross reference LR_ACT_Poss_Hoist_603 and find what ever else writes to that address
How is the routine Pos_603_Hoist called?
if you are able (allowed) you can Zip the source file and attach it here