Modbus input for absolute encoder position?

Ran a test qModMaster to see the RAW data and they are not signed. I can not adjust the range.

RAW Capture:

Positive values
[TCP]>Tx > 16:35:10:639 - 00 A3 00 00 00 06 01 03 08 44 00 01
[TCP]>Rx > 16:35:10:644 - 00 A3 00 00 00 05 01 03 02 00 06
Value: 00 06 = 6

Negative values:
[TCP]>Tx > 16:30:28:065 - 00 88 00 00 00 06 01 03 08 44 00 01
[TCP]>Rx > 16:30:28:070 - 00 88 00 00 00 05 01 03 02 FF FC
Value: FF FC = 65532 (65535 -65532= -3)

Found this note below that might explain why some ModBus implementations like ModPoll interpret the number differently - in my case correct.

Quote from MinimalModBus Website:
Negative numbers (INT16 = short)
Some manufacturers allow negative values for some registers. Instead of an allowed integer range 0-65535, a range -32768 to 32767 is allowed. This is implemented as any received value in the upper range (32768-65535) is interpreted as negative value (in the range -32768 to -1).

I did set up something that somehow works but not great yet.

MB2MAL axis load data -> Mux2.in0
MB2MAL axis load data -> Offset (-65535) -> Mux2.in1
stepgen.00.velocity-fb -> Abs.in to determine if X is going positive or negative direction -> used as selection pin for Mux2

This is working but when changing directions I get spikes in the 65000 range - instead of 20 - 100 for normal load. Especially with short moves I get spike all the time. Probably a timing issue between ModBus data (too slow) and the direction change signal.

You made my day Andy. Working, with bitslice as the input select pin.

Agree, it would be great if MB2HAL would allow configuration for that.

Thanks!

Mb2hal, is a rather crude tool. I found myself running much of what it reads through Classicladder to deal with similar issues.

I think for me personally it wouldn't matter and both options would be fine. As long as there is a way to configure it per transaction.

I am still looking into the absolute encoder position with MB2HAL but I will wait until I have my new milling machine next week. Initial tests were just on my bench. Funny thing for the absolute encoder trial, MB2HAL behaved in my favor this time - as it counts 0-65536. Tools like ModPoll would read 32768 to 0 (half turn) and then -32768 to 0 again for the other half turn.

andypugh wrote:

...

full-turns -> convert to float
initial counts -> convert to float
turns and counts -> sum2, turns scaled by 65536

sum2.N.out -> sample-hold.0.in

(Magic here to latch the sample-hold)

sample-hold.0.out -> sum2.M.in0 (gain 1)
encoder.N.position -> sum2.M.in1 (gain 1)
sum2.M.out -> PID feedback, axis position feedback, etc.

Then specify HOME_ABSOLUTE_ENCODER in the INI.

Hello,

wanted to provide a quick update and ask a question. I did exactly as suggested by Andy. All working but there is one remaining issue.

The sum2.M.in1 (encoder.N.position) or in my case (stepgen.00.position-fb) is "0" when I start LinuxCNC for the first time. Machine on triggers the sample hold. Then I jog the machine in X +50mm and set machine to off. When I turn it back on the position is off by 50mm.

The encoder.N.position or stegen.00.position-fb counters remains at 50.00000mm or whatever value it has and does not reset to 0 between machine off and on. Sum2.M adds the actual absolute encoder position + the encoder.N.position stored for the new absolute position.

Example:
Turn on machine: x-pos = 200mm + 0 = 200mm, jog X+ 50mm, turn off machine -> new x-pos = 250 + 50 = 300 but the real position is 250.

I might be able to add a mux2 or use an offset to subtract the encoder value but maybe someone has another idea. I already have to use quite a # of rt modules: offset, abs, conv_float_s32, sample_hold, conv_s32_float, 2 x sum2 to make it work and I am starting to see "Insufficient memory for pin.." warnings.

I will take your advise and never turn the machine off

I am at a point where I can't turn it back on due to following error warning in LinuxCNC. This is actually better than having the actual position all wrong.

I think in closed-loop setup I would be able to reset the encoder feedback to 0 before I turn the machine back on. I found a reset bit in the hostmot2 manual. I might give that a try later, when I have glass scales on the machine.

I could use the servo encoder feedback but when I set them up in closed-loop (with the servo's auto-tuning on) I have some strange behavior in LinuxCNC PID tuning. Might need to turn auto-tuning off.

Anyway, thanks for all your help so far. I will provide an update, if things change.

Hello, any progress here? I have servo motors with 24bit absolute encoder. I want to read the current starting position and axle load when the machine is moving .Again, communication is modbus. Can you share your config files . I am new to LinuxCNC. Thanks!