Using motorcortex api to stream position commands.

MankaranSingh

Hi,

So I am trying to use the c++/python mortorcortex APIs to give motion to the motor. I am able to engage and stream velocity and position commands on topic root/Control/jointManualVelocity and root/Control/jointManualPositions. There are many more attributes regarding position and velocities, are these documented somewhere ?

My goal is to control the acceleration, velocity and position. Right now the motion is quick with very high starting and stopping accelerations and decelerations and drive goes into fault if these the positions and velocity commands are too high. Are there some attributes that we can set to make the motion a bit smoother (less initial and stopping jerk) or do we need to write a custom trajectory generator for this ?

Thanks.

Philippe

Obviously you are using the Generic-App.
What you are doing is correct. Currently you can only command either position or velocity (not at the same time).
Velocity setpoints are integrated to a position command. This happens in root/Control/jointJogIntegrators/. These are integrators with limiters on the inputs and outputs. So make sure your limits are setup correctly for your application.

Once the signal is a position command this is fed into PVA-Limiters. These smooth and limit the signal. You can find the settings of the PVA limiter in the Desk tool.

Here you can set for instance the maximum velocity and acceleration.

root/Control/actuatorLimiters/pvaLimiter01/maxVelocity

root/Control/actuatorLimiters/pvaLimiter01/maxAcceleration

MankaranSingh

Thanks for pointing out the limits. The velocity one works well, but on setting the maxAcceleration to 10, the motor starts to oscillate slowly after reaching the setpoint. I was using the generic app gui slider to control it.

philippeH

The maxAcceleration should always be factor 10 higher as your maxVelocity (rule of thumb) . So that with a certain velocity the motor is able to decelerate in time, to prevent overshoot. The oscillation you are experiencing is probably caused by not applying this factor 10.

MankaranSingh

Thanks, that rule of thumb seems to fix the oscillations.

The robotic manipulator we are working on has a gear ratio of 100:1. I set the gearbox/nLoadSide to 100 and now the outer shaft's degrees matches with what's displayed on the gui. I am using 10 deg/s and 100 deg/s as maxVelocity and maxAccel in pva limiter. Now on pressing the manual jog up/down arrows, it commands +- 40 degrees and the motion is discontinuous, there are stops in-between (I could hear the ticking sound).

How do I make the step size to be low (around 2 degrees) ? and solve this discontinuity/ticking sound ?
Thanks!

philippeH

If you have a step of +-40deg you probably filled in the gearbox incorrect.
Gearbox ratio 100:1 so it is an reduction, n-Motorside = 1 and n-loadside = 100
I checked the documentation site, how this is currently described is a bit unclear. This part of documentation will be improved.

n stands for number of teeth of your gear.
n-Motorside you define number of teeth moterside
n-loadside you define number of teeth loadside

MankaranSingh

Hi phillippe,

The gear ratio and stuff seems correct now, had to set the velocity limits to 30 deg/s to avoid it from getting into uncontrolled jerk/motion when starting. Although, there are still 3 more issues.

1) In velocity mode, there is still a constant 'hitting sound' from motor while in motion as I mentioned above. (I can send you video over email if you are unsure about what I am talking about)
2) Motor does not move at all while streaming positions to root/Control/jointManualPositions. I am giving positions in radians at 20 Hz from python api code.
3) When running the motor through code, only sometimes, in start the motor moves very fast uncontrollably and stops after that. This has happened few time. I'd get you the logs next time this happens.

Thanks & Regards,
Mankaran Singh

philippeH

I think I know what you mean, could you send me a video to philippe.habets@vectioneer.com.
Does this happen each time at the same frequency, regardless of the speed. So for slow motion and fast motion the ticking frequency is the same.

1) What you can do, check if the signal send to the drive is smooth, https://desk.motorcortex.io/, path: root/Ethercat/Robot/Axis1/In/Target position. Don't forget to set the update rate to 1khz (Settings). https://docs.motorcortex.io/docs/motorcortex-io-tools/motorcortex-desk/#screen-lay-out

2) You are sending to root/Control/jointManualPositions this is incorrect.
You have to stream your positions to jointAutoSetpointGenerator/input
set fade time to 0 jointAutoSetpointGenerator/fadetime = 0
root/Control/gotoPauseMode = 0
root/Control/gotoManualMode = 0
root/Control/gotoMotionplayerMode = 0

These should be the settings to stream position with the generic app

3) If you run the motor though code, you should not forget to reference, else these jumps will occure.

MankaranSingh

Thanks, the position mode works now. had to set maxVel > 0 ofc to make it move.
The ticking sound is there in position mode also. the frequency is same at all speeds.

philippeH 3) If you run the motor though code, you should not forget to reference, else these jumps will occur.

I set the limits and reference is well between -360 and 360 degree limit. This happened with velocity mode. I am not sure if I did something wrong. Will get logs next time if this happens again.

MankaranSingh

One last question,
I set velocity using root/Control/jointManualVelocity which has channels, so we set an array of velocities.
req.setParameter("root/Control/jointManualVelocity", [1, -1]).get() for giving velocity to both axis at same time.

I want a separate interface for both motors something like jointAutoSetpointGenerator0x/input so I can work individually with both axis.

Right now I have to get velocities for all the motors, populate the velocity array with all axis' current velocity for all channels, and then feed it to motor-cortex.

Is there a better way to set individual channel value instead of array ?
I hope I am clear here.
Thanks

philippeH

Hi,

I was checking the drive issue, the continue ticking sound.
For elmo drive you always have to set a DC clock.

In ECAT-Tool, could you set these values and deploy it to your controller?

Let me know if this solved your problem

philippeH

Can you update your motorcortex python version to 0.22.2
In this version you can specify an offset:
set_param_reply_msg = req.setParameter('root/Control/jointManualVelocity', 1.2, offset=1).get()
Now you only write to axis 2.

MankaranSingh

thanks for 0.22.2 update!

I also tried the distributed clock setting, but now the ticking sounds are even more and the frequency of it is uneven.

philippeH

MankaranSingh

Which kind of Elmo firmware version are you using, did you update elmo drive to the newest firmware version?
Are you just using one or multiple Elmo drives, for each drive you have to configure the distributed clock.

Philippe, will respond to the other questions about streaming the position commands

MankaranSingh

Some more questions regarding streaming the position commands.

1) I see the jointAutoSetpointGenerator interface is more of a profile position mode type where you give one position and motor stops only after reaching that target. Is there some other parameter path also that allows me to work more like a cyclic synchronous position mode ?

2) In jointAutoSetpointGenerator interface I queue a trajectory of 10 points. How do I stop executing trajectory midway and make motor to stop / provide another trajectory ? Something like a cancel motion bit that empties the setpoint queue.

Thanks

Philippe

MankaranSingh

1) I see the jointAutoSetpointGenerator interface is more of a profile position mode type where you give one position and motor stops only after reaching that target. Is there some other parameter path also that allows me to work more like a cyclic synchronous position mode ?

2) In jointAutoSetpointGenerator interface I queue a trajectory of 10 points. How do I stop executing trajectory midway and make motor to stop / provide another trajectory ? Something like a cancel motion bit that empties the setpoint queue.

ad 1)
Yes it is meant for just setting targets and the app then creates a nice smooth profile with given velcity, acceleration and jerk constraints to that target. If you set the constraints high eneough you should be able to stream setpoints at any rate to root/Control/jointAutoSetpointGenerator0x/input
There is some implicit behavior in this SetpointGenerator module. If the System Mode is in Pause or Manual mode the module will be resetting its input to track the actual position. This is to avoid any sudden jumps when this module is connected to the active signal path again. So when your write to root/Control/jointAutoSetpointGenerator0x/input is not changng, you know that the module is being reset internally.

ad 2)
if you want to stop in the middle of the execution of the cue, you can do several things:

switch the state to pause, the setpoint generator will then be reset and the que will be emptied
if you just want to pause you can use the timeScaleFactor input. As the name suggest you can scale time with this parameter. If you set the timescale to 0.5, your motion will be twice as slow. If you set it to 0 it will stop. But beware, you should probably fade this value nicely, because it is applied immediately.

I hope this answers your questions

MankaranSingh

Hi @philippeH any tips on this ?

MankaranSingh

Tested with both single and multiple drives with distributed clock enabled and disabled.
Today for some weird reason, the sound is gone for no reason. Nothing much changed from my side. I'd investigate more into what changed. The distributed clock is off

MankaranSingh

Philippe
Thanks, The pause mode to reset queue is working. Although, I had to set a delay before disabling pause mode (0.1s) otherwise the drive does a weird motion and seemed like was still trying to reach the previous target. The timescale thing also works as expected.

For the 1st point, I need to do a very precise motion of let's say 500 ticks on motor side and execute a sequence. And mind that we have a gear ratio of 100:1 . Will this still work ?

Philippe

MankaranSingh

For the 1st point, I need to do a very precise motion of let's say 500 ticks on motor side and execute a sequence. And mind that we have a gear ratio of 100:1 . Will this still work ?

This should work. Internally the values are doubles (64 bit). So in case of joint angles in radians this should give you plenty of resolution to drive on tick-level precision.