if top speed = requested speed motor stalls???

CNCModeller

Ok I'm stumped. I'm using g1 Gcode entries in the console to find max acceleration and velocity settings for each axis on my printer. If i set a feedrate that is unachievable all is well, but if I set a lower feed where requested speed = top speed on the web display the motor just jitters when it should be running fixed rate at the requested speed... the motor is fine while accelerating and will run quicker as long as it's always accelerating. I'm using external drivers via a duet expansion breakout board.

Thoughts?

Any helpfulsuggestionswouldbemuchappreciated...

Cheers
Barry M

edit: I'm running rrf 2.03

A Former User

might help with some specs on motor, supply voltage, micro stepping etc, and what feeds you're trying to achieve.

CNCModeller

@bearer said in if top speed = requested speed motor stalls???:

might help with some specs on motor, supply voltage, micro stepping etc, and what feeds you're trying to achieve.

@bearer
tb6600 drivers, 16 microstep, 2a
nema 17, 200 step/rev motors
just trying to find the limits on my test bench at 15v, will be re tuning to run at 36v, only trying to get the tuning process sorted now and figure out why this is happening. It happens at very low feeds, stable during accel / decell but not at fixed demanded speed.

Have had the drivers and motors running faster driven from a simple pulse generator, so not sure what's going on...

bondus

Dig out that old 'scope and measure the signals. If you have one,

dc42

Most likely the issue is that at high speeds, the Duet is sending step pulses faster than the external driver can accept them. Check that the timings in the M569 command T parameter are sufficiently high for your stepper drivers.

thomas_rudloff

Put 3.3V on the TB6600 control inputs and measure the current. The current is probably more than the Duet can drive reliable and results in lost steps.

I replaced the optos in the TB6600 by Toshiba TLP2x61. Where x is for the different packages and need some "creative" soldering to fit.

Maybe I can send a picture this evening.

CNCModeller

@thomas_rudloff said in if top speed = requested speed motor stalls???:

Put 3.3V on the TB6600 control inputs and measure the current. The current is probably more than the Duet can drive reliable and results in lost steps.

I replaced the optos in the TB6600 by Toshiba TLP2x61. Where x is for the different packages and need some "creative" soldering to fit.

Maybe I can send a picture this evening.

I was aware of the restrictions, but I thought that was what the expansion breakout board I'm using is for? Also it doesn't explain why an accelerating stepper is fine but a steady speed one at a lower feed rate isn't.

CNCModeller

@dc42 said in if top speed = requested speed motor stalls???:

Most likely the issue is that at high speeds, the Duet is sending step pulses faster than the external driver can accept them. Check that the timings in the M569 command T parameter are sufficiently high for your stepper drivers.

Will check that out, thanks. But that doesn't explain that if i run the stepper really slowly well below speeds its worked during acceleration and allow the move to be long enough to get out of acceleration and into steady state top speed i.e. the requested speed the motor still just jitters... but then works again as it decelerates...

CNCModeller

thanks for the info folks.

I'm going to rewire to drive the stepper from the on board driver to do some more testing.

hopefully that'll confirm it's a driver integration issue.

Thanks
BarryM

CNCModeller

@cncmodeller said in if top speed = requested speed motor stalls???:

thanks for the info folks.

I'm going to rewire to drive the stepper from the on board driver to do some more testing.

hopefully that'll confirm it's a driver integration issue.

Thanks
BarryM

Yup it works fine on the integrated drivers so I'll stick with them to start with.I need to figure out how to configure the external stepper pulses so that it works at constant speed as well as during acceleration.

thomas_rudloff

The opto coupler in those TB6600 are a mess. I read from many people that they had problems and from supporters, that those TB6600 take the most support effort. I pulsed them with an AWG (Waveform Generator) and hat lost steps at constant rate. Note: You need to discharge the LED in an opto coupler to minimize pulse width distortion. It seems the protection circuit in those drivers prevents this, what leads to lost steps. This could also explain why accellaration works fine because the DC point might get shifted.

The transfer ratio of these opto couplers is very bad. This might result in the fact, that even if you are overloading the controller you do not have enough current to drive them propperly.

I know they are only EUR6.00 in China. Adding the same amount of money for quality couplers make them work. But at this point the difference to the dual stepper add on board is not very much. You should consider how much money your life time is worth while. Meanwhile I see the mod as a proof of concept and hopefully get the add on cards today by DHL Express.

CNCModeller

@thomas_rudloff

Sounds good. I'm only using the tb6600s for prototyping as I had them to hand but I'd like to understand the issue as part of the Duet3d learning process.

The tb6600s are a stepping stone to integrating much more expensive, and more complicated to set up, closed loop external drivers once I have the kinematics sorted.

Do you know what the max pulse rate and min pulse durations are for the TB6600s? performance isn't an issue yet, I just want to get an external driver working properly for now.

Cheers
Barry M

thomas_rudloff

This is the chip in the black boxes:

1_1564651639261_TB67S109AFTG_datasheet_en_20140327.pdf 0_1564651639261_TB67S109AFTG_application_note_en_20170922.pdf