Using stepper motors and the DAC

[tmoodley]

Hi,

we have recently acquired the UMI - 7774 Controller, in the hopes of using it in conjunction with some parker electo-thrust cylinders in order to control a stewart-gough platfrom for a vehicle simulator.

I have been trying to use the UMI for torque control purposes i.e. loading a DAC value (voltage) into each motor which in turn powers the actuators, the main problem that has arisen is this, firstly the motors being used are servo motors, and originally during testing i used position and velocity control of the servo motors with an encoder set as feedback, however i found that in the servo axis configration i could not load values onto the DAC, thus i changed my axis confguration to stepper motor, at which point i found i could control the DAC.

The main problem however with this is i no longer have any capabilities of using position and velocity control which would be very useful, is there any way around this problem in which i could use the load DAC fcnt, and also use postiion and velocity control. I have found that if i set the stepper loop mode to open loop, i am able to read the position values and the velocity values with my fle_read_pos_rtn functions, however if i set it to closed loop i cannot.

Also perhaps you could recommend a better way in which to use torque control of the actuators.

sincerely,

tmoodley

[Brian_Beal]

I can't comment on the details of that controller, but in general you have to decide what mode you want to be in.  Either torque, position,or velocity.  What you usually can do is limit torque output while in position mode.  If you could explain in more detail what you want to do, we might be able to come up with a solution.

[tmoodley]

Hi again...

thank you for the speedy reply...

well basically we are tryin to use the controller in order to control 4 parker actuators which in turn will basically drive a stewart platform.

We have tested the actuators for position and velocity control using the servo motor configuration however we ideally want to use torque control for these actuators and intended on doing this by using the load DAC fn, this works fine when we change the setup of the axis to stepper motor, and change the following error to zero (i.e. to ignore it), also it was found once in stepper mode we no longer have the ability to use position and velocity control.

The main problem which has occurred is that due to the internal friction of the actuators, they do not move as expected under small torques, thus it occured to us that we need to use a closed loop controller and acquire some feedback on the actuators. Thus we ideally want to set up the drives for torque control, however also capable of retuning the position and velocity of the actuators via the encoder. This was achieved by setting the axis configuration to stepper and closed loop control, however a problem has occured when we do this, firstly the reset position button in the measurment and automation explorer does not want to reset the encoder to the correct position, also it would be ideal to be able to initially use position control to initially send the actuators to their neutral positions, and then switch to torque control.

I hope this has been helpfull, and please notify me if you need any more information.

Also if you could suggest any alternative configurations that you have heard of for control of a stewart platfrom this would prove very helpfull.

sincerely,

T. moodley

[Brian_Beal]

The UMI 7774 is just an interface, what controller are you using.  Maybe somebody from NI can answer the questions about their controller, but from what I can see, their controllers can't do what you want.  I have done something similar using Delta Tau PMAC controller.  They can switch beween position and torque modes on the fly, and get encoder feedback the whole time.  They are pretty complex, but very capable.  Labview drivers are available from Delta Tau. 

[Jochen]

T.moodley,

it's correct that you can't control the DAC by yourself when the DAC is assigned to an axis. Thus the only solution to control the DAC manually is configuring the board in open loop stepper mode as you have done already. As you have found in this mode the board works just as a "stupid" analog output board without any control capabilities which is typically not desirable.

I think the solution is to run the board in analog feedback mode. Please have a look at this link where I have discussed this feature in detail. With the NI 7350 ou also can switch between position mode and force/torque mode on the fly, that means without reinitializing the whole board. Of course you will also have to change the PID parameters when you switch the feedback source.

I hope that helps,

Jochen Klier
National Instruments Germany