Breakaway Torque and RPM Control

[FLAnatic]

I have an upcoming application which I need to apply torque and measure the torque required to begin rotation.  This breakaway torque will be less than 20 in-lbs.  After movement occurs shaft speed must be ramped up to 1200 rpm and then ramped down to 0 rpm. Ramp up and down must be completed within 1 second and I must be able to have precise control of the total number of revolutions including ramp up and ramp down portions of the movement. I currently have torque sensors and rotary encoders from a previous application that I can use for this, but I would like to know if NI-Motion controllers, drivers and a stepper motor would provide the possiblity to perform these tasks without this extra hardware? Thanks for any input.

[Jochen]

FLAnatic,

a stepper motor doesn't seem to be the right choice for your application, as it doesn't provide a good way to increase the torque in a controlled way, so I strongly recommend using a servo motor for your task. Instead you should use a servo motor with a drive configured in torque/current mode. You could use an NI-Motion board like a PCI-7340 for the motion control task and a multifunction DAQ board to acquire the data of the torque sensors. Your acceleration requirements are quite high, so you will have to carefully select the drive and the motor with appropriate capabilities.

Could you please provide some information about your timing and accuracy requirements? E. g. do you strictly need a triangular velocity profile with 1200 RPM at the peek or is it also ok to run a trapezoidal profile. These requirements determine the appropriate programming approach .

Kind regards,

Jochen Klier

National Instruments

[FLAnatic]

Jochen, thanks for your assitance.  I'll try to provide you with further information based on what I've been told. A trapezoidal profile will be nescessary, when the motor reaches 1200 rpm it will remain at this speed for ~8.6 seconds.(172 revs), I don't know how critical having acceleration to 1200 rpm in 1 second is, but for now I'm going to assume that is required.   Being able to keep track of the revolutions is important because I can not exceed a certain number of rotations, the entire trapezoidal profile must be completed within 190-94 revolutions without exceeding 194 so there is a 4 revolution window to come to a complete stop.

[FLAnatic]

Would this be a possible method for measuring the breakaway torque using a stepper motor? Instead of increasing the torque until there is movement,  I could monitor the torque when setting a speed on the servo motor, and just measure the torque at the time movement occurs, would this also provide me with an accurate breakaway torque?

[Jochen]

For safe and most accurate operation I recommend using the contouring mode. In contouring mode you can download an array of position data to the board that is used as the trajectory data for the move. This provides full control over position, acceleration, deceleration and velocity at any time of the move. Contouring is supported by NI-7340 and NI-7350 motion control boards.

I really feel very uncomfortable with using a stepper motor for this application. With a stepper the breakaway torque needs to be reached within a single step of the motor, so you will need to sample the torque data at a very high rate. I also would expect significant oscillations in the torque signal after the breakaway torque is reached and finally your dynamical specs will require a significantly oversized stepper motor which might result in a serious mismatch between the motor's inertia and the load's inertia, causing even more oscillations and power reflections. Overall this setup smells like trouble.

Jochen

[FLAnatic]

Would an NI PCI-7342 with NI MID-7652 drive provide the control that I need?  I would have to just find an appropriate Brushed DC Servo motor.  Does NI carry or recommend any?  Is this link the best resource for this?  Thanks again.

[Jochen]

NI's offering for servo drives is quite limited, but if the MID-7652 meets your power requirements, it's a good completion for the 7342 with very simple wiring. Please note, that the motor's inductance shouldn't be below 100µH.

If the MID-7652 is oversized or undersized, you could choose a 3rd party drive and motor in combination with a UMI. The motion advisor should help you to select a fitting product.

Jochen