Configure 73xx Controller and UMI-7764 to Drive 3rd Party Stepper Motor

Issue Details

How do I get the 73xx controller and UMI-7764 to drive a third party stepper motor?

Solution

A common situation when connecting third party stepper amplifiers or drives to NI motion controllers is that these amplifiers usually come with a Step+ signal connection and a Step- signal connection to drive their motors and these are opto-isolated. Reading the labels on the UMI-7764 interface would usually indicate that we connect the Step/CW signal to Step+ on the third party amp and the Gnd signal to the Step- connection on the drive. This configuration, however, will not usually work with opto-isolated amplifiers and the motors will not move.

The problem lies in the fact that the step and direction signals on the motion controller are open collector (this holds true also for the direction, breakpoint and inhibit out signals). Open collectormeans that the output stage circuit on the motion controller can source very little current (max of 1.5 mA for our controllers), but it can sink a lot of current. Refer to the diagram below. 

On the amplifier side, opto-isolation is usually implemented by means of a diode which limits current flow in only one direction. When this diode turns on, it sends a signal to a photoreciever and this signal is used inside the amplifier to drive the motors. This functionality prevents any high current signals from flowing back into the controller and damaging it. The problem this presents is that in order to turn on, diodes typically require higher currents (15 mA or more), so connecting the board signals directly will not provide sufficient current. 

Workaround:
Refer to the picture below. Instead of connecting the Step+ on the amplifier to the Step/CW signal on the NI UMI-7764, get a +5v power supply capable of sourcing enough current to turn on the diode and connect the Step+ signal to it. Next, connect the Step/CW signal from the UMI-7764 to the Step- signal on the amplifier. This will ensure that as the UMI-7764 Step signal switches between +5 V and GND, the voltage drop across the diode will toggle between 0 V and +5 V. The voltage drop across the diode will allow the current from the source to flow and switch the diode, thus moving the motors. Perform the same operation for the direction, breakpoint and inhibit out signals.

 

Additional Information

When connecting to an external power supply, you need to close the circuit. If you choose to connect the power supply to a digital ground on the 73xx board, you need to be cautious of potentially creating a ground loop. To prevent this, make sure to check the voltage across the digital ground and the ground of your power supply. If you measure zero volts, then this configuration will suffice. Use an isolated power supply to guarantee that you do not create a ground loop.

If you do produce a voltage other than zero across the grounds, an alternative setup is to use the +5 V power of the 73xx board itself to power the drive. With this configuration, however, the maximum current for the ENTIRE board is 100 mA as documented in the product specifications.