Analog ±10 V speed and torque control: scaling, offset and noise

Basics 01

What is analog speed/torque control, and when is it used?

In analog control the drive receives a voltage in the ±10 V range and interprets it as a speed or torque reference. It is used where an external motion controller closes the position loop, in machine retrofits, and in torque-controlled applications such as winding. The SD-LP series supports analog speed and analog torque modes.

• Speed mode: voltage → rpm
• Torque mode: voltage → torque (winding, tensioning, pressing)

Commissioning 02

How do I scale the analog reference?

The drive has a scale parameter that sets which speed (or torque) 10 V produces. Choose it so the controller's full command matches the highest speed the machine will really use; using the whole range improves resolution and noise immunity. For example, with a top speed of 3000 rpm, 10 V = 3000 rpm, so 300 rpm per volt.

• Too large a scale turns a little noise into a lot of speed
• In torque mode, always set an overspeed limit

Troubleshooting 03

At a 0 V command the motor slowly creeps. How do I fix it?

Creep at zero command means the drive sees an offset of a few millivolts instead of 0 V, coming from the controller output, a ground difference or noise on the cable. Use the drive's zero offset adjustment first and, if needed, define a small deadband. The lasting fix is a differential analog input with the common reference wired correctly.

• If the position loop closes outside, the axis must hold in servo lock at rest
• Use shielded twisted pair for the analog cable, shield grounded at one end

Commissioning 04

If the position loop closes in an external controller, where does the encoder signal come from?

In this arrangement the drive closes the speed loop and the external controller closes the position loop. The controller reads position from the drive's encoder output (the A/B/Z pulse output) or from a separate encoder of its own. If you use the drive output, set the output division ratio to suit the frequency limit of the controller's counter input.

• Tune the drive's speed loop first, then the external position gain

Basics 05

Should I move from analog control to a fieldbus?

On a new machine, usually yes. A fieldbus carries the reference digitally, so there is no offset or noise problem, and the drive's alarms, current and temperature come back to the controller. Analog still makes sense in retrofits where the existing controller can only give an analog output, and in simple single-axis torque applications.