# Thread: -10V to 10v circuit

1. I want to control servo speed-torque & vfd from mach3/linuxcnc with analog voltage -10v to 10v.
My servo accepts digital step/dir - cw/ccw - quadrature for position control and analog -10v to 10v for speed/torque control.
So iam searching a circuit to do this thing..
Knows anyone something about that ?
Last edited by vre; 13-01-2014 at 10:53 AM.

2. Sorry I can't be much help here, it's something I've not looked into with Mach3

using -10v to 10v is normally carried out with a joystick control and a VFD, -10v is the reverse direction whereas +10v is forward direction.

To generate the -10v to 10v you could use a simple potential divider across a 20v supply with the centre tap connected to zero or you
could search for a better circuit on Google for example using the search term "Dual rail power supply"

There's some info here that might help.

http://www.machsupport.com/forum/ind...?topic=18151.0
Last edited by EddyCurrent; 13-01-2014 at 11:44 AM.

3. Mach outputs a PWM signal to control spindle speed, so that's 0-5V (assuming 5V parallel port). You therefore need a circuit to convert 0-5V to -10 to 10v. You can use op-amps to subtract 2.5V from the signal (so it's -2.5 to 2.5v), and apply gain of 4 to get the correct range. Use an RC filter to get an analogue voltage level from the PWM signal.

Something like this circuit:
Non-Inverting Op-Amp Level Shifter

The op-amp would need a +-10V supply... one easy way, though probably not the cheapest way to get that is with one of these DC-DC converters.

Or you could get a breakout board which does it for you.

If you're using LinuxCNC then you could control the spindle speed using any of the position control (step/dir, cw/ccw etc) inputs, which is probably easier as it saves making the above circuit.

4. Think we need clarification of OPs needs. He doesn't mention spindle in OP only control of speed of a servo. Everybody's made the assumption of spindle speed control by virtue of mention of VFD but VFD is not a servo thing. If using step/direction controls then speed is related to step rate, so its not clear what analogue voltage is doing.

Creating -10 -> +10v from step (PWM) and direction (polarity) isn't hard but you need some skills in design, electronics assembly and testing. Jonathan has given part of a solution but its of little use without the skill/knowledge to do the rest.

I may be wrong but I know of no BOB that does -10v out... the speed output is invariably 0 -> +10v and direction goes to fwd/rev input on VFD.

@vre: please be more specific in your requirements; are you referring to motion control or to spindle speed/direction control?

5. Originally Posted by vre
I want to control servo speed-torque & vfd from mach3/linuxcnc with analog voltage -10v to 10v.
My servo accepts digital step/dir - cw/ccw - quadrature for position control and analog -10v to 10v for speed/torque control.
Vre can you post the Make and model of your Servo drives because Like Irving says it's unclear what your saying and I think you maybe misunderstanding the requirements of your Servo drives.?

Also if using a VFD what Make and model of that also.

6. delete it.......
Last edited by vre; 13-01-2014 at 08:24 PM. Reason: double post

7. Man that's one crap manual and very hard work reading.

Anyway you don't need +/-10v it just means the drive has the abilty to use Analog signal if that's what your control used. Lot's of older Machines used analog controllers so they provide it for compatibilty reasons but Step/dir is the more modern way.

In your case you'll use Step/Dir from the motion Control device you use, whether that be Parallel port or External Motion controller. If your using Mach3 then I highly suggest you get a motion control Card has the Parallel port will not be fast enough for servos with 2500cpr encoders.

To control the VFD spindle speed you just need a 0-10V signal which can come from either an external speed control card or one built into a BOB. Can also use RS485 I believe but not done it my self can't help here.

8. OK definitely some confusion here...

Servo controller is nothing to do with VFD. VFD is standard 0 - 10v speed and FWD/REV run/stop inputs and can be driven from normal spindle control board or BOB with 0 - 10v speed control output.

Servo controller can be programmed to operate in different modes - parameter Pn002. One of these modes is step/direction for positional control (like a stepper) using wiring diagram 2.3.1. Another mode is analogue speed or torque control using wiring diagram 2.3.2. These modes are mutually exclusive, you can use one or the other but not both.

@vre: what are you using servos for?

9. Assuming the OP still wants -10 to +10.
With the method proposed previously by Jonathan there exists a problem, if the reference wire to the servo driver goes open circuit, the drive would go to full speed reverse.
Would it be possible to use a standard spindle board with 2 relays such that the output would always be 0-10v but one relay would reverse the polarity of the 0-10v output ? opto isolators would need to be used to prevent grounding issues. Within Mach3 the user would use S for speed and M3,M4 for direction.
In additiaon a Mach3 Brain could be used to read the Spindle Speed DRO and only operate the direction relay outputs if the speed was above a particular figure, this would give a deadband around the zero position.
Then the logic could also include a test to see if the requested speed (via S command within Mach3) was odd or even, if it was even this would indicate forward, if odd it would indicate reverse.
The speed would then be programmed in Mach3 such that a value ending with zero would be forward and ending in 1 would be reverse i.e. S1000 would be 1000rpm forward, S1001 would be 1000(almost)rpm reverse. The Mach3 Brain would operate the polarity switching relays automatically. It's clear that the code could move from full forward to full reverse e.g. S10000, S10001 implying that an integrator be used but this is normally a function of the VFD or Servo Driver that employ ramps.
Last edited by EddyCurrent; 14-01-2014 at 10:57 AM.

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