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| Servo Driven 4th Axis for CNC Mill |
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| awemawson:
The Heidenhain TNC355 (which is very similar to the TNC150/151/155 ones) outputs an analog signal in the range +10v to -10v representing the motor speed and direction. This goes to the Bosch servo driver card that beefs it up to the 140 volt max and 26 amp absolute max peak that the motor will take. Into the Bosch servo card goes the tachometer output that is integral to the motor (7 volts per 1000 rpm). At this point the TNC355 has no idea if the motor has moved at all so it takes in signals from a rotary quadrature encoder (also made by Heidenhain so conforming to the same spec) fixed to the servo motor (in my case through a belt drive as I need to change the ratio) giving it 36,000 counts round the 360 degrees of table rotation. Also on the table is a 'reference point' switch. When the controller is first switched on, all axis, including the rotary one, have to transverse across their reference points. From then on the TNC355 keeps count of where the table is. The servo card has some more sophistication in that it has a form of 'three term control' (proportional, differential and integral) that when set up right allows it to respond to, for instance, a step function increase in speed without either overshooting or approaching too slowly. All this is ahead at the moment until the mechanicals are sorted :lol: Andrew |
| mgnbuk:
You will have a very hot motor if you manage to stuff 26 amps into it ! IIRC the peak current value is the maximum the motor can tolerate before de-magnetisation occurs - continuous stall current is 4.4 amps for an MT30 H4-44, which I think is the one you are using ? Regards, Nigel B. |
| awemawson:
Yes that's the one Nigel. I am amazed that at 5 volts on a psu that limits at 1 amp it is turning the table, and while it's doing that (remember the encoder is removed) I cannot stall it by grasping the rear end of the shaft with my fingers - the current just goes up from 250 mA to about half an amp. Incredible torque. Presumably the Bosch servo driver limits the current to a safe level ? Andrew |
| awemawson:
Pulleys and belt for the encoder reduction drive came in the post this morning, no time to fit as we have guests for lunch Andrew |
| mgnbuk:
There should be some method of adjusting the current limit on the drive. I am not familiar with the Bosch drives - I've tweaked them & swapped one or two, but can't recall the setup - usually I was just fine tuning the standstill offset or the speed to get the trailing error correct. It maybe a potentiometer, where you set a voltage on a test pin to a particular value for a particular current limit, but some drives require a resistor changing to set the limit. Most of the Bosch drives on Bridgeports would have been set up for the axis motors, which were 3.5 or 4.5 Nm, so the current limit will probably set much higher. If you Google the Bosch drive part number, you may be able to find a copy of the installation / set-up manual. HTH Nigel B. |
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