Servos on a shopbot

[rob_taboada]

I'm looking at used prs alphas right now and have found one with servos instead of steppers. I'm assuming this is an after purchase addition(?), anybody got an opinion? I'm guessing it was done to improve cut speed/quality, but I'm wondering if the shopbot frame can withstand much more force than it gets, also wondering if it might cause problems with shopbots controller. I'm new to CAM, so thanks (and apologies) for responding if I'm asking foolish questions

[knight_toolworks]

I may be mistaken but it would need a whole new controller box and not really be a shopbot anymore. for the most part servos would be slower unless they are really expensive ones.

[rob_taboada]

That's what I was wondering. Looks like they're paired with a Yaskawa V74X Drive. So it's basically a shopbot frame with whole new setup? Would this thing then be running g-code as opposed to the shopbot language?

[knight_toolworks]

it would have to be gcode. but it would also have to run one of the other controller software's out there. it could not run the shopbot software.
some of the cnc routers in the same price range as the shopbot can only more 3 or 4 ips. that includes jogging speed. the shopbot can jog at 25ips or more.

[knight_toolworks]

so this machine you could not really get any help with since it is no longer really a shopbot.

[rob_taboada]

makes sense. The specs they list seem to be cut and pasted from the shopbot website, so this machine sounds a little like a crapshoot.

[bill.young]

I don't know anything about servos but that's the same VFD that runs many of the spindles that ShopBot sells including mine.

I wonder if they're just confused and calling them servos, but that it's really a regular PRS?

Bill

[rob_taboada]

Huh. That makes it interesting again. I'll have to give the broker a call and see if I can get an answer out of him. It's a bank repo, and I don't know if he's even seen it in person. Could be a regular prs and he's mistaken.

[rb99]

Get pictures of the control box...open if possible.

RIB

[rob_taboada]

will see what I can do about pictures.
so I'm clear (remember I'm a beginner here): the VFD controls the spindle only, correct? and then the controller directs the xy and z motors and directs the VFD for spindle speed?

[rob_taboada]

machine sold this morning, back to the hunt. They were yaskawa servos, though.

[widgetworks_unlimited]

I don't see why the ShopBot control board/software couldn't run a servo driven machine.

Servos require different drivers (which are housed in the control box and wired to the control board, but are not actually on/in the control board itself.)

The machine would have different power and speed characteristics, just as though you had changed to different stepper motors. Weather these would be better/worse would probably depend on your application.

[richards]

Servos can be run by a Shopbot controller if the controller sends step/direction signals. Geckodrive sells the G3xx series of servo controllers for brush-type servo motors. I have one G340 (that was converted to a G320 by removing the pulse multiplier chip) and a brushed servo motor. It works on the same principles as a stepper motor, i.e., for every step pulse, the shaft rotates one 'position'. The distance or arc of the 'position' depends on the encoder that you use. A 500 position encoder becomes a 2,000 'step' encoder because of the quadrature method of reading the encoder signals.

In other words, if the designer knew what he was doing, a servo or a stepper can be used on a Shopbot.

The advantage of a servo motor is that it produces constant torque at all speeds. The disadvantage is that its torque is much lower when physical size of motors (stepper vs servo) are compared; however, gearing would give the best of both worlds.

[knight_toolworks]

but would it give the position feedback needed that servo's are famous for? myself that's what I really want is my machine to know when it is off.

[richards]

A servo gives feedback through its encoder. The Gecko G320 sends a move signal to the motor (pulse). When the motor has moved, the encoder sends a signal back to the G320 (encoder pulse). Depending on which model of servo driver, the driver will 'fault' when the number of step pulses exceeds the number of encoder pulses by a preset number. I think that the G320 faulted at six mismatched steps, but I may be wrong about that number.

Because there is a little 'slop' between pulses being sent to the motor and encoder pulses being received by the driver, a servo motor acts a little like the Alpha motor/driver on the PRS-Alpha machines; however, the Alpha can recover many more 'steps' than a servo motor.

When power is removed from the servo, it looses all track of positioning. It has no way of knowing if an axes moved. That stands to reason because the encoder does not send pulses when there is no power and the driver cannot receive pulses when there is no power.

A clever operator could leave the encoders powered up and, using a simple microprocessor, could count encoder pulses when the machine was turned off. Then when the machine was powered on, the microprocessor could move the axis the correct number of steps to return to its original position. That seems like a lot more work than just running an XYZero and ZZero routine.

Geckodrive publishes a FAQ giving reasons to use either a stepper motor or a servo motor. Generally, they recommend a servo only when the weight being moved and/or the speed at which the load is being moved exceeds the abilities of a stepper based system. The Shopbot falls near the high end of the scale, but as we all know, steppers work just fine.

Even though the cost of a servo, encoder and driver is just about the same as a stepper and stepper driver, the servo will (almost always) require gearing to get enough torque. My servo motor, an ID33004, weighs 10.5 lbs, is about 8" long (not including front or rear shaft) and puts out only 225 oz*in of continuous torque. In other words, it is much larger and heavier than a stepper motor. Because of its lower torque, it would have to be geared at least 3:1 to equal the torque of the original Alpha motors on my PRT-Alpha machine and about 5.5:1 to equal the 7.2:1 motors now furnished with the PRS-Alpha machines.