Analog speed signal

Hello,

Does anybody knows if there is an analog signal which represent the speed of each axes?
The reason why I ask this is the following.
I have 3 servo controllers and motors available which has an analog input to set the speed of the servo motor.
What I would like to do, if it is possible, use the controller card of the reprap project but control the motion with servo's instead of steppermotors.
Is there a signal between the CPU on the controller card and the steppercontroller which is analog or convertable to an analog signal?

Greets Dirk

No and you need servos that control position, not speed.

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[www.hydraraptor.blogspot.com]

That totally depends on the type of servo controller. I have a Kollmorgen 200serie VT version. The motor and drive keeps itself in controll with the close-loop control. I don't know for sure if this is done by position or speed. The setpoint however is a speed setpoint. De version VT stands for velocity. The input at the controller is an analog input.

But you answered my main question: there are no analog signal on the card available.

Is it possible to converter the stepper signal to an analog signal?

If all your have is a velocity control how can you accurately control position? It seems that your controllers are for spindle control rather than axis positioning.

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[www.hydraraptor.blogspot.com]

It is true that it is impossible to make a accurate positioning loop. The only thing you will know is that the motor is following the speed set point properly. So if you assume that you will not lost positions in the first stage (because position = speed * time) you will have not a bad positioning system. But it is the same as a steppermotor. You will never know for sure that the motor will reach its position.

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position = speed * time

Yes but if speed was represented by an analogue value then it would need to have a precision of about 0.1%, which is very hard to achieve with analogue electronics, and since position is the actually integral of velocity you would slowly accumulate errors, which would be disastrous for FFF. (Speed is not constant because you can't change it instantaneously).

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But it is the same as a steppermotor. You will never know for sure that the motor will reach its position.

If the motor is used within its specification you can assume it will reach the correct position. I make that assumption hundreds of thousands times a day.

To use servos you need closed loop position control. Some servos have this built in and take step and direction signals. To make a servo with velocity control work you have to do the following with firmware or an FPGA or discrete logic / analogue electronics: -

Use the step and direction signals to make a target position variable, with for example an up down counter
Use a quadrature encoder to create an actual position variable.
Take the difference between the two and use it as the input to a PID controller, the output will be your velocity signal.

Then you have to tune the controller to give a clean response without overshoot or ringing. All in all, a lot of work.

Edited 2 time(s). Last edit at 10/27/2011 08:15AM by nophead.

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[www.hydraraptor.blogspot.com]

I've retyped this a bunch of times. I can't get the wording to come out nicely:

Stepper motors will work better for our applications. The benefits of higher efficiency and higher RPM range are moot for our users. It can be done, it is a bad idea.

And to answer to topics question: Servos require a sepearte encoder, which our boards would not provide. There may be a few Arduino sketches for CNC purposes for servos, but I'm fairly sure RepRap has dropped Servos as an option.