I want to run a big servo

[jnnewton]

Hello, I would like to operate a servo drive from AMC. I want a minimum of 1.5kW motor and drive combo. I think that I want to use it first in analog velocity control mode. My questions to start are:

1. Will using a low pass filter on a pwm from a pic be sufficient or do i need an analog ic (if so, a recommendation on serial or parrallel comm would be great)?
-If so, how can i tell from a data sheet on the drive how low the low pass filter needs to be?
-Would it be easier to just get a pwm version of their drives?
-I belive that in order to use the +/-10 V input to their drive (assuming i go analog velocity mode), i will need a op amp with a gain of 2 to get from my 5 v pwm on the pic to the 10v, and an inverting op amp to get the negative side of the signal. What would be the easiest way to switch between the positive signal and the negative (another output pin switches something (mosfet, etc.)
- how can i get the high dc voltages reqired (80V @ 2 kW) from 120VAC? rectifier, cap and resistor(shunt)?

 

[dknguyen]

Hello, I would like to operate a servo drive from AMC. I want a minimum of 1.5kW motor and drive combo. I think that I want to use it first in analog velocity control mode. My questions to start are:

1. Will using a low pass filter on a pwm from a pic be sufficient or do i need an analog ic (if so, a recommendation on serial or parrallel comm would be great)?
-If so, how can i tell from a data sheet on the drive how low the low pass filter needs to be?
-Would it be easier to just get a pwm version of their drives?
-I belive that in order to use the +/-10 V input to their drive (assuming i go analog velocity mode), i will need a op amp with a gain of 2 to get from my 5 v pwm on the pic to the 10v, and an inverting op amp to get the negative side of the signal. What would be the easiest way to switch between the positive signal and the negative (another output pin switches something (mosfet, etc.)

Huh? WHat on earth are you talking about?

If you are thinking about driving the motor from a linear device like an op-amp- generating a straight adjustable analog voltage source to drive the motor, don't. It barely works for things under a watt. Anything larger (up to a few hundred watts) can be done with very specialized, very expensive op amps designed for that purpose. Just use a bridge driven by PWM, where the transistor gates are driven by gate drivers that are controlled by the PIC's PWM signal. Or you can just get a DAC or resistive divider controlled by a PIC (or a potentiometer) that works with a 10V source.

If you are not referring to how to drive the motor, but how to control the driver (ie. send a certain speed to it with an proportional analog signal), then what you are looking for is a DAC- which can be generated using an IC or using an RC filter driven by PWM from a PIC- the filter doesn't really matter, just make sure it's cutoff is much lower than your PWM's frequency. THen yeah, use an op-amp to step up the DAC voltage or make it bipolar or whatever yo need.

- how can i get the high dc voltages reqired (80V @ 2 kW) from 120VAC? rectifier, cap and resistor(shunt)?

[/quote]
A step-down transformer that feeds into a large diode rectifier, cap and maybe inductor. BUt...I don't think you can draw 2kW from a standard wall socket. 1500W is about the max.

 

[jnnewton]

Huh? WHat on earth are you talking about?

I am talking about controlling a servo amplifier from AMC (link in previous post) from a dsPIC microcontroller. I realize that I can't drive kilowatt+ motors directly from an analog voltage out of a microcontroller. What I am trying to get to is a system controlled by my dsPIC that will run a 1.5kW servo motor. so, i decided that building the circuitry between the pic and the motor would be too big of a step for me at this point so i found AMC who sells low cost servo amplifiers. They offer PWM control and Analog control (+/- 10Vdc). I chose to first pursue the analog option due to the fact that the microchip could be swapped out for a pot or a PLC control system in the future if needed. I was hoping someone could point me to some info on designing a low pass filter to create an analog output from my pwm or tell me that I am better off using a DAC chip controlled by I2C or SPI. Any thoughts on using a pwm controlled drive from amc would also be apprecieated.

THen yeah, use an op-amp to step up the DAC voltage or make it bipolar or whatever yo need.

I need +/- 10 Vdc for control, but i am not sure the arrangement of op amps to get there. I think i know how i can get from my pwm filtered signal to 0-10 Vdc or -10 to 0 Vdc, I am not sure how to get to -10 to 10 Vdc. The zero crossing confuses me.

 

[Nigel Goodwin]

For a start get the 0-5V PWM to DC sorted out - if you search on MicroChip they have two application notes explaining all about it, and how to calculate the filters required.

Why didn't you just get the PWM version?.

 

[jnnewton]

If i had gone with the PWM version, I would have to close the velocity looop at the microcontroller. I do not know how to do this, but if anyone has any Ideas, i would be very interested in hearing them.

 

[jnnewton]

Nigel,
I read the app notes and some other filter information, and downloaded the microchip filter lab software. What I can't get past is the start of the design process where i need a low pass filter to turn my pwm signal into analog. I understand how to go through the component selection process once i know what my filter parameters are, but how do i know for instance what the passband and stopband attenuation and frequencies should be for a specific application?

I can choose my pwm frequency and the duty cycle, and was planning on simply varying the duty cycle and leaving the frequency constant, but I don't know how to wisely choose a frequency.