Servo motor control for a machine tool - Page 1
Forum section: electronics / projects

author: chrisalbertson
author_rank: Newbie
timestamp: April 12, 2017, 12:48:17 am
content: This is a question for someone with experience with CNC machine tools.I'm converting a small milling machine from manual to CNC control. The software is LinuxCNC/Machinekit. I was going to use stepper motors on all axis but I was given a box of DC brushed servo motors and might use one of them on the Z-axis, if I can figure out how to interface it.It is your basic DC brushed motor with a 250 pulse per revolution quadrature encoder.I can configure the software to produce a digital PWM signal and a direction pin to control the motor speed/direction and the software can read the quadrature encoder.What is needed is something to go between the PWM and the motor and a power supply. I'd say a dual H-bridge built with mosfets would do it. But it gets complex when you add the required safety circuits for back EMF, overshoot, current limit, interface to an E-Stop system, optical isolation and I'm looking at power in the 48 to 60 volt range and current maybe 8 or 10A peak.I can design smaller, simpler motor drivers. What I'm looking for here is a known and proven design or better a ready made product I can get on eBay.OK, I've seen servo controllers that accept "step" and "direction" inputs that make a servo look like a stepper motor. These servo controllers close the loop internally and run a PID algorithm. I can configure the software to produce step pulses rather then PWM but I want to "close the loop" in software not the controller.So,.. I'm looking for pointers to reading materials, schematics and products.I you haven't guessed, this is a hobby budget project. I'm using used motors and making most of the metal parts from raw stock. There are many easy solutions but with four digit price tags.Thanks.Chris

author: amitchell
author_rank: Regular Contributor
timestamp: April 12, 2017, 01:12:51 am
content: I Converted a bridge mill to a 3 axis with servos, glass scales, Anilam drives and Mesa Electronics control hardware running LinuxCNC. I think most people are using the Gecko or Mesa stuff. This was about 9 years ago, so I am a bit rusty. This looks like a decent overview of what needs to happenhttp://www.cnctablebuild.com/mesa-setup-part-1-hardware/http://www.mesanet.com/http://www.geckodrive.com/products-order.html/

author: H.O
author_rank: Frequent Contributor
timestamp: April 12, 2017, 05:37:40 am
content: What type of interface card are you using? Or are you planning to use the LPT-port?Geckodrive does step/direction and closes the loop in the drive so you can't use those if you want/need to close the loop in LinuxCNC. MesaNet does all kinds of interface boards, I'd suggest you get one that can give you an industry standard +/-10V signal and feed that into something like a AMC 30A8 servo amplifier which are readily available on EBAY. If you ARE planning a "no hardware" approach - ie using the LPT-port only I suppose you could build a simple circuit to convert the PWM/DIR signal to a +/-10V signal for the servo amplifier.

author: awallin
author_rank: Frequent Contributor
timestamp: April 12, 2017, 05:55:19 am
content: Quote from: amitchell on April 12, 2017, 01:12:51 amMesa FPGAs seem to be quite popular. They used to be always PCI or PCI-E cards, but there might be something over Ethernet working now.This is one option for a PWM-input brushed servodrive:http://pico-systems.com/pwmservo.htmlAn alternative to geckodrives, taking step/dir, might be:https://granitedevices.com/digital-servo-drive-argon/

author: W9GFO
author_rank: Regular Contributor
timestamp: April 12, 2017, 09:37:51 am
content: Quote from: chrisalbertson on April 12, 2017, 12:48:17 amSimplest way I can think of is to use aGecko Servo driverand drive it with the same step and direction signals as the stepper drivers.

author: 691175002
author_rank: Regular Contributor
timestamp: April 12, 2017, 04:55:14 pm
content: Generally people use servo drives which close the complete position loop.If you want to read the encoder yourself and close the position loop in software you are looking for a servo amplifier. Amplifiers generally take a 0-10V analogue signal in and produce a corresponding output current (torque). They should be available fairly cheaply used since.