Hello all. As there is some traffic on a servo board being designed by Xerxes, it seems like a good time to start discussion about a similar effort that I have underway, instead of cluttering his thread.
The purpose is a low-cost sinusoidal brushless motor controller capable of driving a range of AC servo motors commonly available on surplus sites and Ebay, in the 100w - 1000w range. The only difference in that range (besides the external motor and power supply) is heat sinking requirements.
The controller itself will be "open source" in that its schematic, board layout, and microcontroller source code will be freely available and licensed under the usual open source licenses.
I do not really intend to make this a "business" in the sense of trying to make a profit or selling complete assembled units. However, I will likely make partially assembled kits with all required parts available. I do not have full pricing worked out yet for that (especially since the design could change due to the demands of potential users), but I expect that kit to cost between $80 and $100, which does not include an enclosure or heat sinking.
Features:
---------
Computer I/O: optically isolated step/direction/enable connections to a PC. The current design does not input and pass through limit switches, reasoning that it is more properly the function of higher level control to do this. I could be convinced otherwise if there is a strong opinion that the extra expense and complexity is worth it. The basic idea is to work with TurboCNC, Mach, and other hobby-level PC-based CNC control systems using standard step/dir interfaces up to approximately 100,000 steps per second.
Motor Inputs: The board requires standard Hall sensor outputs and A,B incremental encoder signals. For now, this leaves out the frequently-available motors that use resolvers for position feedback. Supporting resolvers would add significantly to the expense and a better option would be the development of a resolver-to-encoder interface board. The motor connections are NOT optically isolated in the current circuit. If that is strongly desired, an external isolation board (which would need its own power supply to truly isolate it from the HappyServo board) could be designed. The board can handle encoder pulses up to 500,000 transitions per second, and that can be increased to about 1,000,000 per second with the installation of an on-board jumper. The surplus center motor series gives 8000 transitions per revolution; at the max speed of 4500 rpm that's 600,000 transitions per second. Most other motors in the "surplus/ebay" category have encoders with fewer counts per revolution.
Input power: The board requires two power supplies. The first supply is for motor power and ranges from about 48 to about 170 volts DC. With the change of a few components the input range can be extended to 350V DC; you can often see motors with that high voltage range, though building a power supply for them is not simple. Additionally, an input of 18-30 volts is required to power board logic; this supply shares a ground with the high voltage supply and has low current requirements.
Software: The board requires a PC-based configuration utility that talks to the board over a serial port connection. This utility sets board settings and is used to tune the control algorithm. This program is written for Windows and is freely available. I will eventually make the code for the PC side open source as well.
The circuit board is a standard Eagle 4x3 board. Fitting all the components onto a board this size is a challenge but it is necessary to make sure that everybody who is interested can get the free version of Eagle and mess around with the board however they like.
The board has a hardware-based hard current shutdown limit of about 30 amps to guard against most short circuits. It also has "de-saturation" protection to guard against ground fault short circuits. Current use is monitored to provide soft current limits that can be set to the operating conditions of the motor being used.
A circuit driving an optional externally-connected braking resistor is supplied to assist with deceleration of high-inertia loads.
Technical Details
----------------
PWM Frequency: 16 kHz
CPU: Atmel ATMega64 running at 16 Mhz
Gate driver: International Rectifier IR2137
Power Transistors: ST Micro STGW20NC60VD
Heat sinking: depends on power needs, exact requirements to be determined. The smallest 100w motors will require very little heat sinking; motors up to 1 kW will require careful heat sinking. My current thought on the extreme cases is to adapt PC CPU coolers which are often available cheaply and have very good heat transfer.
Status
------
Schematic: Undergoing some final touches
Board: Design roughly complete except for trace routing.
Circuit: Prototype assembled and functional enough that it works. Braking is not in place yet, nor is voltage regulation, and some of the component values are still being decided.
Microcontroller Software: over half done. All PC and motor signal code is done and working. Commutation is functional enough to begin exercising the power section. PID loop not yet complete.
PC Software: about half done. Able to communicate with the board and graph positions/velocities streaming off the board.
Timeline
--------
Roughly, here's what to expect (these are estimates only)
November 15: Schematic posted for comments. I need to build an actual board instead of a ratsnest prototype to really evaluate and tune the board, but I'd like feedback on potential stupid things I did before doing that. In the meantime I want to get all the circuitry functioning properly in the prototype and get the commutation code finished.
November 22: while the board is off being fabricated, I need to finish building my test harness. I want 8 different makes or models of servo motor (I have 6 so far). The test harness consists of an adjustable-torque clutch to simulate different torque requirements, and also a variable-weight disk to simulate different inertial loads. So far I have acquired the clutch but need to actually build the test stand. I also need to finish a 150v 20A power supply which will be needed to drive the 1kW size motors.
December 1: presentation of initial stress-testing results on various motors. The software should be complete at this time.
The timeline after that depends on the results of testing. If redesign is needed, there will be a delay.
I'll be making a web site to organize all this data more effectively.
Any comments are welcome. I am not attempting to compete with existing commercial products or other hobbyist controller efforts. I have no intention of making money off of this so no motivation to do marketing efforts or comparative analyses with other efforts. There's lots of room for different options (look at stepper driver boards, there must be 50 different products or designs out there). However, with the exception of circuit boards and transistors, I have enough parts on hand to build about 20 boards, so I'm motivated to get this thing to work :-)