The scopedrive site has information on my Electronics for automating telescopes and other odds and ends. Most of my ATM work has used stepper motors in order to keep things simple and low cost.

Some thoughts first on stepper motors, gearing and mounts.
I am using my electronics on a Losmandy G-11 mount. The G-11 is a classic German mount, both the RA & Dec axis are stepper driven with a 360 tooth worm gear on each axis. As delivered, the worm was driven by a Hurst Stepper motor with 15 degree steps and a 1:150 gearhead, by half stepping the stepper Losmandy achieved 0.5 arc sec. drive resolution. I replaced the original Hurst stepper motors with another model Hurst motor that has the same mounting, but with 7.5 degree steps and a 1:15 gearhead, this along with 8x micro-stepping now gives the mount 0.625 arc sec. resolution. These replacement motors are fairly low cost and drop right in without requiring any mechanical mods to the mount.
I have have been asked a time or two as to why I have maintained a drive resolution of around a half arc sec. when, for most of us, seeing of even a couple of arc sec. is a rare event. If picking a step size that was a fraction of the seeing confusion was the governing factor, I would guess that for most of us one arc sec. more or less would work just fine. But that's not what drives my step size selection, I want a step rate that is well over 10 to 15 steps per second while tracking in order to avoid running into mount resonances. If the drive step rate happens to fall close to the mount's natural resonant frequency you will end up "pumping" the mount and the result will be a bad case of the "fuzzies."

For the past few years I have been using/testing a two axis microstepping drive using the Allegro MicroSystems 3955 Full-Bridge PWM Microstepping Driver. The board design is based on a two axis stage controler I did some time back, but with the addition of a drive 'idle power' feature that allows over driving the steppers while slewing, but dropping the drive current back when slow stepping or idling. The PC boards were designed with the free design software from ExpressPCB. You can view a parts list as well as the solder mask silkscreen and the ExpressPCB design file for the PCB board itself (right click, and select "save link as" to download). The source code for a low level dual axis driver is here
History: I did most of this work around 2000, but did not post it until late 2003. I integrated the driver for my board into the GPL'd software by Mel Bartels. Mel's design does everything in software and works well. My aim was to offload the processor by moving the microstepping current control onto the A3955, this worked OK, but I did not get the gains I expected because the RTC interrupt rate had to be increased by more than I had thought in order to get good stepper performance while slewing. Driving the replacement steppers at rated voltage I was able to get slew rates of just under half a degree per second. While I felt fairly good about this design, it still used more processor that I liked, and I wanted faster slew rates.... so on to plan 'B'.

Moving right along! While my A3955 driver worked OK, I was unhappy with the slew speed and I still felt the processor load should be much lower. So... I have started working on a new design that (a) has a much lower processor load and (b) uses a higher power driver chip, not that I needed more drive for my steppers, but so that I would not have to worry so much about cooling. My new design uses the Allegro MicroSystems 3977 Microstepping DMOS Driver with Translator, the A3977 has a 2.5A output rating vs 1.5A for the A3955. In order to offload the processor I am working on a controller chip based on the Xilinx XC9500XL PLD. My design is losely based on the microstepper controller outlined by Mark Trueblood in Telescope Control (1st edition). Right now the design is fairly well frozen, the step rate will run from 2 steps/sec to well over 100K steps/sec, the step time resolution at tracking rates is about one part in 10**6. The open loop position counters will be 24 bits which will support steps of less than 1/10 arc sec (which is insane, but bits are cheap). In addition I am hoping to include support for automatic rate ramping, I think I have the extra logic to do it, and I have the "hardware" in mind, but I have not yet totaly reached a conclusion as to how useful it will realy be. Right now I am in the early stages of actual controller design, I expect to have breadboard copper done before mid year '06. Stay tuned.
Update March '06 : The controler design is finished and software debugged, here is the VHDL design code. The reality of working with chips with lead spacings of about 0.020" has forced me to do the design in a much more limited chip than originally planned, as a result each axis will require a chip and the open loop position counters will only be 16 bits. As long as the counters are read more often than once in 32767 counts there will not be any problems. Given that I don't expect to see a slew rate of over 6,000 to 9,000 micro-steps per second I don't think this will be a issue. I have also dropped the automatic rate ramping feature, a bit of quick and dirty simulation convinced me that a real solution was more complex than I had expected. I don't know if I will revisit this feature, if I do it will not be in this iteration. I have started on the first breadboard PCB. This will be strictly for chip design testing and will not include the actual A3977 driver chips.

I also have been using a high power discrete full bridge driver based on a design by Chuck McManis. This driver will carry up to 5 amps with good heat sinks, and the driver can put the motor into dynamic braking mode, but will not allow free wheeling. Two drivers are laid out on a single small board such that both may be used via a single connector, or they may be cut apart for two stand alone boards. The TIP transistors may be mounted from the bottom and then bent out 90 degrees for mounting to a single large heat sink. You can view a parts list as well as the solder mask silkscreen and the ExpressPCB. design file for the PCB board itself (right click, and select "save link as" to download ).

This will be a long running project, so visit this page later for any updates.
You are visitor number
Send me mail