Hi All,
I've been avidly following this thread as I've been working on a stepper control system without computer control for a month, and thought I'd share my findings.
Below is a control system for a forkmount as I converted my 8" dob to a forkmount. Pics to come soon.
First of all, I'm not sure if its been mentioned before, but i found an excellent stepper driver from Allegro Systems on this aussie web site
http://www.littlebirdelectronics.com.au (A4983)
it is amongst the best value for money i've seen so far and works a treat. (I had a look at ocean controls as well, but that was a bit too much for my budget)
The A4983 has built in chopping circuitry to limit current draw when idle, microstelling upto 1/16th step (3200 steps/rev for a 1.8deg motor), drives upto 3A, has a current limiting pot), extremely small footprint, digital logic inputs for EN, SLEEP, DIR, microstep, reset, etc and the list goes on.
I wanted to start motorizing my scope using discrete control and not software, so I worked on a control system using digital logic.
I came across some really good electronics and hardware suppliers (australia-rsonline.com , farnell.com.au, littlebirdelectronics.com.au) for all rare components and handy kits.
I started with a Silicon oscillator and found the LTC6900 to be perfect. It is a 5 pin SOT-23 IC, has 3 settable divisions for its master freq, uses just one external resistor to set the frequency, and is precise, extremely small (SOT-23, had to buy a breakout board from littlebird).
My Concept was simple. the Silicon oscillator generates square waves at a fixed freq (variable with a trimpot to arrive at final freq), this is fed to a freq divider CD4060, three frequencies (1/5, 1/6, 1/8th) are fed to a multiplexer (CD4051) that has three channel selection control inputs and 8 channel inputs. (i used only two bits as I needed only one of three frequencies).
I planned to use a hand held controller that has 5 buttons
CW Hi Speed (high speed slew)
CW Lo Speed (micro slew)
CCW Hi Speed
CCW Lo Speed
Tracking (fixed dir)
(only 3 frequencies are reqd as CCW requires a change only in DIR bit)
Dilemna was switch control as I had to control five different variables with one spst button each.
I therefore built a truth table to see what 5 bit nibble I needed for each of the above buttons.
the 5 bit nibble was
X,Y - Mux channel selection
EN - Stepper driver enable (when not enabled, driver disables output circuitry internally thereby conserving power)
INHIBIT - inhibit / strobe input for Mux
DIR - Simple High/Low at driver's DIR input for CW/CCW
Therefore, I settled on a 20key encoder (MM74C923) that I bought from Farnell. This has 5 bit outputs which is what I wanted, built in debounce circuitry, built in multiple key control, (if you press two buttons ).
So from the datasheet, i just chose the 5 bit outputs that I wanted as per my truth table, found the corresponding key number, wired it up to spst switches and it worked.
My criteria was that when no key is pressed, the stepper and Mux enable inputs are disabled. I achieved this by setting the correct high/low states on the EN / INH pins using the encoder.
I salvaged a 1.8deg hybrid unipolar motor from a fax machine which proved to be perfect as it came with a timing driver and belt. just wired as a Bipolar. still testing on whether to use one half winding for higher torque instead of full winding.
a general purpose PCB from jaycar, and I got the whole controller and driver working.
I use a 9V battery with a 7805 regulator for logic supply, and a 78T12 3A fixed 12v regulator for the motor supply (2 x 9V batt at this stage)
I made the fork mount entirely out of wood ( i know its not the best choice), but I'm handy with wood, so it worked well.
I got two self aligning bearings from a bearing supplier in dandenong, a 20mm steel shaft ($12) from CSC in Clayton.
I used 35mmx90mm pine sleepers (bunnings) for the fork, but had double sided cladding with 8mm ply to reinforce it, and its extremely rigid with no flex under full load.
I bought two 20mm flanges from australia-rsonline.com on either side of the fork that slides onto the shaft so that minimizes perpendicular deflection due to load.
I'm just waiting to get my timing wheel from small parts and have to figure out how to mate it to the steel shaft.
I setup jumpers for microstepping selection on the PCB and can easily switch between full, half, 1/4, 1/8th and 1/16th microstepping depending on what speed I need.
I had a few posts on Pololu's web site as I was using their driver, and they gave the idea of using a PICAXE microcontroller, and programming it with the free software or BASIC.
Thats my next step to computerize it or atleast add position sensing, gradual acceleration for slew, etc.
I have learnt a lot from this exercise that took me about a month, and although there are fully functional systems out there like the Bartels, I thought I'd share my ideas for someone who might be looking for a system like this.
To achieve accurate tracking, I need to ensure there are no missed steps, the motor is not overloaded, clock is stable, and adjust the master freq until the tracking rate is achieved.
One more bit that I wanted to share is that I built a Digital Oscilloscope from a Kit sold at littlebirdelectronics. I bought the $55 kit that comes with everything with the MCU already soldered, but all else including around 40 SMD parts need to be soldered.
after 6 hours of soldering the 40 odd SMD's, (desoldering braid was invaluable), it worked straight away, and was well worth the effort and price.
It has a built in frequency counter so I can see what my clock and divider frequencies are, and more oscilloscope features that I don't really use. they sell a no soldering required kit for 70 odd.
I'd welcome feedback on my ideas and criticism as well.
Thanks
