Hardware: Maybe this is obvious to you, but assuming that you have a motor or other actuator that can move the lift, the most important thing you need to make this work is some sort of feedback sensor that tells you how high your lift is. You could get the AndyMark "string potentiometer", but it's limited to about 30" of travel. You could use one with a second string tied off to the top of the shaft and the lift, then passing through a pulley at the top of the potentiometer string to reduce the travel by a factor of 2, but that seems prone to tangling. If you're interested in a design which we have built and calibrated but not yet used operationally, PM me and I can send you build directions for the "electrified tape measure" we're using do do this; the only required tools are a hack saw, drill press, vise, wrenches, and screwdriver, though a chop saw makes most of the cutting easier. Apart from the 10-turn potentiometer (about $15 from Jameco) and a bit of scrap aluminum that we cut off the KOP drive chassis, all the parts and a 1-1/8" spade bit were bought locally (Lowe's, Radio Shack, Hobby Lobby, Ace Hardware) for about $25.
Software: You need to have a variable that keeps track of how high you currently would like the lift to be -- on the ground, one tote up, two totes up, etc. You may have intermediate stages as well; last I checked we had four additional positions associated with: flipping totes, scoring on the platform, pulling things off the step, and doing the coopertition stack. The value of this variable is the "state" of the "state machine". You then develop a set of "state transitions" (probably using case statements) to handle rules like:
- If you're at ground level, and "up" is pressed, got to level one
- If you're at level one and "up" is pressed, go to level two
- If you're at level one and "down" is pressed, go to level zero
Then, you drive your lift to the desired level using your favorite algorithm, whether it's a PID or something fancier or simpler.