Nuts & Volts - September 2004

Stamp

looks simple on paper — with circles and dots — it takes a bit of thinking. Let’s look at the code that determines rotational direction and work through it:

Move_Spout:
  IF (angle > pos) THEN
    span = angle - pos
    IF (span < HalfRev) THEN
      stpDir = MCW
    ELSE
      stpDir = MCCW
      span = RevSteps - span
    ENDIF
  ELSE
    span = pos - angle
    IF (span > HalfRev) THEN
      stpDir = MCW
      span = RevSteps - span
    ELSE
      stpDir = MCCW
    ENDIF
  ENDIF

Here we go: If the new angle is greater than the current position, then we get the span by subtracting the old position from the new. Next, we check to see if that span is less than half a revolution. If it is, then we set the stepper direction to clockwise; otherwise, it will rotate counterclockwise.

Notice that we have to make a correction in the span if we determine that the shortest path is counter-clockwise. If we don’t make this correction, the spout will overshoot the desired position. We can use a similar set of logic when the new angle is less than the current position — with adjustments, of course, to make sure that we turn in the direction that provides the shortest path.

Okay, let’s finish up and actually reposition the stepper motor:

stpDelay = MoveTime / span

DO
  stpIdx = stpIdx + stpDir // 4
  READ (Steps + stpIdx), Coils
  PAUSE stpDelay
  span = span - 1
LOOP WHILE (span > 0)

pos = angle

RETURN

The rest of the subroutine starts by calculating the delay between steps. The value is set by dividing the total movement time (500 ms in our program) by the number of steps in our move. Note that we must have some delay; the stepper motor needs it. Also, we’ll want to experiment with this a bit. The largest possible move is half a revolution. Setting the time for that move to 500 milliseconds seems reasonable — fast enough to get there swiftly, but not so fast as to sling coffee out of the spout.

A DO-LOOP is used to send the step data to the

SEPTEMBER 2004

motor. We’re back to our old tricks with the modulus operator; this allows us to set the direction with a variable. The coil data is pulled from a DATA table with READ and placed right on the pins. Of course, we cannot drive a stepper directly with BASIC Stamp IO pins; we need a high-current buffer to handle the coil load. Figure 3 shows how to use our old friend, the ULN2003, to handle the load for us. Finally, we need to update the pos variable with our new position and start the whole process again.

Short and Sweet Rules!

Okay, I know this project wasn’t a big mental challenge, but I do think it’s useful — even if we don’t automate our car’s cup holder. I’m sure you’ll figure out something fun to do with it and, when you do, don’t hesitate to share your results with me.