Page 248 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

‘Limit delay to 250 ms
if ti > 250 then hold1
‘Check switch status
peek porta,b0
if bit0 = 0 then loop1
‘Still increasing delay?
goto check ‘If not, jump to main switch status
loop2: ‘Decrease delay
poke portb,0 ‘Turn off transistors
ti = ti 5 ‘Decrease delay by 5 ms
pause 50 ‘Pause a moment
if ti < 20 then hold2 ‘Limit delay to 20 ms
peek porta,b0 ‘Check switch status
if bit1 = 0 then loop2 ‘Still decreasing delay?
goto check ‘If not, jump to main switch status
hold1: ‘Limit upper delay
ti = 245 ‘to 250 ms
goto loop1 ‘Go back
hold2: ‘Limit lower delay
ti = 25 ‘to 25 ms
goto loop2 ‘Go back
hold3: ‘Stop stepper motor
poke portb,0 ‘Turn off transistor
peek porta, b0 ‘Check switches
if bit2 = 0 then hold3 ‘Keep motor off? 227
goto check ‘If not, go to main switch status
check
The schematic for this program is shown in Fig. 10.13. In the photo-
graph of the circuit (see Fig. 10.14), the four switches are difficult
to make out. There are the four bare wire strips behind the PIC
microcontroller. The top sides of the bare wire strips are con-
nected to 5V through 10K-ohm resistors. A wire from each
switch is connected to the appropriate pin on port A. A single wire
is connected to ground and is used to close any of the switches by
touching the bare wire strip.
Half stepping
Half stepping the motor will effectively double the resolution. In
this instance it requires 400 pulses to complete one rotation. Table
10.4 shows the switching logic needed in a program. When you
reach the end of the table, the sequence repeats starting back at
the top of the table.
The ti delay variable
The ti variable used in each PICBASIC program controls a delay
pause whose purpose is to slow down the output sequence to port
B. Without the pause, the sequence may run too fast for the stepper
motor to respond, causing the stepper motor to malfunction.
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