Servomotor Control Program

In our first program , wewill simply sweep the servomotor from CCW to CW and then sweep back.The program will be kept simple as to demonstrate the priniciples ofcontrolling a servo with a the PIC Basic language. The schematic can beseen in figure 2 (below).


Thevariable pw controls the pulsewidth, and is started at 100 (extremeleft, -45 degrees). The program sends the pulse out to the servo, andthen is increased by a value of 1 until it reaches 200 (extreme right,45 degrees), at which point it will reverese the rotation.

Listing 1

' First servomotor program
' Sweeps left to right, then reverses
Symbol B1 = pw
pw = 100
sweep: pulsout 0,pw
pause 18
pw = pw + 1
if pw > 200 then back
goto sweep
back: pulsout 0,pw
pause 18
pw = pw - 1
if pw < 100 then sweep
goto back

' create a variable pw
' start at extreme left
' send pulse to motor
' set frequency to about 50 Hz
' increase pw by 1
' at extreme right, turn CCW
' otherwise, continue
' send pulse to motor
' set frequency to about 50 Hz
' decrease pw by 1
' at extreme left, turn CW
' otherwise, continue

End of Listing 1

Ifdesired, we could extend the rotation of the servomotor to a full 180degrees (-90 to 90 degrees) rotation by decreasing the minimumpulsewidth to below 1 ms and increasing the maximum pulsewidth to over2 ms. This can be accomplished with our previous program by modifyingthe occurances of 100 and 200 to your desired minimum and maximumpulsewidths, respectivly.
However, a note of caution: thepulsewidth required for servos varies from brand to brand. One motormay require a 2.8 ms pulsewidth for maximum rotation, while another mayonly need 2.4 ms.
Furthermore, servomotors have end stopsthat limit its rotation. If you send the motor a pulsewidth that isbeyond the end stops, the motor will keep trying to turn. A motor inthis stalled condition not only draws more current, but also puts wearon the internal gears, shortening the lifespan of the motor.

Manual Servo Control

Our next program will allow youto control the direction of the servo manually via a SPDT switch (witha center-off position) connected to ports B1 and B2. Without acenter-off position, you will have to use two switches. A schematic isshown in figure 3 (below).


Withthe switch in the center position, the servo will not turn. When it ismoved into forward, it will turn one way. Moving the switch down willmake it turn the opposite direction. This program as-is will limitrotation to 45 degrees off-center, but can be modified to extendrotation through the aforementioned methods.

Listing 2

' Manual control of servo direction via
' an SPDT switch.
Symbol B1=pw
pw = 150
check: if pin1 = 0 then left
if pin2 = 0 then right
Pulsout 0,pw
pause 18
goto check
left: pw = pw + 1
pulsout 0,pw
pause 18
if pw > 200 then max
goto check
right: pw = pw - 1
pulsout 0,pw
pause 18
if pw < 100 then min
goto check
max: pw = 200
goto check
min: pw = 100
goto check

' create a variable pw
' begin at center position
' is pin 1 active?
' is pin 2 active?
' send current pw
' set frequency to about 50 Hz
' check again
' increase pulsewidth
' send current pw
' set frequency to about 50 Hz
' dont go over 2 ms
' go back and check again
' decrease pulsewidth
' send current pw
' set frequency to about 50 Hz
' dont go under 1 ms
' check again
' cap pw at 2 ms
' check again
' cap at 1 ms
' check again

End of Listing 2

Multiple Servomotors

Using a modified version of thelast program, we can control as many servomotors as we have I/O lineson port B. In the next listing, we will control two servos in the samemanner as we controlled a single servo in the previous program. Thecircuit is shown in figure 4 (below).


Theprogram uses two pulsewidth variables, pw1 and pw2; and two sets ofroutines, left1 and left2, right1 and right2; one for each motor. Asyou can see in the schematic, the first servo is wired as per theprevious circuit. The second servo is now using B3 as it's pulse out,and B4 and B5 for the SPDT switch.

Listing 3

'Manual control of two servomotors using 2 SPDT switches
'Use B1 to hold pulsewidth variable for servo 1
'Use B2 to hold pulsewidth variable for servo 2
'Initialize Variables

B1 = 150
B2 = 150

'start servo 1 at center position
'start servo 2 at center position

start:
   IF pin1 = 0 Then left1
   IF pin2 = 0 Then right1
   IF pin4 = 0 Then left2
   IF pin5 = 0 Then right2
   PulsOut 0, B1
   PulsOut 3, B2
   Pause 18
   GoTo start

'check for switch closures
'is sw1 left active?
'is sw1 right active?
'is sw2 left active?
'is sw2 right active?
'send current servo 1 position out
'send current servo 2 position out

'Routines for Servomotor 1
left1:
   B1 = B1 + 1
   PulsOut 0, B1
   PulsOut 3, B2
   Pause 18
   IF B1 > 225 Then max1
   GoTo start
right1:
   B1 = B1 - 1
   PulsOut 0, B1
   PulsOut 3, B2
   Pause 18
   IF B1 < 75 Then min1
GoTo start
max1:
   B1 = 225
   GoTo start
min1:
   B1 = 75
   GoTo start

'increase the pulse width
'send current B1
'send current B2
'set frequency update about 50 hz
'maximum 2.25 millisecond


'decrease the pulse width
'send current B1
'send current B2
'set frequency update about 50 hz
'minimum .75 millisecond


'cap max B1 at 2.25 milliseconds


'cap min B1 at .75 millisecond

'Routines for Servomotor 2
left2:
   B2 = B2 + 1
   PulsOut 0, B1
   PulsOut 3, B2
   Pause 18
   IF B2 > 225 Then max2
   GoTo start
right2:
   B2 = B2 - 1
   PulsOut 0, B1
   PulsOut 3, B2
   Pause 18
   IF B2 < 75 Then min2
   GoTo start
max2:
   B2 = 225
   GoTo start
min2:
   B2 = 75
   GoTo start

'increase the pulse width
'send current B1
'send current B2
'set frequency update about 50 hz
'maximum 2.25 millisecond


'decrease the pulse width
'send current B1
'send current B2
'set frequency update about 50 hz
'minimum .75 millisecond


'cap max B2 at 2.25 milliseconds


'cap min B2 at .75 millisecond

End of Listing 3

Catalog Page for PIC Microcontrollers
(a kit containing all necessary parts is available)
Catalog Page for Servo Motors