In our first program , we will simply sweep the servomotor from CCW to CW and then sweep back. The program will be kept simple as to demonstrate the priniciples of controlling a servo with a the PIC Basic language. The schematic can be seen in figure 2 (below).
The variable pw controls the pulsewidth, and is started at 100 (extreme left, -45 degrees). The program sends the pulse out to the servo, and then 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
If desired, we could extend the rotation of the servomotor to a full 180 degrees (-90 to 90 degrees) rotation by decreasing the minimum pulsewidth to below 1 ms and increasing the maximum pulsewidth to over 2 ms. This can be accomplished with our previous program by modifying the occurances of 100 and 200 to your desired minimum and maximum pulsewidths, respectivly.
However, a note of caution: the pulsewidth required for servos varies from brand to brand. One motor may require a 2.8 ms pulsewidth for maximum rotation, while another may only need 2.4 ms.
Furthermore, servomotors have end stops that limit its rotation. If you send the motor a pulsewidth that is beyond the end stops, the motor will keep trying to turn. A motor in this stalled condition not only draws more current, but also puts wear on the internal gears, shortening the lifespan of the motor.
Manual Servo Control
Our next program will allow you to control the direction of the servo manually via a SPDT switch (with a center-off position) connected to ports B1 and B2. Without a center-off position, you will have to use two switches. A schematic is shown in figure 3 (below).
With the switch in the center position, the servo will not turn. When it is moved into forward, it will turn one way. Moving the switch down will make it turn the opposite direction. This program as-is will limit rotation to 45 degrees off-center, but can be modified to extend rotation through the aforementioned methods.
Read more: First Servomotor Control Program