Datasheet

Assemble and Test your BOE Shield-Bot • Chapter 3
Robotics with the BOE Shield-Bot • 93
Activity 4: Test Speed Control
The transfer curve graph on the next page shows pulse time vs. servo speed. The graph’s
horizontal axis shows the pulse width in microseconds (µs), and the vertical axis shows the
servo’s response in RPM. Clockwise rotation is shown as negative, and counterclockwise is
positive. This particular servo’s graph, which can also be called a transfer curve, ranges from
about -48 to +48 RPM over the range of test pulse widths from 1300 to 1700 µs. A transfer
curve graph of your servos would be similar.
Three Reasons Why the Transfer Curve Graph is Useful
1. You can get a good idea of what to expect from your servo for a certain pulse width.
Follow the vertical line up from 1500 to where the graph crosses it, then follow the
horizontal line over and you’ll see that there is zero rotation for 1500 µs pulses. We
already knew
servoLeft.writeMicroseconds(1500) stops a servo, but try some
other values.
o Compare servo speeds for 1300 and 1350 µs pulses. Does it really make
any difference?
o What speed would you expect from your servos with 1550 µs pulses?
How about 1525 µs pulses?
2. Speed doesn’t change much between 1300 and 1400 µs pulses. So, 1300 µs pulses
for full speed clockwise is overkill; the same applies to 1600 vs. 1700 µs pulses for
counterclockwise rotation. These overkill speed settings are useful because they are
Rotational Velocity vs. Pulse Width for Continuous Rotation Servo
-60
-40
-20
0
20
40
60
1300 1350 1400 1450 1500 1550 1600 1650 1700
Pulse Width, µs
Rotational
Velocity,
RPM
Right Servo