Overview
Servos are a very handy resource for people involved in robotics. Servos are basically a
small geared motor with a controller. You send an electrical pulse to the servo to tell it
where to turn to. Normally a servo can only turn in a 120 to 180 degree range and is used
for positioning, but a servo can be taken apart and modified so that it turns the full
360 degrees, esentially creating a geared motor. This article is about how to position the
servo using pulses, not how to hack the servos.
What good it it?
Servos are normally used by hobbiests to position rudders, flaps, etc.. on hobby
airplanes. What can we use them for? Well, as I describe in my
GP2D02 article,
servos can be used to sweep sensors arrays back and forth. They can also be used
to create legs on a robot. There are many other uses. They come in all different sizes,
from less than an inch in length to several inches long. The larger ones are very
strong and can be used to move very large loads.
Alright, how do we use it?
A servo has three wires. A signal line, power line and ground line. Different
manufacturers have a different orders and colours for the wires, but they are
fairly easy to identify which is which. Red and black wires are the power wires, and
the third is the signal line. I have servos with either orange or white signal lines.
You can test a servo by connecting +5V and GND to the appropriate wires, then just
tapping the control wire on +5V quickly. This simulates the control pulse, and the
servo will move left or right.
The control pulse?
A servo is controlled by a series of pulses with certain high and low times.
The high time controls the position of the servo. A typical servo accepts a 1 ms
to 2 ms high pulse in order to position itself over a 110 degree sweep. The low
time can vary from 10 ms to 20 ms, and plays no role in the position of the servo.
The control signals are linear, so that a pulse of 1.5ms will position the servo
very close to centre. Some brands of servos will accept pulses lower than 1ms
amd higher than 2ms. This lets you position the servo over a greater angle. I've
had servos which can sweep 180 degree with servo pulses ranging from 0.7ms to
2.4ms. Just how far you can sweep a servo is determined by experimental means.
Just be sure to gradually push the servo rather than feeding it extreme values to
see how far it can go. You can physically damage the servo gears by trying to
move it too far in one direction or the other. You'll know when you are going too
far by the sounds that the servo makes. <grin>
Below is a sample servo pulse.
Servo Pulse Width
A typical servo that I use is a Cirrus CS-70 Standard Pro Servo. It is rated 0.15 sec/60
degrees at 4.8 volts and 0.12 sec/60 degrees at 6.0 volts. It is powered by 5
volts. In order to position it accurately, I actually need to give it 6 pulses,
each around 12 ms long. Usually 2 pulses are enough to get the servo in the general
position that I want it to be, and the 4 additonal pulses are just fine tuning.
If accuracy is not important to your application, you can cut back on the number
of pulses that you send. If you are sweeping the servo over a great distance, you
need to send it more pulses.
Something to note is that the servo is only powered when you are actually sending
it a pulse. When it is not receiving pulses, the motor will not hold the servo
arm in place. The gearing helps to hold it in place, but if you are creating a
walker which needs the servo to physically hold it off the floor, you need to
continuously send pulses to the servo. If you are just sweeping a sensor back and
forth, you only need to send the pulses while you are moving the servo.
So, how do I generate that pulse?
Below, I present some C code for the CCS compiler for the PIC processors. The
code is fairly readable, even though I use some functions specific to the CCS
compiler. You should be able to easily move this code to another compiler or
language. Basically I loop 6 times. Inside the loop I output a high on pin B0 of
the PIC, delay for 1.5ms, output a low on pin B0, and delay for 10ms. It's really
that easy!
Sample Servo Code
#define SERVO_CONTROL pin_B0
// Move the servo to the centre
// send 6 pulses to make sure it gets there accurately
for (counter2=0;counter2<6;counter2++) {
output_high(SERVO_CONTROL); // high pulse
delay_us(1500); // 1500us = 1.5ms
output_low(SERVO_CONTROL); // low pulse
delay_ms(10);
}
You don't need a microprocessor to control the servo. You can easily build
a 555 timer circuit or other circuits which will do the same thing as above.
All you need is something that can output pulses.
Conclusion
Well, hopefully by now you understand how to use a servo. They are really
easy to control, and very useful for many things. Now, if only I could justify
to my wife buying 12 of them to build a 6 legged walker. <grin>
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