What's a Servo?
alexoprice
Posts: 20
If anyone can point me to a page or something explaining servos i would appreciate it. I don't understand why the third wire is coming out of the servo and why it needs pulses and the motors in my rc cars just use dc (or do they?). Anyway if anybody could point me in the right direction on how to find out about servos (different types too and why not just a dc motor) then i· would appreciate it.
Comments
A servo is a nice little electric motor, with some control electronics and some gears, which runs off 5 volts to 6 volts, in a small package.
Originally intended for "RC Control", an RC airplane (or car) would have an RC reciever, which would recieve signals from a hand-held controller. The RC Reciever would then output repetitive pulses to the Servo to hold throttle positions, position control surfaces, etc.
So, the on-servo electronics work by comparing a pulse-width in the servo (generated by the postion of a variable resistor hooked to the output shaft) to the pulse-width of the control signal. The on-servo electronics then move the motor (and the gearing, AND the resistor) until the two pulse-widths match. The on-servo electronics expect the signal to repeat every 20 to 50 mSec, if you want the Servo to hold the position.
The result is a relatively inexpensive, easily interfaced motor. If you "modify" the servo by disconnecting the internal resistor and letting the output shaft rotate continuously, then you have a reversible motor which is easily interfaced.
Now, the BS2 'stands-in' for the RC Reciever. Thus, it's output signals MUST MATCH those once generated by the RC Reciever. Thus, for a Servo you have three wires -- +5, Gnd, and Signal. And it turns out it is very easy for the BS2 to use the 'PULSOUT' command to generate the Servo control signal.
D.C. Motors are much harder for a single-tasking processor (like the BS2) to control.· Typically it requires an 'H-Bridge', and some Pulse-Width-Modulated (PWM) speed control, for each motor.