Pulse width control of radio controlled servo motors

Most servos have 3 leads, negative power supply, positive power supply and a control signal. The control signal is a pulse of 5 volts with a constant Frame rate F and a pulse width P.

Typical frame rates are 20mS, typically values of P vary from 1 to 2 mS.

Futaba S148 RC Servo-motor

Mechanical Characteristics

Dimensions	19.5mm x 40mm x 40mm
Weight 		40 grams (1.5 oz)
Torque		0.3 Nm (42 oz/in)
Transit Speed	.22 seconds for 60 degrees
Travel		90 degrees

Connections
  red		Positive power supply 4.8-6 volts
  black		Negative power supply
  white		Control signal

Control signal

  P = 2.5 mS
  Min Ph= 300 mS
  Max Ph = 2.2 mS

Device is linear between P max and P min. Thus target angle q (in degrees) given by P= 0.01 q +0.3 mS

Tower hobbies TS-51

Mechanical Characteristics

Dimensions	1.6"x.8"x1.4"
Weight 		40 grams (1.5 oz)
Torque		0.27 Nm (38 oz/in)
Transit Speed	.21 seconds for 60 degrees
Travel		90 degrees

Connections

Control signal

  Probably the same as the Futaba

References

The computer applications journal Oct 1994 No 51, pg 16

William Harwin