31st Jan to 1st Feb 2008

TECHNOLOGY ROBOTICS SOCIETY

Indian Institute of Technology, Kharagpur

SERVO MOTORS Has a shaft which can

provide a high value of torque.

Angular Position on the shaft can be maintained by signals on the input line. This provides precision.

Has a high torque/weight ratio.

SERVO MOTOR vs STEPPER MOTOR

GENERAL CHARACTERISTICS Servo Motors have

Amplifiers, Gear Reduction, motor and the feedback pot in a small Case.

They can be direction within a range of -90 to +90 degrees.

They have three wires-Vcc, Ground and control input.

CONTROL OF A SERVO MOTOR The angle is determined by

the duration of a pulse that is applied to the control wire.

A 1.5 millisecond pulse, for example, will make the motor turn to the 90 degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, then the motor will turn the shaft to closer to 0 degress.

EXAMPLE:ROTATION BY 30degrees 1.5 ms = 0 deg. 1.75 ms = 90 deg Linear Relationship 1.59 ms = 30 deg If there is an external force trying to

move it away, the servo will resist it.

CONTROL CIRCUIT

CIRCUIT : SCRUTINIZED

The output signal at pin 3 is inverse.Hence it is inverted by a transistor in Common collector configuration in saturated mode.

NE 555 is the timer IC. The circuit is astable. That is it generates

repeated pulses at regular time intervals. The length of each pulse may be calibrated by

using the pot P1 and an oscillator.

POWER SUPPLY Standard Power Supply > 4.5 volts. May operate on lower voltages but the

response is sluggish. 7805 IC may also be used. Current requirements depend on Torque

and the specific Servo motor.

THE FLIP SIDE

The flip side to Servo motors is their cost, motors being expensive.

Complicated Driver Circuits.