Agamem Microelectronics Inc.

AA51882

PRELIMINARY

SERVO MOTOR CONTROLLER

©Copyright Agamem Microelectronics Inc.

www.agamem.com.tw

2008/5/5

AGAMEM MICROELECTRONICS INCOPERATION RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE

TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. AGAMEM DOES NOT ASSUME ANY

LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCTS OR CIRCUIT DESRIBED HEREIN; NEITHER

DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS.

5

■ FUNCTIONAL DESCRIPTION

(1) Voltage Regulator

This circuit is composed of a band gap circuit, and outputs a 2.5-volt temperature

compensated reference voltage. This reference voltage is stabilized when the supply voltage

is variable.

(2) Sawtooth wave & One-Shot Generator

Connect a capacitor to CT pin will generate a triangular wave by constant current charging. A

typical value is 0.1uF. The constant current is determined by a resistor connected to RT pin. A

typical capacitor of approximately 0.1uF should be connected in parallel with the resistor to

increase stability.

(3) Servo Position & Over-Time logic

This circuitry consists of a pulse width comparator, which compares the servo position

detection signal (POT pin) from the one-shot timer (CT pin) whose period depends on the

resistance of a potentiometer connected to the servo's drive shaft. This feedback provides the

stability for the control circuitry.

(4) Pulse Stretcher

The difference between the servo control signal and the feedback signal is the error signal.

This error signal is used to toggle the direction of the current flows through the servo. The

function of this pulse stretcher is to “stretch” the small error signal long enough and increases

the duty cycle to the motor for it can maintain sufficient holding force. The circuit also

implements a “dead band” function that prevents servo jitter and hunting. This is a range over

which differences between the input and reference signals will not cause servo operation.

When the signal differences exceed this “dead band” range, the servo will start to shift. The

dead band will change according to the value of capacitor connected to the CDB pin.

(5) Directional Logic Control

The comparator circuit compares pulses from the servo control input (VIN pin) and the voltage

controlled pulse generator. It provides either a positive or a negative output depending on

whether the signal pulse width is larger or smaller than the position generator’s output pulse

(POT pin). Application of this error voltage to the motor driver circuit causes the motor to turn in

a direction that will minimize the error until both pulse widths are the same.

(6) PWM Control

This signal generated by directional logic control block is used to control a flip-flop that toggles

the direction the current flows through the motor. The outputs of the flip-flop drive an external

H-Bridge output driver that handles the high current going through the motor. This output of

this block will be turned on or off with each input pulse based on the status of the directional

logic. The PWM drive techniques provide the benefits of reduced power dissipation, improved

servo motor performance and positively affect system efficiency.