SYSTEM DESCRIPTION

The L290, L291 and L292 are intended to be used

as a 3-chip microprocessor controlled positioning

system. The device may be used separately - par-

ticularly the L292 motor driver - but since they will

usually be used together, a description of a typical

L290/1/2 system follows.

Figure 1. System Block Diagram

The system operates in two modes to achieve high

speed, high-accurancy positioning.

Speed commands for the system originate in the

microprocessor. It is continuosly updated on the

motor position by means of pulses from the L290

tachometer chip, whitch in tur gets its information

from the optical encoder. From this basic input, the

microprocessor computes a 5-bit control word that

sets the system speed dependent on the distance

to travel.

When the motor is stopped and the microprocessor

orders it to a new positio, the system operates

initially in an open-loop configuration as there is no

feedback from the tachometer generator. A maxi-

mum speed is reached, the tachometer chip output

backs off the processor signal thus reducing accel-

ering torque. The motor continues to run at rop

speed but under closed-loop control.

As the target position is approached, the microproc-

essor lowers the value of the speed-demand word;

this reduces the voltage at the main summing point,

in effect braking the motor. The braking is applied

progressively until the motor is running at minimum

speed.

At the time, the microprocessor orders a switch to

the position mode, (strobe signal at pin 8 of L291)

and within 3 to 4 ms the L292 drives the motor to

a null position, where it is held by electronic "de-

tenting".

The mechanical/electrical interface consists of an

optical encoder which generates two sinusoidal

signals 90

° out of phase (leading according to the

motor direction) and proportional in frequency to the

speed of rotation. The optical encoder also provides

an output at one position on the disk which is used

to set the initial position.

The opto encoder signals, FTA and FTB are filtered

and are supplied to the FTA/FTB inputs on the L290.

The main function on the L290 is to implement the

following expression:

Output signal

dt

•

FTA

| FTA |

−

dt

•

FTB

| FTB |

Thus the mean value of TACHO is proportional to

the rotation speed and its polarity indicates the

direction of rotation.

The above function is performed by amplifying the

3/12

L292