L292 datasheet(4/13 Pages) STMICROELECTRONICS

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L292

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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 op-

tical 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 maximum 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 microprocessor 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 pro-

gressively until the motor is running at minimum speed.

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 out-

put at one position on the disk which is used to set the initial position.

The opto encoder signals, FTA and FTB are filtered by the networks R2C2 and R3 C3 (referring to Fig.4)

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 (TACHO) =

Output signal (TACHO) = dVAB · FTA - dVAA · FTBdt | FTA | dt | 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 input signals in A1 and A2 to obtain VAA and VAB (typ.7

Vp). From VAA and VAB the external differen-tiatior RC networks R5 C6 and R4 C4 give the signals VMA and

VMB which are fed to the multipliers.

The second input to each multipler consists of the sign of the first input of the other multiplier before dif-

ferentiation, these are obtained using the comparators Cs1 and Cs2. The multiplier outputs, CSA and CSB,

are summed by A3 to give the final output signal TACHO. The peak-topeak ripple signal of the TACHO

can be found from the following expression:

Vripple p - p =

· Vthaco DC

The max value of TACHO is:

Vtacho max =

· Vthaco DC

Using the coparators C1 and C2 another two signals from VAA and VAB are derived - the logic signals STA

and STB.

This signals are used by the microprocessor to determine the position by counting the pulses. The L2910

internal reference voltage is also derived from VAA and VAB:

Vref= | VAA | + | VAB |

This reference is used by the D/A converter in the L291 to compensate for variations in input levels, tem-

perature changes and ageing.

The "one pulse per rotation" opto encoder output is connected to pin 12 of the L290 (FTF) where it is

squared to give the STF logic output for the microprocessor.

The TACHO signal and Vref are sent to the L291 via filter networks R8 C8 R9 and R6 C7 R7 respectively.

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FTA

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FTB

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