L292(1993) View Datasheet(PDF) - STMicroelectronics
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L292 Datasheet PDF : 12 Pages
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L292
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.
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
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.
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
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)
=
dVAB
dt
•
|
FTA
FTA
|
−
dVAA
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
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.
3/12
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