GS-C200 datasheet(30/31 Pages) STMICROELECTRONICS

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The mechanical damping is obtained by the intro-

duction of a viscous friction between the motor shaft

and the load. The friction system must be elastic

and it will recover the original relative angular shaft

alignement to assure the correct final positioning.

The response time of the damping system must be

quite fast, and it must be active just for rapid speed

changes otherwise a severe limitation in the maxi-

mum speed will occur.

The electronic damping is obtained by the proper

driving of the motor phases that are switched on

and off in such a way to generated a negative torque

to decelerate and stop the rotor smoothly. Let’s

assume the motor is moving from position 1 to the

detent position 2, i.e. the phase A is switched OFF

and the phase B is switched ON.

The rotor starts moving at t0 instant (see fig. 22),

and after a time t1, the phase driving is reversed

(phase A ON and phase B OFF) generating a

braking torque that will allow the rotor to approach

the final detent position at a very limited speed.

Before the zero speed is reached, (t2) it is neces-

sary to switch back the phase driving to its original

condition in order to stop the system at its target

position.

Leaving the phase driving unchanged will cause the

motor to stop a step earlier of the correct position

because the motor, after the zero speed is reached,

will accelerate in the reverse direction returning to

the starting position.

The deceleration time as well the damping level is

easily adjusted by changing the timing i.e. t1 and t2,

but it can be quite complicate to compensate a

system where large load variation occurs.

Figure 22. Single Step Response with Damping.

In fact, an heavy load variation causes a large

variation of the single step response time of the

system, and it could be that a system compensated

in a no load condition will stop one step behind

when fully loaded, while another compensated at

full load will probably exibits erratic positioning at

no load.

If the load condition isknown it is possible to introduce

a compensation circuit that can be conveniently

driven by one or more User outputs. Fig. 22 shows

the motor response to a single step pulse with elec-

tronic damping and the relative phase driving. This

phase switching reversal method is also known as

the bang-bang damping method, and it can be easily

implemented by using the GS-C module.

The RAMP and MOV signals allow the user to

detect when the last pulses are issued, and to

generate, by a simple logic circuit, the delayed

phase reversal commands necessary to implement

the sequence of fig. 23.

The circuit uses a last pulse detector (G1), and on

the falling edge of the A signal (synchronous to the

last step command), a timing generator is triggered.

The various delays can be trimmed to the values

requested by the operating conditions, and the

pulse sequence reported in figure 23 (A, B and C

signals) in generated.

The Aand B signals are used to reverse the motion

direction (G2) while the C signal steps twice the

motor (backward and forward).

steps

GS-C200 / GS-C200S

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