AD2S83

REV. E

–14–

The small signal step response is shown in Figure 7. The time

t

1

f

BW

t

5

f

BW

×

R

12

where R = resolution, i.e., 10, 12, 14 or 16.

TIME

Figure 7. Small Step Response

The large signal step response (for steps greater than 5 degrees)

applies when the error voltage exceeds the linear range of the

converter.

Typically the converter will take three times longer to reach the

first peak for a 179 degrees step.

In response to a velocity step, the velocity output will exhibit

the same time response characteristics as outlined above for the

position output.

THE AD2S83 AS A SILICON TACHOGENERATOR

Position Control Using the AD2S83

The AD2S83 has been optimized for use as a feedback device

for velocity as well as position. A traditional position control

loop shown below compares a demand position with an actual

to derive a position error and hence a velocity demand.

+

–

POSITION

DEMAND

ACTUAL

POSITION

MOTOR

FEEDBACK

SOURCE

CONTROL

TERMS

POSITION

ELECTRONICS

Figure 8. Position Control

Quality of control may be reduced if the load on a motor varies

dynamically. System reaction and compensation for a sudden

change in the loading depends on how rapidly the system can

update the velocity demand to the motor. This can cause rapid

acceleration of the motor until the loop updates with a new

velocity demand.

The only effective way to compensate for dynamic loading

effects is to introduce a 2nd order term which will provide the

motor with an acceleration or deceleration demand signal (see

Figure 9).

CONTROL

TERMS

POSITION

ELECTRONICS

+

–

POSITION

DEMAND

ACTUAL

POSITION

MOTOR

FEEDBACK

SOURCE

VELOCITY

ELECTRONICS

Figure 9. Position Control and Velocity Control

Traditionally this would need to be implemented by using sepa-

rate position and speed feedback transducers, e.g., an encoder

or resolver and a dc tachogenerator. The AD2S83 can decode

the resolver to provide both velocity and position information.

DC Tachogenerator

The DC tachogenerator is a small permanent magnet dc

generator. The output is a dc voltage which is proportional to

the speed of the rotor and whose polarity is determined by the

direction of rotation. Physically they are similar to a resolver.

Velocity Error Derivation

The velocity error is the difference between the synthesized dc

velocity demand derived from the actual and demand positions

and the feedback from the tachogenerator or the AD2S83. The

velocity demand is usually derived via a DAC so apart from any

quantization noise it is clean. The velocity feedback, therefore,

needs to be as close to a pure dc level as possible. The errors

which determine the quality of the resultant acceleration demand

to the motor are explained below.

Linearity

Linearity is the maximum deviation from the ideal straight line

velocity characteristic. The line used is given by:

v = mx + c

where

v = velocity

m = gain scaling

x = dc voltage

c = zero velocity dc offset

Linearity is generally a function of the input velocity to the

tachogenerator or resolver.

Reversion Error

Reversion or reversal error is an offset which is dependent on

the direction of rotation of the transducer; e.g., if 10 rps =

1.000 V dc, then –10 rps = 1.003 V dc with +0.3% reversion

error and FSO =

±8 V dc.

Zero Velocity DC Offset

This is a residual dc offset present at zero input velocity. This

can be externally nulled.