AD2S83

–15–

REV. E

Ripple Content

Ripple content is due to several factors. Tachogenerators suffer

from ripple due to the speed of rotation, commutator segments

and the number of poles. The resolver/RDC combination has a

predominant ripple at twice the resolver reference as a result of

the synchronous demodulator and at a frequency twice per

revolution due to the resolver windings mismatch.

Motor torque pulsations which are a consequence of excessive

velocity ripple have a detrimental effect upon the quality of

speed control in servo systems.

The resultant “cogging” effect will be particularly noticeable at

low speed and when the motor is in the low torque region.

Other undesirable side effects such as the increase in acoustic

noise from a motor and a temperature rise in the motor stator

windings are possible results of the presence of torque ripple.

For more detailed information of the causes and sources of

errors see the Velocity Errors section.

AD2S83 COMPARISON WITH DC TACHOGENERATOR

Comparative tests of the AD2S83 and a dc tachogenerator were

carried out. The tachogenerator was connected at the nondrive

end of the motor shaft with the resolver located behind the drive

shaft of the motor. The AD2S83 was located remotely. The

AD2S83 was set up with a 200 Hz bandwidth, reference fre-

quency of 2.6 kHz and resolution of 14 bits.

The comparative analysis can be summarized:

AD2S83

DC Tacho Conditions

Linearity %

0.1

0.1

0–3600 rpm

Reversion Error % FSO

0.3

0.25

Note the typical operating range of dc tachogenerator is

0 rpm-3600 rpm. The resolver/AD2S83 combination will oper-

ate up to speeds in excess of 10000 rpm.

Ripple Effects

The comparative analysis of the output ripple from the tacho-

generator and the AD2S83 is illustrated below.

Minimization of the AD2S83 output ripple is discussed in detail

in the Velocity Errors section.

Other Factors

Other factors concerning choice of feedback source have to be

addressed. On average the MTBF of a tachogenerator is 347

days as opposed to typically 8 years for a resolver. Resolvers are

relatively insensitive to temperature whereas a tachogenerator

will be specified up to a maximum of 100

°C with a ±0.1%/°C

(above 25

°C) degradation in output voltage. The brushless

resolver requires no preventative maintenance; the brushes on a

tachogenerator, however, will require periodic checking.

ACCELERATION ERROR

A tracking converter employing a Type 2 servo loop does not

suffer any velocity lag, however, there is an additional error due

to acceleration. This additional error can be defined using the

K

Input Acceleration

Error in Output Angle

The numerator and denominator must have consistent angular

may be specified in degrees/sec

to acceleration. The maximum acceleration allowable before the

converter loses track is dependent on the angular accuracy

requirements of the system.

Angular Accuracy

given input acceleration. For example for an acceleration of

100 revs/sec

Error in LSBs

=

Input acceleration [LSB/sec

2

]

K

–2

]

=

100 [rev /sec

2

]

× 2

12

2.7

× 10

6

= 0.15 LSBs or 47.5 seconds of arc

used to define the dynamics of the converter the following

should be used.

K

4.04

× 10

11

2

Where n = resolution of the converter.

R4, R6 in ohms

C5, C4 in farads.