AD2S83 Analog Devices, Inc., AD2S83 Datasheet - Page 15

• Manufacturer Part Number: AD2S83
• Description: Variable Resolution, Resolver-to-digital Converter
• Manufacturer: Analog Devices, Inc.

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:
Linearity %
Reversion Error % FSO 0.3
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.
AD2S83
0.1
DC Tacho Conditions
0.1
0.25
0–3600 rpm
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
acceleration constant K
The numerator and denominator must have consistent angular
units. For example if K
may be specified in degrees/sec
K
to acceleration. The maximum acceleration allowable before the
converter loses track is dependent on the angular accuracy
requirements of the system.
K
given input acceleration. For example for an acceleration of
100 revs/sec
To determine the value of K
used to define the dynamics of the converter the following
should be used.
Where n = resolution of the converter.
A
A
=
does not define maximum input acceleration, only the error due
can be used to predict the output position error for a
100 [rev /sec
Error in LSBs =
2.7 × 10
R4, R6 in ohms
C5, C4 in farads.
2
, K
Angular Accuracy × K
K
A
2
= 2.7 × 10
K
A
] × 2
6
A
=
=
2
A
A
12
n
Error in Output Angle
is in sec
of the converter.
× R6 × R4 × (C4 + C5)
Input acceleration [LSB/sec
Input Acceleration
= 0.15 LSBs or 47.5 seconds of arc
6
A
sec
4.04 × 10
based on the passive components
2
and the error output in degrees.
–2
–2
, then the input acceleration
and 12-bit resolution.
K
A
= Degrees/sec
A
[sec
11
–2
]
AD2S83
2
2
]