Page 34 - Keubler Motion Sensors

Basics

Encoders Installing encoders

Loading of encoder shaft bearings using With all spring couplings (shaft coupling, stator cou- This force does not occur with torque stops for hol-
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coupling forces pling, fixing bracket), alignment and axial errors are low shaft encoders, where the encoder is prevented
converted to a force that corresponds to the spring from turning also by means of a pin or rod.
constant of the coupling. Although the encoder is prevented from rotating due
This force has to be absorbed by the encoder shaft to a rigid interlock, the encoder is still free to move in
bearings. When installing an encoder, this should be any other direction. This is of course dependent on it
done with as little force as possible, i.e. without any being mounted in such a way that it has freedom to
unnecessary initial tension on the coupling. If this is move radially and especially axially (thermal linear
adhered to, then with all Kübler couplings adequate expansion of the drive shaft!).
tolerance compensation is guaranteed for the whole
service life of the encoder bearings.

Possible errors in accuracy due to 1. Deviations in accuracy caused by torsion of a spring coupling (in particular shaft couplings)
couplings This deviation in accuracy is defined by the torque The following applies:
to be transmitted (bearing friction and mass moment Max. error max. torque [Ncm]
of inertia) and by the torsional spring constant of the =
(degree) torsional spring constant [Ncm/Grad]
torque stop.
The following table serves to estimate the ratio
between such an error and the smallest increment
of an encoder:

Relationship between the resolution of an encoder in bit and the smallest increment in angular degrees:
binary 10 bit 11 bit 12 bit 13 bit 14 bit 17 bit
Resolution
ppr 1024 2048 4096 8192 16384 131072
degrees 0.352 0.176 0.088 0.044 0.022 0.0028
Increment degrees:min:sec 0:21:06 0:10:33 0:05:16 0:02:38 0:01:19 0:00:10
sec 1266 633 316 158 79 10

2. Deviations in accuracy caused by radial play in the drive shaft with asymmetrical mounting of the couplings
Here one has to differentiate between couplings that With asymmetrical couplings deviations in accuracy
are mounted in an axially symmetrical manner round can arise due to radial movements of the drive shaft
the shaft (all shaft couplings, many stator couplings) (radial runout/play); this is determined by the system.
and asymmetrically mounted couplings (many stator These deviations are dependent on the amount of
couplings, all mounting brackets and pin-based the radial play and the distance of the torque stop
torque stops). locating point from the drive shaft.
The relationship is shown in the following diagram:

Maximum permissible radial runout to achieve an accuracy >1/2 LSB when using an asymmetrical 1 point
torque stop
100
90 10 bit / 1024 ppr

] 80
m 11 bit / 2048 ppr
µ
[ 70

l
e
i
p 60
S
-
/
g 50
a
l 12 bit / 4096 ppr
h Max. permissible radial runout/play [µm]
c 40
s
l
a
i
d 30
a
R 13 bit / 8192 ppr

. 20
l
u
z
. 10 14 bit / 16384 ppr
x
a
M 0
10 20 30 40 50 60 70 80 90 100
Abstand zwischen Drehmomentstützen-Fixpunkt und Antriebsachsen-Mittelpunkt [mm]
Distance between torque stop locating point and mid-point of drive axle [mm]
32 www.kuebler.com © Fritz Kübler GmbH, subject to errors and changes. 02/2015