ABB AV2123000 Positioner User Manual

APPENDIX C - CAM CHARACTERIZATION
INTRODUCTION
This section provides cam shaping information. Cam shaping is
the process of changing the shape of the cam by cutting or forming
so the control signal produces the desired control characteristic.
The standard cam normally does not require shaping.
CAM CHARACTERIZATION
Selecting or shaping these cams allows you to obtain a piston (or
valve) position versus control signal characteristic that will produce
a desired controlled medium versus control signal characteristic.
An example is a desired flow rate of air, water or steam
through a valve for each control signal pressure applied to the
positioner.
The control characteristics for which the cams are shaped are
listed in Table 3-1 and are shown in Figures C-1, C-2 and C-3. The
figures show a family of curves for each cam whose boundaries
are established by a span adjustment.
100
90
80
LIMITED
TRAVEL
70
SCALE = % OF
SIGNAL RANGE
60
50
40
30
20
LIMITED
SIGNAL
10
0
0
10
20 30 40 50 60 70
SOLID LINE INDICATES FORWARD TRAVEL
80
90
100
100
90 80 70 60 50 40 30
DASHED LINE INDICATES REVERSE TRAVEL
20
10
0
SCALE = % TRAVEL
T00791A
Figure C-1. Cam A, Square Root Relationship
INTRODUCTION
C - 1