ABB AV2123000 Positioner User Manual

CAM CHARACTERIZATION
3. Using values determined in Steps 1 and 2, plot a curve for the
exact control signal versus piston (or valve) characteristic that will
produce the results of Figure C-6 from a cylinder or actuator that
performs as shown in Figure C-4. In the examples shown, the
required curve is a mirror image of the Step 1 curve taken about
the Step 2 curve (Fig. C-6).
100
90
80
70
60
%OF
SIGNAL
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100
% OF PISTON OR VALVE TRAVEL T00796A
Figure C-6. Cam Characteristics
4. Compare the curve plotted in Step 3 with the cam curves
shown in Figures C-1, C-2 and C-3. Select a standard cam shape
(in this case a C cam) whose characteristic most closely matches
the control signal versus piston (or valve) characteristic plotted in
Step 3.
5. If necessary, adjust the positioner zero and span as outlined
under Zero Adjustment and Span Adjustment in Section 4 to
make the control signal pressure versus piston (or valve) characteristic
conform better to the curve plotted in Step 3.
6. If the required characteristic cannot be obtained using this procedure,
or if more exact characteristics are required, alter the
shape of a standard A, B or C cam or cut a new cam from a blank
cam as outlined under CAM SHAPING.
CAM SHAPING
To assist in the alteration process, a graph is imprinted on each
cam. This graph consists of radial lines and concentric arcs. The
first and last radial lines mark the angles at which zero and 100
percent of cam rotation occurs for control purposes; the radial
CAM SHAPING
C - 5