ABB AV2123000 Positioner User Manual

CAM CHARACTERIZATION
lines in between are equal intervals of rotation. These lines are
indexes of percentage of piston (or valve) travel.
The concentric arcs on the cam correspond to actual control signal
pressure values shown in Table C-1 for the particular positioner
being used. Together, the radial lines and concentric arcs
form a polar diagram on which a new cam shape (and characteristic)
can be plotted.
NOTE: Before cutting any cam, make sure that shaping will
involve removal of cam material and not building up of cam
material. For example, if the characteristic plotted lies between
the A and B cams (Figs. C-1 and C-2), cut the A cam away.
1. Use the cam selected in Step 4 under CAM SELECTION. Also
have available a graph of control signal versus actuator travel for
the characteristic desired. (Such a graph may be derived from the
cam characteristic curve developed in Step 3 of CAM SELEC-
TION). Replace the percent values on the vertical axis with the
corresponding actual signal from Table C-1 for the positioner
being used.
2. Refer to Figure C-7 for the method of transferring data from
the graph to the cam. For each radial line on the cam (representing
a percent piston or valve travel), find the value of signal on the
graph that corresponds to that percent piston or valve travel. Proceed
along the radial line to the concentric arc that represents the
signal pressure for the radial lines percentage piston or valve
travel. Mark the cam at that point.
3. Continue in this manner until all the radial lines are marked
with a signal pressure point. For example: In Figure C-7, the graph
calls for a signal of 10.4 milliamps at 60-percent travel position. On
the 60-percent travel radial line of the cam, a mark is placed on
concentric line four, representing that pressure.
4. Draw a line through the points located in Step 2. This will be
the desired cam shape.
NOTE: If a cam shape has too steep a rise, the cam follower
may become locked. A cam with too steep a rise may be
replaced with a cam with a gentler rise if sufficient angularity is
introduced into the drive linkage of the actuator or cylinder.
Optional blank cams are available if alteration of the standard
cam is not desired.
CAM SHAPING
C - 6