Delta MMC120 - Manual (Page 70)

70
Curvilinear valves - The flow through these valves increases slowly as the input signal
increases for the first 20% of range. Beyond 20% the flow increases rapidly as the input
increases. This is equivalent to having two different gains for different signal levels. The
low gain at low flow causes poor response at slow speeds, and the high gain at high speed
can cause instability. These problems are more pronounced when heavy loads are moved
by relatively small cylinders.
Slow-Response Valves
Valves with slow response cause the Motion Control Module (MCM) to overcompensate
for disturbances in the motion of the system. Since the system does not respond
immediately to the control signal, the MCM continues to increase the drive signal. By the
time the system begins to respond to the error, the control signal has become too large
and the system overshoots. The MCM then attempts to control in the opposite direction,
but again it overshoots. These valves can cause the system to oscillate around the set
point as the MCM overshoots first in one direction, then the other.
Hoses
Long hoses between the valves and cylinder act as accumulators and make the system
respond as if it has a spring in it (imagine trying to control the position of one end of a
Slinky™ by moving the other end!). The lines between the valves and cylinders must be
as short and rigid as possible.
Pumps and Accumulators