Delta MMC120 - Manual (Page 69)

MMC120 Linear Motion Control Module Hydraulic System Problems
Delta Computer Systems, Inc. 360/254-8688 69
HYDRAULIC SYSTEM PROBLEMS
These hydraulic system problems can make system tuning difficult or impossible.
Nonlinear Valves
A valve is linear when the flow through it is directly proportional to the input signal over the entire range of
the input signal. It is nonlinear when the output is not directly proportional to the input. You may find two
types of valve nonlinearity:
Overlapped valves - Oil does not start to flow through these valves until the spool has moved some distance.
This causes a dead band in the system, where small amounts of drive do not produce motion. Overlapped
valves are designed for manual and on/off type control and are not suited for servo control. These valves
should be replaced with non-overlapped valves.
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.
Linear Overlapped Curvilinear
Flow Flow Flow
Input Signal Input Signal Input Signal
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.