Ace PWM Series Technical File

PWM Technical File
PWM (Pulse Width Modulated) control systems are being used widely in modern liquid
applications. The use of this technology is driven by the need in agriculture for precision
application of fertilizers and chemicals. The goal is to apply what is needed at the correct time
while minimizing input cost, preventing runoff which may contaminate water supplies, and
eliminate drift.
The PWM signal is an efficient technique to
control current to a proportional electrical
hydraulic valve. The PWM signal switches on
and off to achieve the required control current
(see Figure 1). The duty cycle "D" refers to
the "on" portion of the cycle. The duty cycle
can be anywhere from 0 (signal always off) to
1 (signal always on).
Dither is a rapid, small variation in the control
signal designed to keep the valve spool in motion. This movement is intended to avoid
stiction and average out hysteresis.
Stiction keeps the valve spool from moving when control signal changes are small. When
the valve spool finally moves it can overshoot the correct position.
Hysteresis is the tendency for the spool movement to be different if the signal is
increasing or decreasing. This can happen even with the identical control signal.
I-Min or Minimum PWM is the minimum control
current induced into the control valve. This is
typically set to the point where the control signal
creates a response from the valve spool. For
Ace Pumps, this is typically set to the point when
our pump starts to turn or where a minimum
application pressure is achieved. This eliminates
the Deadband which is typical for all control valves
(see Figure 2).
I-Max or Maximum PWM is the maximum control
current supplied to the control valve.
typically set to the point where the control signal
results in maximum performance. For Ace Pumps,
set this to achieve the maximum shut-off pressure
recommended for the pump model. (see p. 3)
P.O. BOX 13187  1650 CHANNEL AVENUE  MEMPHIS, TN 38113
PWM Control Basics and Terminology
Valve Settings & Performance
This is
Figure 1
Deadband

Figure 2
Typical Valve Performance Graph