Page 518 - Mechatronics with Experiments

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The blocked-load condition data plotted in Figure 7.78b is also conveyed in Figure 7.78a.
The data for the port pressure versus current (Figure 7.78b) is same as the data on Fig-
ure 7.78a for flow rate equal to zero cases (Q = 0) for different values of i and ΔP . These
L
are the data points obtained for ΔP and i along the x-axis intersections. The data for the
l
no-load flow versus current on Figure 7.78b is obtained from Figure 7.78a for zero values
of load pressure, ΔP = 0. These are the data points obtained for Q and i along the y-axis
L
intersection.
The current versus the no-load flow, and the current versus the pressure drop across the
valve when the load is blocked are shown in Figure 7.78b. Notice that the K is the no-load
q
case current-to-flow gain (or flow gain), and K is the blocked load case current-to-pressure
p
gain (or pressure gain). The flow-pressure gain (leakage coefficient) is K pq = K ∕K .
p
q
Some of the non-ideal characteristics of a proportional valve are illustrated in Fig-
ure 7.77. They are
deadband due to friction between the spool and sleeve, and leakage,

hysteresis due to the magnetic hysteresis in the electromagnetic circuit of the solenoid

(or torque motor), and
zero-position current bias due to manufacturing tolerances in the spool and feedback

spring.

In fact, the source of deadband and hysteresis in the valve cannot be exactly separated into
friction and magnetic hysteresis. It is their combined effects that create the deadband and
hysteresis in the valve input–output behavior.
Valve control can be based on regulating flow, pressure, or both. The control variable
of an EH valve is the current. The transient response relation between the control signal
(solenoid current command) and flow rate (or spool displacement) for a valve is character-
ized by small signal step response and frequency response (magnitude and phase between
current command for an EH valve) and output signal (flow rate or spool displacement) as
a function of frequency (Figure 7.79). The frequency response of a valve is also a function
of the magnitude of the input signal as well as the supply pressure. The frequency response
is typically measured under no-load conditions. Notice that the small signal step response
of a servo valve is in the order of a few milliseconds and the bandwidth in the order of a
few hundred Hertz. As the valve flow rating gets larger, the bandwidth of a given valve
type gets smaller. The dynamic bandwidth between the electric actuator (solenoid, torque

100
3 0 Mag
Control flow - % rated 60 Amplitude ratio dB –10 180 Phase lag degrees
80
–3
–5
40
45
20 –15 90
–20 Phase
0
0 1 2 3 4 0 10 100
Time - milliseconds
Frequency Hz
FIGURE 7.79: Dynamic characteristics of a servo valve. (a) Step response and (b) frequency
response.