Page 142 - Servo Motors and Industrial Control Theory -

8.3 Properties of ER Fluid in Flow Mode 137

14
+ 2 lin/min
4 lit/min
12 # 12 lit/min
16 lit/min
+
10 +
"ER Pressure (bar)" 8 6 + +

4 +

2

0
0 2 4 6 8 10 12 14 16
Current (mamp)
Fig. 8.11 Steady state pressure drops at different supplied current

in the electronics. For fast response requirement applications, the power supply
must be able to generate large voltage with little time delay.
Another important property of ER fluid in flow mode is the steady state pressure
drop across the valve at various current. This is shown in Fig. 8.11.
The above diagram shows that there is approximately a linear relationship be-
tween the current and the pressure drop across the valve at steady state. The flow
rate has little influence on these phenomena. It would be interesting to investigate
the transient behavior when a current is applied across the valve. This is shown in
Fig. 8.12.
It can be seen that there are two distinct phenomena in the transient response.
One is a first order lag type which increases exponentially and the other one is the
response of a second order transfer function. When the transfer functions cascaded
together produces this kind of response.
The above analysis shows that there is a transfer function between the current
and input voltage and there is another transfer function between the pressure drop
and the current. The transfer function between pressure drop and current can be
obtained with assumption that the current response is much faster than the pressure.
This approximation can be made because the current response is very fast and in
Fig. 8.12, it can be assumed that it is pressure response to the applied current.
From Fig. 8.8, the transfer function between current and voltage can be obtained
as
( RCs +1 )
I := v (8.1)
R