Page 140 - Servo Motors and Industrial Control Theory -

8.3 Properties of ER Fluid in Flow Mode 135

20
18

16
14
12
I/mA 10
8

6
4
2
0
0 0.005 0.01 0.016 0.02 0.025
t/s
Fig. 8.7 A typical current response to a step input voltage

Fig. 8.8 A simple model for I
the electrical properties of
ER fluid

V R
C

The small variation of the flow rate on the resistance is negligible. It is clear that the
variation of current on flow rate is very small and may be ignored. This is useful in
design of power supply for fluid in flow mode.
To design a power supply, the dynamic relationship between the current and volt-
age must be known. This can be found from the step input of voltage and observing
the current. This is shown in Fig. 8.7. It can be seen that as the voltage is applied,
the current rises rapidly to a large value and then it reduces to its steady state values.
The sinusoidal input superimposed to a constant base value of voltage relationship
(not shown here) shows that the current leads the voltage.
This indicates that there is a charging phenomenon and this point can also be
predicted from the step input voltage response. Therefore, an adequate electrical
model would be in the form of a capacitance parallel to a resistance.
This is shown in Fig. 8.8. It was stated that above the capacitance and resis-
tance are variable with respect to the applied voltage frequency. An identification