Page 500 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
486 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
The performance specifications of a proportional directional EH valve includes:
1. Flow rating at a certain standard pressure drop (i.e., 180 l∕min at a standard pressure
drop across the valve, when the valve is fully open. The standard pressure drop for
the flow rating of the servo valves is 1000 psi and of proportional valves is 150 psi).
2. Maximum and minimum operating pressures, p max , p min .
3. Maximum tank line pressure, p t,max .
4. Number of stages (single stage, double stage, triple stage).
5. Control signal type: manual, pilot pressure, or electrical signal controlled.
6. Pilot pressure range (p pl,max , p pl,min ) and pilot flow (Q ) required if pilot pressure is
pl
used.
7. Open center or closed center type spool design.
8. Linearity of current–flow relationship at a constant pressure drop (Figure 7.77).
9. Symmetry of current–flow relationship between plus and minus side of current.
10. Nominal deadband.
11. Nominal hysterisis.
12. Maximum current from the amplifier stage if solenoid operated (and PWM frequency
and dither frequency if used).
13. Bandwidth of the valve at a specified supply pressure (w Hz) or rise time for a step
v
command change in flow at a certain percentage (i.e., 25%) of maximum flow rate.
14. Operating temperature range.

The specifications for the electronic driver (power supply and amplifier circuit) include:

1. Input power supply voltage (i.e., 24 VDC nominal, or ±15 VDC) and current rating
to the drive power supply section.
2. Control signal type and range (i.e., ±10 VDC, ± 10 mA, or 4 to 20 mAmp, analog or
PWM signal),
3. Recommended dither signal frequency and magnitude (i.e., less than 10% of rated
signal at a higher frequency than the valve bandwidth) in order to reduce the effect
of friction between the spool and valve body.
4. Feedback sensor signal type (if any for a driver which uses local valve spool position
feedback to control the spool position).
For flow rates under 20 to 50 gallon∕min (gpm), a single stage direct actuated valve
is generally sufficient. For flow rate ranges between 50–500 gpm, a two stage valve is
used. For flow rates over 500 gpm, typically a three stage valve is used. In a single stage
valve, the electrical current sent to the solenoid (or linear torque motor), which creates
electromagnetic force, directly forces the main spool and moves it (Figure 7.63). Multi
stage valves (two or three stage) use pilot pressure (which is a lower pressure than the main
supply line pressure) to amplify the electrical actuator signal to shift the main spool. This
provides a very large amplification gain between the first stage electrical actuator signal
and second stage pilot force. This gain cannot be matched by any direct drive electric motor
technology currently. The large pilot stage gain generated shifting power also makes the
valve less sensitive to main stage contamination problems since the large shifting pressure
of the pilot stage is very likely to able to force through contamination problems. However,
the contamination problem is more likely to occour at the pilot stage of the valve since the
orifices at the pilot stage are much smaller than those in main stage.