Page 528 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
514 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
Assume i = i max , and using Equation 7.266, calculate P and add 10% safety margin
s
to it. Then calculate the necessary servo valve flow rating,
√
Q ∕Q = 1 − (ΔP ∕P ) (7.286)
s
L
r
nl
2
2
2
2
ΔP ∕P = 1 − (Q ∕Q ) = (Q − Q )∕Q 2 nl (7.287)
L
s
r
r
nl
nl
Q 2 nl
P = 2 ⋅ ΔP L (7.288)
s
Q − Q 2
nl r
= 11.42 MPa = 1658 psi (7.289)
√
ΔP v
Q = Q nl (7.290)
v
P s
√
= 240 ⋅ 1000∕1658 l∕min (7.291)
= 186 l∕min (7.292)
where we assumed a pressure drop of 1000 psi across the valve for its flow rating. Since
we know the maximum flow rate needed and the input speed to the pump, we can calculate
the pump displacement, D , from Equation 7.259,
p
D = 0.140 l∕rev (7.293)
p
In summary, the following component sizes are required: cylinder with bore diameter d =
c
2
0.05 m (bore cross-sectional area A = 0.002 m ), valve with flow rating of Q = 200 l∕min
c
v
or higher at 1000 psi pressure drop, and pump with displacement D = 0.140 l∕rev and
p
output pressure capacity of P = 2000 psi or higher value.
s
Finally, an amplifier with proper power supply is needed to drive the valve. An
amplifier type which has a current feedback loop is preferred so that the current sent to the
solenoid is maintained proportional to the signal from the ECU regardless of the variations
in the power supply or solenoid resistance as a function of temperature. A commercial
amplifier has tunable parameters to adjust the input voltage offset, gain, and maximum
output current values (i.e., Parker BD98A or EW554 series amplifiers for servo valves
plus a ±15 VDC power supply, such as Model PS15, which provides a DC bus voltage of
±15 VDC up to 1.5 Amp to the amplifier using a regular 85–132 VAC single phase line
power). Similarly, if a variable displacement pump is used and the valve that controls the
pump displacement is EH controlled, we would need an another amplifier-power supply
matched to the size of the pump control valve. Notice that if the EH motion axis was to
be controlled only through the joystick inputs, and no programming was necessary, the
closed loop control algorithm can be implemented with analog OP-AMP circuits in the
amplifier and the need for the ECU can be eliminated. The drive can have a PID circuit
where the command signal from the joystick sensor and the feedback signal from the
cylinder motion connects to the the input of the OP-AMP circuit (Figure 7.86). The error
signal output of the OP-AMP is amplified as a current signal to the solenoids. However, the
software programmability of the motion system is not possible in this case. One such drive
is the Parker Series EZ595 used together with a Series BD98A amplifier, where the EZ595
implements the PID function and interfaces and BD98A implements the voltage to current
amplification function.
List of components selected for this design:
1. Pump and Reservoir: Oil Gear model PVWH-60 axial piston pump, with a pressure
rating of 2000 psi, and flow rate of 60 gpm when driven by an electric motor (i.e., AC
induction motor) at 1800 rpm. The reservoir should handle 150 gallons (3 to 5 times
the rated flow capacity per minute of the pump) of hydraulic fluid, fitted with filters