Page 609 - Mechatronics with Experiments

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JWST499-c07
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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 595
Mode 2: closed loop force control where the force command is obtained from a programmed

command generator. The cylinder force is measured using a pair of pressure sensors on both
ends of the cylinder.
A real-time control algorithm, programmed in the embedded computer, decides when to operate
the axis motion in speed control or force control mode (Figure 7.84).
(a) Draw a block diagram of the components and their interconnection in the circuit.
(b) Select proper components in order to meet the following specifications:
1. Maximum speed of the cylinder under no load conditions is V = 1.0m∕s.
nl
2. Provide an effective output force of F = 1000 Nt at the cylinder rod while moving at the
r
speed of V = 0.75 m∕s.
r
3. The desired regulation accuracy of the speed control loop is 0.1% of maximum speed.
4. The desired regulation accuracy of the force control loop is 1.0% of maximum force.
The components to be selected and sized include an electronic control unit (ECU) with
necessary analog-digital signal interfaces, position and force sensor on the cylinder for closed loop
control (Figure 7.85), pump, valve, and cylinder. Assume that the valve rating is specified for ΔP =
v
2
7 MPa (1 Pa = 1Nt∕m ) pressure drop, and that the input shaft speed at the pump is w = 1200 rpm.
in
8. For the previous problem, write a pseudo-code for the real-time control software in order to
implement the following logic for the operation of this hydraulic motion control system.
(a) First, make a list of all of the inputs and outputs from the controller point of view and give
symbolic names to them.
(b) Then write the pseudo-code.
The objective is that the actuator is to approach the load at a predefined speed until a certain
amount of pressure differential is sensed between P1 and P2 signals. The predefined speed is a
trapezoidal speed, the cylinder is to start from stationary position and accelerate to a top speed, and
run for a while at the top speed while monitoring the pressure sensors. The position range of the
actuator around which that is expected is known. When a certain pressure differential is sensed, the
control system is supposed to automatically switch to force (pressure) regulation mode; maintain
the desired pressure until and move a defined distance at a lower speed. Then reverse the motion, and
return to the original position under another programmed speed profile.
9. Consider the hydraulic circuit shown in Figure 7.120. a) Describe the operation of the circuit. b)
What is the role of the emergency back up steering block and how does it work? c) What is the role
of joystick and wheel steering and how do they work?
10. A Case Study: One Degree of Freedom Hydraulic Motion Control System with an Accumulator.
Consider the hydraulic system shown in Figure 7.121, which has a fixed displacement pump,
a closed-center valve, a cylinder, a relief valve, and an accumulator. Notice the accumulator volume
and flow rate are included in Equations 7.778 and 7.782 below, describing the pressure variation on
the pump-valve line where the accumulator is located.
The dynamic model of the EH system is defined by the following equations in its general form,
when the dynamics of cylinder-load inertia and the fluid compressibility are taken into account. Since
the main valve is closed center type, A (x ) = 0.0, hence Q (t) = 0 always. As a result, we have
PT
s
PT
one less unknown and one less equation compared to the general treatment of this problem in Section
7.8. However, the same number of equations are displayed below to make the comparison easier.
(1) m ⋅ ̈ y(t) =− c ⋅ ̇ y(t) + p (t) ⋅ A − p (t) ⋅ A − F (t) (7.774)
A A B B load
If up motion x ≥ 0.0 (7.775)
s
d
(2) (p (t)) = (Q (t) − ̇ y(t) ⋅ A ) (7.776)
PA
A
A
dt V hose,VA + y ⋅ A A
d
(3) (p (t)) = (−Q (t) + ̇ y(t) ⋅ A ) (7.777)
B
B
BT
dt V hose,VB + (l cyl − y) ⋅ A B
d
(4) (p (t)) = (Q (t) − (Q (t) + Q (t) + Q (t) + Q (t))) (7.778)
dt P V hose,pv + V acc P PA PT r acc