Page 427 - Mechatronics with Experiments

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ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 413
9. Identify the sensors, and the logic by which they are used in the control decisions:
electrical sensors are interfaced to an electronic control module. Mechanical sensing
mechanisms and hydraulic sensing lines are directly used in a hydro-mechanical
control logic in the circuit. For instance, the load sensing control of a pump can be
implemented either as a hydro-mechanical control or a digital control.
10. Determine any hydro-mechanically built-in priority and over-ride type logic in the
circuit. Then, study the control logic software used to digitally control the hydraulic
system.
The control of an EH motion system may involve the control of the valves, the
pump and the motor. Furthermore, the control logic may be based on “open loop” or
“closed loop” concepts. If a human operator is in the loop, a fixed displacement pump with
variable input speed (i.e., variable engine speed) controlled by an operator pedal, and a
valve actuated proportional to the operator lever input (open loop control) is sufficient. The
operator would adjust the pump input speed with pedal, and hence controls the hydraulic
supply power. At the same time, the operator controls the valve shifting with a lever based
on his/her observations, and hence modulates the delivery to the actuator. In automated
systems, closed loop control of the valve and pump may be necessary. The design of a
electro-hydraulic motion control system involves the following steps,
1. Specifications: the first step in any design is to specify the requirements that the
system must meet, that is performance, operating modes, fail-safe operation.
2. System concept design: given the specifications, an appropriate system design concept
must be developed, that is it is an open circuit hydraulic or closed circuit hydraulic
system, open loop or closed loop controlled.
3. Component sizing and selection: the proper component types and sizes must be
selected. Component sizing requires power calculations to make sure the size of
components is properly matched and that they have the capacity in terms of power,
pressure, and flow rate to meet the performance requirements.
4. Control algorithm design: in computer controlled EH systems, there is always a
controller hardware and real-time control software involved in control of the system.
5. Modeling and simulation: if necessary, the EH system hardware and control algorithm
may be modeled and simulated off-line on a computer to predict the performance of
the overall system.
6. Hardware test: finally, a prototype system must be built and the operation of the EH
system, under the control of the designed control algorithm and hardware, must be
tested. If the desired performance objectives are not met, the design process is iterated
for refinements.
Measured variables in an EH system for control purposes may include the following:
1. load position and speed measured at the actuator (cylinder) and/or load end,
2. load pressure measured as pressure differential at the actuator ports or at the outlet
ports (A & B) of the valve,
3. load force or torque,
4. spool displacement,
5. flow rate,
6. pump output pressure,
7. pump displacement,
8. pump speed.