Hydraulic and Pneumatic Control
FLUID POWER GRAPHIC SYMBOLS
The standard icons to graphically represent fluid power components are defined in the Australian standard AS 1101.1-1993 Graphic symbols for general engineering - Hydraulic and pneumatic systems. The following are some of the commonly used components:
The valve designation is as follows:
p/n DCV = Directional Control Valve with n settings and p ports. It can be Normally Open (NO) or Normally Close (CO).
The port numbers on DCVs have the following standard designation:
|1||Pressure port (coming from the pump)|
|3||Exhaust port (back to the tank)|
|2, 4||Output ports|
VARIABLE FLOW VALVES
The great of majority of the valves used in fluid power applications are discrete valves as represented by the above valve symbols. The opening area for these valves are constant and can be either open or closed.
In variable flow valves, the flow area is a function of the spool position. By adjusting the spool position, one can adjust the flow rate.
The flow rate obviously depends on the flow area as well as the pressure difference across the valve. Only the flow area can be controlled by the spool position. Therefore, if precise control of the flow rate is essential, then a control loop has to be implemented that modulates the spool position to keep the flow rate constant. Such valves are called the servovalves.
The choice of the valve is an important consideration in any fluid control application. This choice is a usually compromise between the control requirements and the system cost. In difficult environments (eg mining or construction industry), reliability also is a factor.
Best control performance is usually offered by servo valves due to their good linearity and high bandwidth characteristics. However, these valves have a very low tolerance against contaminants in the hydraulic oil. The standard of cleanliness required by a servovalve may be difficult to achieve and maintain in "dirty" environments. In some instances, capacity may also be a problem. For large machines, it simply is not possible to find a servovalve with the required flow capacity.
Regular directional control valves can be made extremely robust and are available at the desired size and they should be the first option for jobs where control requirements are not too demanding.
Solenoid-driven proportional directional valves (SDPDV) provide a reasonable trade-off. SDPDVs are relatively new additions to control world. They were introduced in the 1980s and they have become very popular since then due to their simplicity and cost effectiveness. Their severe non-linearity (in comparison with the servovalves), however, may offer a significant challenge for the control designer.
EXAMPLES OF FLUID POWER CONTROL
We will now provide some examples of how the fluid power components are connected and controlled to perform simple tasks. The examples all use simple DCVs. The treatment of proportional or servo-valves are beyond the scope of this course.
Control of a single-actuating ram
Example: Draw the hydraulic circuit and the electrical diagram for a hydraulic ram. The piston is to be extended when a manual switch is closed. The piston should return back when the switch is released. Use a single-acting cylinder and a 3/2 valve.
Example: A hydraulic press is controlled by two manual switches placed 1 m apart (switches S1 and S2) and a third switch S3 representing the status of the protective cover. The switch S3 is automatically closed when the protective cover is in place.
The press is to be activated whenever
(a) both S1 and S2 are ON; or
(b) either of the S1 or S2 and S3 are ON.
Once the press is activated, it will stay down until a normally closed lift button switch (S4) is released and breaks the circuit.
Design a hydraulic circuit and the electrical logic to drive this hydraulic press.
There is a deliberate mistake in the PLC diagram. See if you can spot it.
1. The symbol S1 appears twice on the PLC diagram. It refers to signals from the same switch on the hydraulic circuit.
2. ditto for S2.
3. K is the relay that engages the latch switch. The latch switch provides the signal for the solenoid relay X even if S1, S2, and S3 are broken.
4. S4 is the STOP button. When it is broken, K is to deenergise; the latch switch opened; the solenoid relay deenergised and, under the spring action, the valve returns to its normal position, causing the ram to retract. Is this happening in the above diagram? If not, how can you correct it?
|No motion||Extension at normal speed (piston moves to right)||Rapid retraction (piston moving to left)|
Centre position: The piston does not move.
Right position: Retraction. The piston speed is given by
Vpist = Qpump /(Apist - Arod)
Vpist Piston Speed, m/s
Qpump Pump Flow rate, m3/s
Apist Piston area, m2
Arod Rod area, m2
Left Position: Rapid Extension
Apist Vpist = Qpump + (Apist - Arod) Vpist ==> Vpist = Qpump/Arod
A small rod area leads to very rapid extension.
In a hydraulic system, the power is transmitted by pushing the working fluid (usually oil) through the circuit. During this process, some of the power is spent in heating the oil. For example, when oil is pushed through a valve, no external work is done but the pump still has to exert effort to push the oil through the pressure differential over the valve. You can think of this as work done on the oil and all of it is converted into heat:
The work is done on the oil is given by
This is converted into heat and the resultant temperature increase for the oil volume can be calculated by