Designing pneumatic and hydraulic circuits

Pneumatic and hydraulic circuits are used to control manufacturing processes so that they can be automated. In a pneumatic or hydraulic control system the following levels can be seen.

ACTUATING DEVICES Outputs		ACTUATORS Pneumatic cylinders Hydraulic cylinders Rotary actuators Indicators
FINAL CONTROL ELEMENT Control signal		CONTROL ELEMENTS Directional control valves
PROCESSING ELEMENT Processor signals		PROCESSORS Directional control valves Logical elements Pressure control valves
INPUT ELEMENT Input signals		SENSORS Directional control valves Limit switches Pushbuttons Proximity sensors
ENERGY SUPPLY Source		ENERGY SUPPLY Compressor Receiver Pressure regulator AIr service equipment

There is no international standards for circuit presentation and layout. The binary movement expression and labelling method is used. 0 is the retracted or normal position and 1 is the extended or actuated position. For movement with more than two possible positions, 0 is the fully retracted, and the integers from 1 numbered sequentially are used to denote the possible positions. Lower case letters are used to refer to the position sensing valve that senses the position and checks full stroke motion. It is also the valve that initiates the pressurisation of the pilot signals on the power valves. Pilot signals on the power valve that drives the actuator are given the capital letter referring to the actuator and the number that relates to the position of the actuator. The position of the actuator is written under the inverted T symbol corresponding to the position sensing valve that sense it. Others (such as FluidSIM) use a two integers to denote the position.

Some helpful principles when drawing circuits are given below:

• Draw all symbols where possible in the horizontal position and attach lines to the right hand square for two position valves or to the centre square for three position valves.
• Do not mirror-invert symbols, since it confuses the comprehension of signal direction, fluid-flow and valve function.
• Draw all valves in their de-actuated, unpressurised rest position, except for valves that are actuated either by a lever, cam or machine part, when the machine is at rest or at the end of the cycle. In such cases, indicate the cam or similar device that activates the valve and all attachment and pressure lines are to be connected to the left hand square.
• Two-position valves that are not spring biased and can freely assume both switching positions may be drawn in whatever flow-path configuration as demanded by the actuator or initial position. Pressure lines should be connected to the right hand square. An exception to this rule is frequently made for memory valves in sequential control circuits that maintain their last selected position to provide the start signal for a new machine cycle.
• Draw all actuators in their machine rest or initial positions.
• Use a bus-bar for distributed position or pressure sensing valves signals to multiple destinations for circuit interlocking. The bus-bar should be regarded as a manifold line with both extremities closed off.
• Separate fringe condition modules such as emergency stop, cycle selection and start restriction from the sequential control circuit.
• Use binary movement expression and labelling method.
• Sequential control circuits with more than one actuator should include the traverse-time diagram, that depicts the sequence and if possible all specifications and sequence equations derived for the design of the control.

The diagram above illustrates a simple problem where clamp A holds a piece of work, drill B drills the hole, and offset C positions the piece for two drill holes. The resulting control diagram can be seen below.

From Rohner, P. and Smith, G. 1990 Pneumatic control for industrial automation. John Wiley & Sons

Notes on drawing principles on the diagram above:

1. The cam valves are lined up in a row at the base of the diagram for ease of identification. In an actual circuit, the cam valves are placed beside the actuator that trigger them.
2. The symbols are all drawn in the same direction to ease identification. Usually the trigger is on the left side and the return mechanism on the right side of the control valve.
3. All valves are in the unactuated position unless they are actuated during the circuit rest position. The valves listed as a₀, b₀ and c₀ are in their activated state as this cam valves are depressed when the actuators are in their retracted state. Note the last control valve indicated by the black arrow is not in its rest position but is activated. This is necessary to ensure that the circuit ends at the last step and restarts with the first step.
4. The bus bar, being the lines numbered 1 to 4, is a closed circuit and a solid circle is used to denote a connection into the bus bar while lines crossing the bus bar do not connect.
5. The start and stop (guard, machine, etc.) valves are all placed on the left side for ease of identification. They are also connected through as they are lined up to ensure safe operation of the circuit.
6. The positions of the actuator A is given as a₀ or a₀ using binary labelling using the smaller letters for the positions and capital letters for the control signal. This is similarly applied to actuators B and C.

There are many ways to design and construct a circuit for the control of automated systems. Long ago circuits were put together more or less intuitively without applying a logical and proper design method, but rather by trial and error. The traverse-time diagrams and step-motion diagrams were used to systematise the search. Two methods will be introduced here, the step-counter and cascade circuit design methods. The step counter method is also ideally suited for programming electronic programmable controllers (PLC).

 

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