TechArchive

Original article date: October 1997

Mannesmann Rexroth’s Pneumatic Trainer manual covers all aspects of pneumatic circuit design including this outline on pneumatic sensors.

For contactless sensing of positions and for measuring purposes flow elements functioning on the “dynamic” principle can be used (fluidlogic). In these elements (sensors) the pressure conditions in the air flow are utilised. Depending on the way they work they are divided into:

• Reflection principle
• Barrier principle
• Dynamic pressure principle

As the graphical symbols for these devices are not standardised individual manufacturers and users employ different symbols which should be standardised within a company. In practical applications it should be taken into account that low pressure is used to attain a defined flow. These low-level control signals must therefore be amplified.

Reflection eye (screen nozzle)

Reflection eyes are flow elements where the interaction between an annular spreading flow (scren) and the reflux pressure in the centrally positioned measurement channel is utilised. They are used for contactless transmission of signals and are also suitable for measurement purposes.

The annular air flow causes a negative pressure in the centrally positioned receiver channel if the airflow is unobstructed. When an object approaches the pressure becomes positive and reaches the supply value when the nozzle is completely obstructed.

Factors influencing the output signal are:

• Supply pressure
• Construction

The switching point of the system and hence the sensing distance depends on the response point of the secondary amplifier. Normal sensing distances lie between 2 and 6mm depending on the design. Special designs for distances of up to 15mm are available using devices of corresponding size.

Air barrier

Air barriers are flow elements which sense changes between the two nozzles. They consist of:

• Transmitter nozzle
• Receiver nozzle

In the simplest case the receiver nozzle has the form of a normal nozzle. To reduce the sensitivity to external influences the receiver nozzle can be pressurised.

The principal application areas are for contactless transmission of signals over shorter sensing distances. The disadvantage of the air barrier principle is the sensitivity to external influences. The advantage is the simple construction.

With two-piece air barriers and with good coverage and use of pressurised receiver nozzles sensing distances up to 80mm are possible higher in certain cases.

In one-piece air barriers otherwise known as fork-shaped air barriers both transmitter and receiver nozzles are integrated in one part. They can be used with pressurised and unpressurised receiver nozzles. Owing to the small distance between the nozzles the susceptibility to faults is significantly lower than with normal air barriers.

Practical applications include checking of thin parts sensing of information codes and monitoring meters such as pointers and vanes.

Air barriers are pressurised with low pressure (0.1-0.5bar) in order to obtain a usable output signal and to limit the air consumption. The usable output signal (from 0.5mbar) must be amplified. Through the back pressure it will be positive if the gap between the nozzles is not obstructed. It must therefore be inverted.

Since air barriers operate with low supply pressures and deliver low pressure output signals the supply pressure must be reduced and theboutput signals amplified on connection to a pneumatic circuit.

Pilot tubes and dynamic pressure switches

Pilot tubes and dynamic pressure switches are characterised by the airflow from a nozzle having a back pressure on a branching channel. The back pressure is dependent on the distance of an object positioned in front of the nozzle.

When the nozzle is completely covered an output pressure corresponding to the supply pressure is obtained which needs no amplification if the supply is at normal pressure.

In a pilot tube the free flow of the output nozzle causes a pressure reduction in the control channel through suction. By covering the output channel in the case of a completely covered nozzle a back pressure is generated which is equal to the supply pressure.

By choking the air supply and by appropriate channel shaping it is possible to use normal pressure as supply pressure to the pilot tube. In this case an amplification of the output is unnecessary and at the same time the air consumption is reduced.

In contract to the pilot tube in the dynamic pressure switch the back pressure channel is integrated into a three-port two-position direction control valve. In this way the sensing point can be designed as a simple nozzle if it is connected to the dynamic pressure switch via a pipe. The length of the connecting pipe can cause switching delays.

The advantage of the dynamic pressure switch is the possibility of using normal supply pressure and in the case of a covered nozzle of obtaining an output signal of equal value. A low pressure regulator and amplifier are therefore unnecessary.

• Mannesmann Rexroth
• 01480 476041

October 1997