Proximity Switches

Introduction to Sensors

Sensors have the task of measuring information and passing this on to the signal processing part of the system in a form that can easily be processed. In an electro-pneumatic controller, sensors are primarily used for the following purposes:

• To detect the advanced and retracted end position of the piston rod in cylinder drives.
• To detect the presence and position of work piece.
• To measure and monitor pressure.

The following sensors are widely used in the industry:

• Limit switches
• Proximity switches
• Reed switch
• Inductive proximity switch
• Capacitive proximity switch
• Optical proximity switch
• Pressure switches

Proximity Switches
In contrast to the limit switches, proximity switches operated contactlessly (non-contact switching) and without an external mechanical actuating force.
As a result, proximity switches have a long service life and high switching reliability.
The following types of proximity switches are widely used in industry:

• Reed switches
• Inductive proximity switches
• Capacitive proximity switches
• Optical proximity switches.

Proximity reed switches (magnetic switches).
Proximity reed switches, Fig. 5.1, are magnetically actuated proximity switches. They are fitted to the cylinder body.
They consist off two contact reeds Fig.5.1-a in a glass tube filled with inert gas. The field of a magnet causes the two reeds to close, allowing current to flow.
Reed switches have a long service life and a very short switching time. They are maintenance-free, but must not be used in environments subject to strong magnetic fields.

• Internal design

b) ISO symbol

b) Proximity reed switch

c) Reed switch fitted to cylinder.

Fig. 5.1: Proximity reed switch

Proximity optical sensors
Proximity optical sensors use optical and electronic means for object detection. Red or infra red light is used.
Semiconductor light emitting diodes (LEDS) are particularly reliable sources of red or infra red light. They are small and rugged have a long service life and can be simply modulated. Photodiodes or phototransistors are used as receivers. Red light has the advantage that the light can be seen during the adjustment of the optical axes of the proximity switch.
Three different types of optical proximity switches are differentiated:

• One-way light barrier.
• Receiver light barrier.
• Diffuse reflective optical sensor.

Optical proximity sensors

• One-way light barrier.

The one-way light barrier has spatially separate transmitter and receiver units. The parts are mounted in such a way that the transmitter beam is directed at the receiver. The output is switched if the beam is interrupted, see Fig 5.2-a.

• Reflective light barrier.

In the reflective light barrier the transmitter and receiver are mounted together in one housing. The reflector is mounted in such a way that the light beam transmitted by the transmitter is completely reflected to the receiver. The output is switched if the beam is interrupted, see Fig. 5.2-b.

• Diffuse reflective optical sensor.

In the diffuse reflective optical sensor, the transmitter and receiver are mounted together in one unit. If the light hits the reflective object. It is redirected to the receiver and causes the output of the sensor to switch, see Fig.5.2-c.
Because the function principle, the diffuse reflective optical sensor can only be used if the material or machine part to be detected is highly reflective (for example polished metal surface, bright paint.

a) One -way light barrier

b) Reflective light barrier

c) Diffuse reflective optical sensor

d) Optical sensor and its ISO symbol

Fig.5.2: Optical proximity sensors