Discrete output proximity sensors are most important for industrial applications. They gain importance in such applications where it is necessary to record or count moving objects or work-pieces on machines or conveyors. Based on the connection of the final output amplifier transistor of a discrete proximity sensor to the power supply terminals, proximity sensors can be categorized into two types:
- Sinking (NPN) output sensors
- Sourcing (PNP) output sensors
Sourcing (PNP) Output sensor
The sourcing output sensor has a PNP transistor output with its emitter connected to the +Vcc of the supply for positive switching, as shown in Figure 1. The load is connected between the proximity sensor output and the negative potential. This connection means that the sensor output will be pulled up to the positive potential, and hence the grounded load will be connected to the positive potential through the transistor in the switched state. This connection will allow the current to flow from the positive potential through the sensor to the output (hence sourcing). This sensor is best selected when all electrical devices in the control system use a single source of the supply voltage.
Figure 1 | A sourcing output proximity sensor and its load connection
Discrete Sensor connection to PLC Input
As discussed, the output of a discrete proximity sensor can be sinking or sourcing type. Similarly, a discrete PLC input can be sinking or sourcing type. Remember, it is important to interface a proximity sensor to the associated PLC input correctly for the proper operation of the PLC system. In general, a sourcing proximity sensor should be connected to a sinking PLC input, and a sinking proximity sensor should be connected to a sourcing PLC input to avoid the erratic operation of the system. For the sourcing sensor output, the PLC input circuit is wired with the common terminal connected to the common of the sensor, as shown in Figure 2.
Figure 2 | A sourcing sensor output connected to a sinking PLC input
Electro-pneumatics and Automation
This book explains the functioning of primary solenoid valves and various electrical control components. Many typical single-actuator and multiple-actuator electro-pneumatic circuits are also developed to illustrate various applications of electro-pneumatics.
Fluid Power Educational Series Books