1. What is Promixity Sensor?
Proximity sensors or switches are pilot devices that detect the presence of an object (usually called the target) without physical contact. These solid-state electronic devices are completely encapsulated to protect against excessive vibration, liquids, chemicals, and corrosive agents found in the industrial environment.
Proximity sensors are used when:
- The object being detected is too small, lightweight, or soft to operate a mechanical switch.
- Rapid response and high switching rates are required, as in counting or ejection control applications.
- An object has to be sensed through nonmetallic barriers such as glass, plastic, and paper cartons.
- Hostile environments prevent proper operation of mechanical switches and demand improved sealing properties.
- Long life and reliable service are required.
- A fast electronic control system requires a bouncefree input signal.
2. Principle of operation
Proximity sensors operate on different principles, depending on the type of matter being detected. When an application calls for noncontact metallic target sensing,
an inductive-type proximity sensor is used. Inductive proximity sensors are used to detect both ferrous metals (containing iron) and nonferrous metals (such as copper,
aluminum, and brass).
Inductive proximity sensors operate under the electrical principle of inductance, where a fluctuating current induces an electromotive force (emf) in a target object.
The block diagram for an inductive proximity sensor is shown in and its operation can be summarized as follows:
- The oscillator circuit generates a high-frequency electromagnetic field that radiates from the end of the sensor.
- When a metal object enters the field, eddy currents are induced in the surface of the object.
- The eddy currents on the object absorb some of the radiated energy from the sensor, resulting in a loss of energy and change of strength of the oscillator.
- The sensor detection circuit monitors the oscillator strength and triggers a solid-state output at a specific level.
Most sensor applications operate either at 24VDC. The method of connecting a proximity
sensor varies with the type of sensor and its application.
+ Figure below shows a typical three-wire DC sensor connection. The three-wire DC proximity sensor has the positive and negative line leads connected directly to it. When the sensor is actuated, the circuit will connect the signal wire to the positive side of the line if operating normally open. If operating normally closed, the circuit will disconnect the signal wire from the positive side of the line.
+ Figurebelow shows a typical two-wire proximity sensor connection intended to be connected in series with the load. They are manufactured for either AC or DC supply voltages. In the off state, enough current must flow through the circuit to keep the sensor active. This off state current is called leakage current and typically may range
from 1 to 2 mA. When the switch is actuated, it will conduct the normal load circuit current.
3. Capacitive proximity sensors
Capacitive proximity sensors are similar to inductive proximity sensors. The main differences between the two types are that capacitive proximity sensors produce an electrostatic field instead of an electromagnetic field and are actuated by both conductive and nonconductive materials.
Capacitive proximity sensors will sense metal objects as well as nonmetallic materials such as paper, glass, liquids, and cloth. They typically have a short sensing range of about 1 inch, regardless of the type of material being sensed.