In virtually every Industrial machine application, you will find a sensor (or many) providing feedback to the machine control system. But do you know how to choose a suitable sensor type for your needs?

Sensors come in many different forms and technologies. Choosing the appropriate sensor for your application can make the difference between a seamless machine operation and a problematic one.

Sensor Types

Sensor types consist of both contact and non-contact devices. Contact-type sensors include limit switches, float switches, pressure switches/transmitters, and Encoders. Non-contact type sensors include photoelectric, inductive proximity, capacitive proximity, ultrasonic technologies, and RFID (Radio Frequency Identification).

Contact Types

Limit Switches

Limit switches use mechanical levers or plungers to detect targets by coming into contact with objects to send signals to the control system. One benefits of mechanical limit switches is that they will sense targets of any shape, size or color. The drawback is that they function slowly compared to other technologies. Also, they are limited to approximately one operation per second. Because they physically come into contact with targets, wear and tear in high-frequency applications may cause sensor lifespans to be shorter than those of other sensor types.

Float and Pressure Switches

Float switches use a very simple principle to provide information regarding the liquid level in a tank, vat, or sump. With these switches, an attached float on a rod pivots the switch. As fluid levels rise and fall, the float moves up and down on the rod. This causes the electrical contacts in the switch to open or close at specific points.

These electrical signals indicate fluid levels in the machine control system. This information determines whether to add or remove liquid from the monitored vessel.

Pressure switches, in turn, monitor pipe or tank pressure by using switches that either open and close at specific pressure points. Or, they use variable (analog) output signals that provide actual measurement readings to the machine control system.

Encoders

Encoders sense either the rotation of a shaft or movement along a specific axis. They provide electrical feedback by sending pulses or waveforms containing target speed and position. These devices use optical or magnetic circuits to determine detailed information about a given process. Encoders are motor mounted or placed directly on the machine you wish to monitor.

Non-Contact Type

Photoelectric

Photoelectric sensors use light transmitters and light receivers to sense when targets are within a specific range. These types of sensors detect objects that “break the beam” by passing between the transmitter and receiver. This results in blocking the path of the light’s travel. Several methods transmit and receive light beams and include through-beam (as in the example previously mentioned), retro-reflective, and retro-reflective with background suppression. The benefits of using photoelectric sensors include the inherent ability to sense objects at long distances (60 meters or greater), fast response times, and speedy recovery times that allow for counting and identification applications that are incredibly fast-paced. Drawbacks to photoelectric sensors include that dirty environments, highly reflective or transparent surfaces, and adjacent objects may interfere with their operation. As a result, take sufficient care when selecting a suitable model sensor.

Proximity

Inductive proximity sensors generate magnetic fields to indicate changes to the field when metallic objects enter the sensing range. These sensors are ideal for sensing metallic objects near (up to 60 mm) the sensor face. These sensors operate with swift response times and have excellent resistance to industrial environments. Drawbacks to inductive sensors include that they are affected by nearby metallic machine structures. Therefore, consider these limitations.

Ultrasonic

Ultrasonic sensors send a series of sound pulses to detect return signals of the sound bouncing off of the target object. The benefits of ultrasonic sensors include that they can sense the presence of irregularly shaped objects more consistently than other types of sensors, regardless of color, transparency, or other surface factors. Additionally, they can measure levels or distance from target applications that consist of monitoring the level of products in bins or tanks. These sensors, however, are sensitive to high airflow, humidity, or rapid temperature changes and may have difficulty detecting sound-absorbing materials such as sponges and fabrics.

Capacitive

Capacitive sensors measure the energy it takes to build a charge on the surfaces of targets. Based on a dielectric constant and correction factor, the information collected affects a resultant change in the state of the contacts of the switch. Engineers use capacitive sensors for applications such as detecting whether or not boxes or bottles are filled in automated filling sequences. These types of sensors work with very specific materials. As such, any change in target material may affect the operation characteristics of the particular sensor being used.

Knowing which sensors to use for your applications is critical to your operational processes. Contact a Motion Automation Intelligence Specialist today.