Do You Know Your Sensors?

Do You Know Your Sensors?

In virtually every Industrial machine application you will find a sensor (or many) of some sort providing feedback to the machine control system, but do you know how to choose the right sensor for your needs? Sensors come in many different forms and technologies, and choosing the appropriate sensor for your application can make the difference between a seamless machine operation and a problematic one. If you understand the basics, then selecting the right sensor is fairly straight-forward, so let’s discuss the basics of choosing the right sensor for your applications.

Sensor Types

Sensor types consist of both contact and non-contact devices. Examples of Contact Type sensors would be devices such as Limit Switches, Float Switches, Pressure Switches/Transmitters and Encoders. Non-Contact Type sensors include Photoelectric, Inductive Proximity, Capacitive Proximity and Ultrasonic technologies, as well as RFID (Radio Frequency Identification).

Contact Type

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. The benefits of mechanical limit switches include the fact that they will sense targets with any shape or size, regardless of composition or color. The drawbacks of using mechanical limit switches include the fact that they are slow in comparison to other technologies, 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, a type of limit switch, use a very simple principle to provide information regarding the level of liquid in a tank, vat or sump. With these types of switches, a float is affixed to a rod that pivots in the switch. As fluid levels rise and fall, the float moves up and down on the rod, causing the electrical contacts in the switch to open or close at specific points. These electrical signals send messages to the machine control system to indicate fluid levels. This information is then used to determine 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 certain pressure points, or 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 that provide information concerning target speed and/or position. These devices use optical or magnetic circuits to determine specific information about a given process, and can be 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 certain range. In their most common applications, photoelectric sensors detect objects that “break the beam” by passing between the transmitter and receiver, thus 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 (at 60 meters or greater), fast response times and fast recovery times that allow for counting and identification applications that are extremely fast-paced. Drawbacks to photoelectric sensors include the fact that dirty environments, highly reflective or transparent surfaces and adjacent objects may interfere with their operation. As a result, sufficient care must be taken when selecting the right model sensor.

Proximity
Inductive proximity sensors generate magnetic fields to indicate changes to the field when metallic objects enter sensing range. These types of sensors are ideal for sensing metallic objects in close proximity (up to 60mm) to the sensor face. These sensors operate with extremely fast response times and have excellent resistance to industrial environments. Drawbacks to inductive sensors include the fact that they are affected by nearby metallic machine structures and must be selected with regard to those limitations.

Ultrasonic
Ultrasonic sensors send a series of sound pulses in order to detect return signals of the sound bouncing off of the target object. The benefits of using ultrasonic sensors include that they are able to 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 be used to measure levels or distance from targets applications that consist of monitoring the level of products in bins or tanks. These sensors, however, are sensitive to high airflow, humidity or rapid changes in temperature, and may have difficulty detecting sound-absorbing materials such as sponges and fabrics.

Capacitive
Capacitive sensors measure the amount of 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. Capacitive sensors are used for applications such as detecting whether or not boxes or bottles are filled in automated filling sequences. These types of sensors are designed to 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. Click here to find your nearest Kaman representative and discuss your specific sensor needs.

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Greg Finley
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