When I first started my engineering career, a sales engineer from a sensor company came to our factory. He placed a well-packaged sample box on the table in the conference room, inside which were neatly arranged sensors. He said to us, "Let us test your parts."
He understood that engineers needed to test sensors with parts before making a selection. Choosing the right sensor for an application required continuously narrowing down the options until a shortlist of sensors was finalized, at which point samples were ordered. Then, the sensor was further tested with parts, actuators, or devices under conditions similar to the sensor's installation requirements.
When selecting a sampling sensor, ensure the device meets the application's basic operating conditions (refer to the manufacturer's datasheet). The six most important operating conditions include: 1) Temperature range; 2) Specifications; 3) Protection class; 4) Voltage range; 5) Discrete or analog output; 6) Parameter variation, i.e., "Is parameter modification beneficial?" When considering sensors with IO-Link configurations, six other factors to consider are: 1) Response speed; 2) Sensing range; 3) Repeatability; 4) Electrical connections; 5) Mounting type; 6) Visual display: Does the application require a visual display on the sensor?
Next, this article will detail some of the most common sensor types used in manufacturing and provide some application tips and insights.
proximity sensor
Proximity sensors can detect the presence of objects in a nearby area without physical contact. They are discrete output devices. Typically, magnetic proximity sensors detect whether an actuator has reached a specific position by sensing a magnet located within the actuator.
Magnetic proximity sensors can detect the position of actuators. (Image source: Festo)
Buying an actuator from one company and a magnetic proximity sensor from another is generally not a good idea. While sensor manufacturers may claim their sensors are compatible with X, Y, and Z actuators, the reality is that variations in magnets and mounting locations can cause sensing problems. For example, the sensor may not energize or at all when the magnet is not in the correct position. If the actuator manufacturer provides a proximity sensor that matches the actuator, that should be the preferred choice.
Transistor-based proximity sensors have no moving parts and a long lifespan. Reed-based proximity sensors use mechanical contacts, have a shorter lifespan, but are less expensive than transistor-based sensors. Reed sensors are best suited for applications requiring AC power and high-temperature applications.
Position sensor
Position sensors have an analog output, displaying the actuator's position based on the position indicator of a magnet on the actuator. From a control perspective, position sensors offer significant flexibility. Control engineers can define a range of setpoints to match component variations.
Analog inductive sensors can be used to measure position.
Since these position sensors are based on magnets (such as proximity sensors), it is best to purchase sensors and actuators from the same manufacturer (if possible). The position sensor data can be acquired via IO-Link functionality, which also simplifies control and enables parameterization.
Inductive sensor
Inductive proximity sensors use Faraday's law of induction to measure the presence of an object or simulate its position. When selecting an inductive sensor, the most critical factor is determining the type of metal the sensor will detect, thus determining the sensing distance. The sensing range for non-ferrous metals is reduced by more than 50% compared to ferrous metals. The sensor manufacturer's product manual should provide information on the necessary sample selection.
For type, position, and rotation direction detection, intelligent compact vision systems are the ideal choice.
Pressure and vacuum sensors
Ensure the pressure or vacuum sensor can meet the measurement pressure range in both imperial (pounds per square inch) and metric (bar) units. Specify the optimal dimensions for the allocated space. During equipment installation, consider whether the sensor should be equipped with indicator lights or a display screen for easy operator access. If rapid changes to setpoints are required, consider using pressure and vacuum sensors with IO-Link functionality.
Presence control can be achieved through pressure or vacuum sensors.
Flow sensor
Similar to pressure and vacuum sensors, flow sensors can be selected based on their flow range, size, and variability in setpoints. Display options can be specified when ordering the sensor. Flow sensors with relatively low flow rates can be selected for applications in a specific area of the equipment or for the entire equipment.
Optical sensors
The most common optical sensors are photoelectric scattering, reflection, and direct beams. Laser sensors and fiber optic sensing devices also belong to the category of optical sensors.
Photoelectric sensors detect the presence of objects by reflecting or interrupting light beams.
Most photoelectric sensors are based on the principle of detecting objects by reflecting or blocking light. Due to their low cost, versatility, and high reliability, these sensors are among the most widely used sensors in manufacturing.
Diffuse reflection photoelectric sensors do not require reflectors. They are cost-effective sensors used to detect the presence of nearby objects.
Straight-beam photoelectric sensors offer the longest sensing range. These sensors have a transmitter and receiver mounted at two separate points. Garage door security sensors are examples of beam sensors. When the beam is interrupted, it indicates the presence of a target. Slotted photoelectric sensors are an interesting variation of straight-beam sensors; they mount a transmitter and receiver on a single, compact unit. Slotted photoelectric sensors are used to detect the presence and absence of small parts.
Reflective photoelectric sensors consist of a sensor and a reflector, used for mid-range presence sensing. In terms of accuracy and cost, they fall between diffuse reflection and direct beam sensors.
Fiber optic sensing devices are used for presence and distance sensing. The parameters on these multi-functional sensors can be adjusted to detect various colors, backgrounds, and distance ranges.
Laser sensors can be used for long-distance presence sensing, but they are the most accurate for short-distance measurement applications.
Vision sensors can be used for barcode reading, counting, shape verification, and more. They are a cost-effective alternative to camera systems, which are often expensive and complex. Vision sensors are used for barcode reading, tracking individual components, and executing process steps to match those components. Sensors can verify the number of functional elements present on a part. They can determine whether a specified curve or other shape has been achieved. Because these sensors need to handle light, it is crucial to test them under conditions that closely resemble the operating environment in terms of ambient light and background reflectivity. In most applications, it is recommended to house the vision sensor within a housing to isolate it from external light sources. Seeking assistance from vision sensor manufacturers is a good idea for sensor testing. Additionally, remember to ensure the appropriate fieldbus is selected.
The signal converter will convert the analog output signal of the sensor into a switching signal on the signal converter, or alternatively, into IO-Link process data.
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