Discrete or digital sensors are ubiquitous in automation systems. Digital sensors were already in use during the relay logic era, even before the advent of programmable logic controllers ( PLCs ), and continue to play a role in simplifying PLC logic. Discrete sensors send on/off (yes/no) signals, typically allowing PLCs to ignore analog thresholds, dead zones, detection speeds, and other complexities.
These signals could indicate "component detected," "equipment pressure above 80 PSI," "actuator has reached the predetermined position," "heater at a certain temperature," or other operating conditions. The realization of robust equipment functionality relies heavily on using appropriate sensors correctly. Each of the above operating conditions may require the use of different types of sensors.
Common discrete sensor types
Below are some common discrete sensors used in automation.
Limit switch
Limit switches, still widely used today, are configured as mechanical switches that open or close when in contact with a part. They vary in shape and size and offer options such as redundant contacts. While mechanical limit switches are simple and practical, many applications are beginning to switch to non-contact solid-state sensors because they are more flexible and have a longer lifespan. Limit switches also have their limitations because they require contact with the sensed part.
Reed switch
Reed switches, primarily used in pneumatic applications, are mechanical switches that are opened and closed by a magnet. They are typically mounted on cylinders, with the magnet inside the cylinder piston. Note that measuring the cylinder position is not always optimal practice. For example, what if the piston slips out of the connection when the cylinder drives the linkage shaft to push the component into a predetermined position? What if there is overflow or backlash in the connection? A better approach is to measure the plate contacting the component, not the cylinder position. Because these are mechanical devices, they, like limit switches, have similar lifespan limitations. Therefore, cylinder switches in the form of solid-state relays can be used as an alternative.
Figure 1: Shielded and unshielded proximity switches. Note the yellow plastic appendage on the unshielded type. Image source: Breen Equipment Automation Services
proximity switch
Proximity switches are another common type of sensor, typically operating on an inductive principle. This requires the use of a metal (preferably ferrous metals) for proper functioning. Non-ferrous metals, such as aluminum and copper, can also be used, but they cannot be detected directly like iron. In this case, the sensing distance is relatively short, and a larger sensing target is needed (sometimes even too large to be usable). In such situations, there are two methods to improve detection:
1. Install steel screws on non-ferrous metal targets so that proximity switches can detect them.
2. Use "large range" or "unshielded" proximity switches. These two types of proximity sensors have less metal coverage at the top, resulting in higher sensitivity.
There are indeed other types of proximity sensors that utilize non-inductive principles (capacitance and ultrasound) to measure non-metallic parts. However, these sensors are not common, so when people talk about proximity switches, they are usually considered to be of the inductive type.
photoelectric sensor
A photoelectric sensor has a light "transmitter" and a "receiver." They can be integrated or distributed. This is often a low-cost method for tracking system components. Sometimes, the light is guided through an optical fiber or used directly in the transmitter/receiver. The detection component accomplishes this by reflecting the light back to the receiver (reflective applications) or by blocking the beam from reaching the receiver (through-beam applications).
Choose the appropriate sensor type
There are several options when procuring discrete sensors. Once a general-purpose sensor matching the mechanical interface type is determined during the selection process, other factors need to be considered:
●PNP vs. NPN: This is a necessary selection for all solid-state devices. It represents the direction of current. In the US, PNP type sensors are generally used, but if the device is from another country, you must know the type of PLC input. If the PLC manual says "sinking input," it is PNP type; if it says "sourcing input," it is NPN type. Some input modules can be configured as either one. In this case, first check the power terminal connected to the "common" terminal. If the common terminal is 0Vdc, it is PNP type; if it is 24Vdc, it is NPN type.
● 2-wire vs. 3-wire: This mainly refers to the choice between mechanical contacts (2-wire) and solid-state contacts (3-wire).
● Quick-Disconnect vs. Integrated Cable: Many sensors offer two options: a permanent connection cable or a quick-disconnect connection. While slightly more expensive, the quick-disconnect type is generally easier to maintain. No new cable is needed if the sensor disconnects.
At one time, discrete sensors were essentially truly digital, such as mechanical pressure switches using spring-loaded diaphragms or mercury-based thermostats, but the lines are blurring. Modern discrete sensors typically measure analog parameters like pressure, temperature, inductance, and brightness, converting them into digital "yes" or "no" signals using a small calculator. It's worth noting that many simple sensors can now transmit analog information back to a PLC via technologies like IO-Link. If the data is already generated and a computer is configured, why not utilize it? This is a recent trend, though not yet widely adopted in the market. PLCs and ladder logic programming languages are based on the concept of discrete signals.
Application Techniques of Photoelectric Sensors
The choice between visible light and infrared light. Typically, users must focus the light. Visible light is relatively easy to focus, so unless there is a compelling reason to use infrared light, visible light should be used whenever possible.
Crosstalk issue. The light from these sensors can interfere with other sensors and light curtains. Considering that light does not remain a straight line but rather a cone, it may affect other devices using the same wavelength (as shown in Figure 2).
Figure 2: Crosstalk problem of photoelectric sensors.
Several methods can help eliminate crosstalk in photoelectric sensors:
Alternating light direction: when photoelectric sensors are close to each other. For example, when two photoelectric sensors are measuring a 6-inch part on a conveyor belt, the first emitter is positioned on the left and the second emitter on the right.
Use different wavelengths: for example, light-colored light curtains typically use infrared light. If a photoelectric sensor is used near the light curtain, use a visible light photoelectric sensor.
An aperture is configured on the transmitter to reduce the size of the light cone.
Light on/off: Does the sensor turn on when it detects light, or when it doesn't see light? This can usually be adjusted using a screwdriver or a button.
Accuracy: Simple photoelectric sensor applications cannot provide precise position detection. For example, a sensor box on a conveyor belt with a reflective photoelectric sensor can only achieve an accuracy of one or two inches. Using optical fibers or apertures to minimize the area of light emission and reception can effectively improve accuracy.
Lasers: Although more expensive, they can perform measurement functions that other sensors cannot, such as measuring distance, rather than simply detecting obstructions/reflections. They can also detect transparent parts, such as plastics or glass.
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