“Discrete or digital sensors are ubiquitous in automation systems. In the age of relay logic, even before the advent of programmable logic controllers (PLCs), digital sensors were used, and they still play a role in simplifying PLC logic. Discrete sensors send on/off (yes/no) signals, often allowing PLCs to ignore analog thresholds, deadbands, detection speeds, and other complexities.
Discrete or digital sensors are ubiquitous in automation systems. In the age of relay logic, even before the advent of programmable logic controllers (PLCs), digital sensors were used, and they still play a role in simplifying PLC logic. Discrete sensors send on/off (yes/no) signals, often allowing PLCs to ignore analog thresholds, deadbands, detection speeds, and other complexities.
These signals may mean “part detected”, “machine air pressure above 80PSI”, “actuator has reached predetermined position”, “heater at a certain temperature” or other conditions. The realization of powerful device capabilities is highly dependent on using the right sensors in the right way. For each of the above conditions, different types of sensors may be used.
Common Discrete Sensor Types
Below are various common discrete sensors used in automation.
Limit switches, still widely used today, are configured with a mechanical switch that opens or closes when in contact with a part. They come in a variety of shapes and sizes and can provide options such as redundant contacts. Despite the simplicity and practicality of mechanical limit switches, many applications have begun to move to non-contact solid-state sensors because of their greater flexibility and longer life. The limit switch also has its limitations because it requires contact with the part being sensed.
Reed switches, mainly used in the pneumatic field, are equipped with a mechanical switch that is opened/closed by a magnet. They are usually mounted on cylinders, which have magnets inside the cylinder pistons. Note that measuring cylinder position is not always the best practice. For example, what if the piston comes out of the connection when the cylinder drives the linkage shaft to push the component into a predetermined position? What if the connection overflows or kicks back? A better way is to measure the plate that touches the part, rather than the position of the cylinder. Since these are mechanical devices, they have similar longevity issues as limit switches. A cylinder switch in the form of a solid state relay can therefore be used instead.
Figure 1: Shielded and unshielded proximity switches. Note the additional yellow plastic on the unshielded type.Image credit: Breen Equipment Automation Services
Proximity switches are another common sensor, usually using an inductive principle, which requires the use of metal (preferably ferrous) in order to function properly. Non-ferrous metals such as aluminum and copper can also be used, but these metals cannot be detected as directly as iron. In this case, the sensing distance is relatively close, and a larger sensing target (sometimes too large to use) is required. In this case, there are two ways to improve detection:
1. Install the steel screw on the non-ferrous metal target so that the proximity switch can detect it.
2. Use “large range” or “unshielded” proximity switches. Because the tip of both types of proximity sensors has less metal covering, the sensitivity is higher.
Certain other types of proximity sensors do exist that utilize non-inductive principles (capacitive and ultrasonic) to measure non-metallic parts. However, such sensors are not common, so when talking about proximity switches, they are usually considered to be of the inductive type.
Photoelectric sensors, which have a light “emitter” and “receiver”. They can be installed integrally or separately. This is usually a low-cost method of tracking system components. Sometimes the light is guided through an optical fiber, or used directly in the transmitter/receiver. Detecting components, either by reflecting light back to the receiver (reflection applications) or by blocking the beam from reaching the receiver (through-beam applications).
Choose the right sensor type
There are several options when sourcing discrete sensors. During the selection process, once a generic sensor to match the type of mechanical interface has been identified, there are other factors to consider:
●PNP and NPN: This is a necessary choice for all solid-state devices. It represents the direction of current flow. In the United States, PNP type sensors are generally used, but if the device is from another country, it is necessary to know the type of PLC input. If the PLC manual says “sinking input”, it is of type PNP; if it says “sourcing input”, then it is of type NPN. Some input modules can be configured as either. In this case, first confirm the power terminal connected to the “common” side. If the common terminal is 0 Vdc, it is a PNP type; if it is 24Vdc, it is an NPN type.
2-wire vs. 3-wire: This is primarily a choice between mechanical contacts (2-wire) and solid-state contacts (3-wire).
● Quick Disconnect vs. Integrated Cable: Many sensors offer two options, permanently attached cable or quick disconnect. Although slightly more expensive, the “quick disconnect” type is generally easier to maintain. If the sensor is disconnected, no new cable is required.
At one point, discrete sensors were truly digital in nature, like mechanical pressure switches using spring-loaded diaphragms, or mercury-based thermostats, but the lines are blurring. Modern discrete sensors, often used to measure parameters such as pressure, temperature, inductance, and brightness in analog, and convert them to a digital “yes” or “no” using a small calculator. It is worth noting that many simple sensors can now transmit analog information back to the PLC through technologies such as IO-Link. If the data is already generated, and a computer is configured, why not take advantage of it? This is the latest trend and there are not many applications in the market yet. PLC and ladder logic programming languages are based on the concept of discrete signals.
Application Skills of Photoelectric Sensors
Choice of visible and infrared light. Usually, the user must concentrate the light, which is relatively easy to achieve if visible light is used, so unless there is a reason to use infrared, try to use visible light.
crosstalk problem. Light from these sensors can interfere with other sensors and light curtains. Consider that light does not remain a line, but a cone, and may affect other devices using the same wavelength (as shown in Figure 2).
Figure 2: Crosstalk issues with photosensors.
There are several ways to help eliminate crosstalk from photosensors:
Alternating light direction: when the photoelectric sensors are close to each other. For example, when two photosensors measure a 6-inch part on a conveyor belt, place the first transmitter on the left and the second transmitter on the right.
Use different wavelengths: For example, light curtains often use infrared light. If using a photoelectric sensor near the light curtain, use a visible light photoelectric sensor.
Configure the aperture on the emitter to narrow the light cone.
Bright “on” vs. dark “on”: sensor on when it detects light, or on when it sees no light? Adjustments can usually be made with a screwdriver or a button.
Accuracy: Simple photoelectric sensor applications cannot provide accurate position detection. For example, sensor boxes on conveyor belts with reflective photoelectric sensors are only accurate to an inch or two. Using optical fibers or apertures to keep the emission and reception areas as small as possible can effectively improve accuracy.
Lasers: Although more expensive, can perform measurement functions that other sensors cannot, such as distance measurement, rather than just detection of obstructions/reflections. They can also detect transparent parts such as plastic or glass.