What Is a Sensor?
A sensor is a device for detecting and signalling a changing condition. And what is this “changing condition”? Often this is simply the presence or absence of an object or material (discrete sensing). It can also be a measurable quantity like a change in distance, size or color (analog sensing). This information, or the sensor’s output, is the basis for the monitoring and control of a manufacturing process.
Contact vs. Non-Contact Technologies
Contact sensors are electromechanical devices that detect change through direct physical contact with the target object. Contact sensors:
- Typically do not require power
- Can handle more current and better tolerate power line disturbances
- Are generally easier to understand and diagnose
Encoders, limit switches, and safety switches are contact sensors. Encoders convert machine motion into signals and data. Limit switches are used when the target object can handle the physical contact. Safety switches incorporate tamper resistant actuation and direct opening action contacts for use as machine guards and emergency stops.
Non-contact sensors are solid-state electronic devices that create an energy field or beam and react to a disturbance in that field. Some characteristics of non-contact sensors:
- No physical contact is required
- No moving parts to jam, wear, or break (therefore less maintenance)
- Generally operate faster
- Greater application flexibility
Photoelectric, inductive, capacitive and ultrasonic sensors are non-contact technologies. Because there is no physical contact, the potential for wear is eliminated, however, there are some rare circumstances where there could be interaction between the sensor and target material. Non-contact sensors can also be susceptible to energy radiated by other devices or processes.
A Practical Example
An example of both contact and non-contact sensor use would be found on a painting line. A contact sensor can be used to count each door as it enters the painting area to determine how many doors have been sent to the area. As the doors are sent to the curing area, a non-contact sensor counts how many have left the painting area and how many have moved on to the curing area. The change to a non-contact sensor is made so there is no contact with, and no possibility of disturbing, the newly painted surface.
Discrete vs. Analog Detection
Discrete sensing answers the question, “Is the target there?” The sensor produces an On/Off (digital) signal as output, based on the presence or absence of the target.
Analog sensing answers the questions, “Where is it?” or “How much is there?” by providing a continuous output response. The output is proportional to the target’s effect on the sensor, either in relation to its position within the sensing range or the relative strength of signal it returns to the sensor.
When specifying sensors, it is important to understand the common terms or “buzz words” associated with the technology. While the exact terms differ from manufacturer to manufacturer, the concepts are globally understood within the industry.
When applying a sensor to an application nominal sensing distance and effective sensing distance must be evaluated.
Nominal Sensing Distance
Nominal sensing distance is the rated operating distance for which a sensor is designed. This rating is achieved using standardized criteria under average conditions.
Effective Sensing Distance
The effective sensing distance is the actual “out of the box” sensing distance achieved in an installed application. This distance is somewhere between the ideal nominal sensing distance and the worst case sensing distance.
Hysteresis or differential travel is the difference between the operate (switch on) and release (switch off) points when the target is moving away from the sensor face. It is expressed as a percentage of the sensing distance. Without sufficient hysteresis a proximity sensor will continuously switch on and off, or “chatter,” when there is excessive vibration applied to the target or sensor. It can also be made adjustable through added circuitry.
Repeatability is the ability of a sensor to detect the same object at the same distance time after time. Expressed as a percentage of the nominal sensing distance, this figure is based on a constant ambient temperature and supply voltage.
Switching frequency is the number of switching operations per second achievable under standardized conditions. In more general terms, it is the relative speed of the sensor.
The response time of a sensor is the amount of time that elapses between the detection of a target and the change of state of the output device (ON to OFF or OFF to ON). It is also the amount of time it takes for the output device to change state once the target is no longer detected by the sensor.
The response time required for a particular application is a function of target size and the velocity at which it passes the sensor.
An industrial control manufacturer has limited or no control over the following factors which are vital to a safe installation:
- Environmental conditions
- System design
- Equipment selection and application
- Operating practices
Presence sensors and switches, like all other electrical equipment, must be installed in accordance with specific National Electrical Codes (NEC). Three primary standards organizations have evolved:
- CENELEC — European Committee for Electrotechnical Standardization
- IEC — International Elecrotechnical Commission
- NEMA — National Electrical Manufacturers Association
Generally, CENELEC specifications are followed on installations in the European market, while installations in North America adhere to the NEMA standards. IEC covers standards on an international scale.
Sensor Selection — A Methodical Approach
Within each system there are many operations or processes: fabrication, assembly, packaging, painting, material handling. Each can be broken down into smaller events like counting, indexing, ejection, spraying, filling, and conveying. A sensor could be of value to detect the changing conditions associated with an action or event.
Determine Where a Sensor May Be Needed
This process involves identifying key operations within the system and defining focus areas where conditions should be verified.
Identify the Functions
Identify what the system does or what you want it to do. Is it necessary for you to count product? Sort? Perform a quality check? Determine part orientation? Specifically:
- What conditions must be met for each function to occur?
- What feedback is required during each function?
- What conditions must be met after each function to verify the function has occurred properly?
Identify the Area of Focus
Focus on the area where an action is taking place. Within this area, you will typically find a work piece and a mechanism that acts upon it. Investigate both to determine what is required for the function to be properly executed.
- Verification of work piece — Are there features or components of the work piece that must be present or in a particular orientation? What is the potential for the work piece itself to be oriented or damaged in a way that could adversely affect the process?
- Verification of mechanism — Is the mechanism or work piece driven by separate systems that could crash if one were present without the other being retracted? Is a particular component prone to breakage or wear?
Determine if a Sensor Should Be Applied
You must now decide how important each of the areas you identified is to the process. The higher the level of automation the more important it is for these functions to execute properly. Specifically, you are asking:
- What is the impact of damage or loss?
- What is the likelihood of it occurring?
- How critical is it to process integrity?
If the answer to any one of these is “high,” you need to consider implementing a sensor to monitor for a condition that, if present, could facilitate a system glitch.
The next step is to define what sensing functions need to be achieved and where the best location is to accomplish them. Are you trying to determine jam-ups in the system, high/low limits, sorting, speed sensing, or part positioning? This determines the location of the sensor and focuses on specific physical limitations. Now is also a good place to consider the following:
- Are there safety or economic considerations?” If failure to detect the condition could result in a person being injured or killed, or if failure could result in a significant monetary loss, you should note the item for special consideration by an expert in these specific applications.
- “Is this the best place to perform the sensing function?” Often, in a sequence of operations, it is the end result that we are concerned with. In many cases, monitoring this end result can provide indication that the preceding actions have occurred properly. In other operations, the environment or space restrictions may prevent us from performing the detection function in the area of focus, but we can perform it more reliably while the work piece is in transit or in a preceding function.
Define the Application
You have identified an application that can benefit from implementing a sensor to detect a changing condition. With this as your focus, you must now determine:
- Available power
- Output/load requirements
- Target characteristics
- Environmental conditions
Identify the Power Sources
What is the available power at the application point — AC or DC?
Based on the voltage commonly available in the field, sensors are generally designed to fall within one of four voltage ranges:
- 10…30V DC
- 20…130V AC
- 90…250V AC
- 20…250V AC/DC
AC sensors and switches can receive power directly from a power line or filtered source, eliminating the need for a separate power supply. AC devices and connection methods are also perceived as being more rugged.
DC sensors require a separate supply to isolate the DC portion of the AC signal. However, with voltages typically less than 30V, DC is considered safer than AC. DC sensors come in current source and current sink versions. Current source sensors supply power to the load which must be referenced to the ground or negative rail of the power supply. Current sink sensors supply ground to the load which must be referenced to a positive voltage that shares the same ground.
A number of manufacturers offer AC/DC devices that operate over a wide range of voltages from either power source. These sensors offer the convenience of being able to stock one device that can operate in a number of applications with different power supplies.
As a matter of general practice, you want to specify that your switches or sensors are powered from a stable source that is free of noise. Typically, this involves specifying an isolated line or separate supply to power the switches and sensors and staying well within the ratings.
Identify the Load Requirements
What will the sensor be affecting? In other words, what device will the sensor control directly and what are its characteristics? The electrical components in series between the sensor output and power or ground constitute what is referred to as the input load of the device and output load for the sensor. This load translates the electrical signals of the sensor output into electrical, mechanical, sound or light energy that initiates a change within the affected device. Key characteristics of the three types of circuit elements that can be found in the load.
- Resistive elements constitute an ideal type of load, dissipating power in direct proportion to the voltage applied.
- Capacitive elements are reactive and can appear to be a short circuit when first switched on.
- Inductive elements like relay coils and solenoids are also reactive elements that can create high voltage transients when switched off abruptly.
Does the sensor need to condition the output in order for it to be useful to the device it is interfacing with? If the event we are detecting is extremely fast, it may be necessary for the sensor or a conditioning circuit to provide a longer output pulse than the duration of the event. In other instances, like when the sensing function and action it initiates occur at two different places in the system, the output signal may need to be shifted by an interval of time.
Determine the Physical Properties of What You Are Detecting
For any sensing function you must identify the item you wish to detect (target); this may be an entire object or a feature of that object. You must also determine the variables associated with the target — presence, position, orientation, etc. — and how these variables affect the process. Finally, we must regard environmental conditions and their effects; insuring that the surroundings do not contain factors that affect the technology is an enormous factor in the reliability of the application.
Properties of the target — size, material, color, opacity, etc. — will dictate the use of a particular technology and define limitations within that technology. For example, inductive sensors will only detect metal targets. However, the size and material of the target affect sensing range and speed. Further target considerations on specific sensing technologies can be found in their respective chapters later in this book.
Identify Environmental Influences
There are characteristics of the target, background and surroundings that influence the ability to differentiate one from the other. Ideally, the changing condition of the target you are trying to detect should be unique from related factors in the background and surroundings. For example, to detect changes in color, we must use light. A sensor that uses light to detect changes (a photoelectric sensor) in the color of our target could have trouble seeing the target if the surrounding was too opaque to transmit the light or if the background reflected more light than the target.
|Size||Proximity to target|
|Reflective properties||Reflective properties||Transmissive properties|
|Accessibility, proximity to sensor, timeframe, amount exposed|
Now that you have documented the application and understand what must be detected, our discussion can be directed toward selecting a sensor. This is a process of determining which technology or technologies best utilize the strongest differentiating traits of the changing condition while being the least affected by background and surrounding conditions. There is rarely a single solution; each technology has strengths and weaknesses that make it a good or poor choice for a given application. It helps to view the overall system and gradually narrow your focus to specific processes. Determine how a sensor could enhance this process and how it relates to the overall system. The information derived through this approach can then be compared to information on available sensor types to determine the best product for the application. Ultimately, the chosen solution provides the best compromise of performance, reliability, availability and cost.