Top considerations for selecting the right sensor

Whether on board or off board, sensors are an essential component of every intelligent electronics system. They receive and interpret data from the physical world and convert it into a readable output. With their help, automated systems can work with greater efficiency by improving reliability, accuracy and optimising productivity.

Top considerations for selecting the right sensor

Engineers will be glad to hear that, when using sensors effectively, companies, students and hobbyists alike are offered the opportunity to move from a reactive approach to a more proactive model, creating greater value for the end consumer. This shift can be the difference that sets effective users of sensing equipment apart.

The range and variety of sensors available on the market is extensive. As a result, trying to choose an appropriate sensor for a particular purpose can be an overwhelming task. To assist with this, purchasers should be guided by the following:

9 key considerations

1. Sensor type

The choice of sensor will be determined by the composition of the property being detected (metal, solid, liquid, gas etc.) This is the primary factor that will determine if a particular sensor is suitable or not.

Commonly used sensors include;

• Temperature Sensors

Temperature sensors are available in a number of forms including temperature sensors, thermistors, thermocouples and resistive temperature devices. Their use is extensive: computers, mobile phones, automobiles and air conditioning systems all make use of various types of temperature sensors.Shop our range of Temperature Sensors

• Proximity Sensors

A proximity sensor is a non-contact sensor that detects the presence of an object. They can be used for Optical, Ultrasonic and Hall Effect techniques amongst others. Proximity sensors are used in applications such as mobile phones, parking sensors in cars and for ground proximity in aircrafts.

• Infrared (IR) Sensors

There are two types of IR sensors – transmissive and reflective:

Transmissive Type: The IR transmitter and IR detector are positioned facing each other so that the sensor detects the object when it passes between them.

Reflective Type: The transmitter and the detector are positioned adjacent to each other, facing the object so that the sensor detects the object when it is in front of the sensor.

• Pressure Sensors

There are three types of pressures that can be measured: gauge, absolute and differential.

1) Gauge: measures pressure relative to the ambient atmospheric pressure. This sensor would be useful for measuring levels of liquid in a vented tank using the difference in ambient atmospheric pressure and hydrostatic pressure.

2) Absolute: measures pressure relative to a perfect vacuum. Absolute pressure sensors can be used to measure atmospheric pressure to determine altitude. Another use is in tyre pressure monitoring systems.

3) Differential: measures the difference in pressure between two points. These sensors are often used to detect the difference in pressure on either side of an object. A prime example of this would be monitoring airflow in air conditioning applications.

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• Light Sensors

There are several types of light sensors including photoresistors, photodiodes and phototransistors.

Photoresistors are components that are sensitive to light. When light falls upon them, their resistance changes. These are the most commonly used photo sensors, with a prime example of their use being street lights.

Photodiodes are light-sensitive semiconductors that are small, light-weight and inexpensive. They can easily measure picowatts to milliwatts of optical power and are very responsive, boasting short response times in picoseconds. They are often used in applications as varied as sensors for door openings, smoke detectors and blood sugar meters for diabetics.

Phototransistors are essentially a combination of a photodiode and an amplification transistor. They convert light energy into electric energy and produce both current and voltage. Phototransistors are used in security systems, encoders and for lighting control (such as in highways).

• Flow and Level Sensors

Flow sensors measure mass and volume flow rates. These sensors are often used in respiratory devices, oxygen concentrators and air conditioning units amongst others.

A level sensor is a device that determines the amount/level of fluids that flow in an open or closed system. There are two types of level measurements: continuous and point level. Continuous level sensors measure to a specific limit providing accurate results, whilst point level sensors only determine if the liquid level is high or low.

2. Intended application

All application requirements need to be considered prior to choosing a sensor. These could include factors such as:

• Distance from the target- this will provide guidance as to the required sensitivity of the sensor in meeting its target and the required range to ensure that it is fit for purpose.
• Location of the sensor- the environment within which the sensor is placed will determine the appropriate size of sensor and will indicate the extent of mounting options.

Regardless of how effective a sensor may be in isolation, it is essential that the environment the sensor is placed in be considered.

3. Accuracy and precision

Accuracy is often a critical specification that a product needs to meet. In the marketing of sensors, accuracy and precision are regularly used interchangeably but in reality represent two different things. It is imperative that the difference between the two is understood when deciding which specification is a priority.

Accuracy refers to how close the sensor output reading is to the true value, whilst precision relates to the sensor’s ability to detect minor changes. A sensor that is more precise has a narrower distribution, whilst a more accurate sensor is closer to the actual value

4. Durability 

When choosing a sensor it is important to consider factors relating to its durability. Questions to consider include:

• How durable is the sensor?
• Will it last for a long time without needing to be replaced?
• Will it withstand harsh environments?

Every sensor has a finite life that is often determined by its operating life, its storage life or the sensor’s expiration date. Factors that may impact upon a sensor’s longevity include; the design, material, manufacturing process, concentration levels measured and the environmental conditions surrounding the sensor.

For instance, certain sensors that consume materials such as oxygen or glucose are likely to have a very short lifespan whilst other sensors, typically temperature sensors, have a service life in excess of 10 years. It is worth noting though that even sensors with greater longevity may age overtime losing sensitivity and decreasing in accuracy as a result.

Durability is important to ensure that sensors are cost-effective, fit for purpose and to reduce the likelihood of drift. Whilst drift (which occurs when a sensor is exposed to conditions that reduce its accuracy) is inevitable over time, this can be minimised by considering the environment and durability of the sensor.

5. Output types  

There are three key output types:

1) Analog voltage

2) PWM

3) Serial digital

There is often a preference for digital rather than analog signals, as the latter tends to be effected by external noise. This can lead to errors in output signal. Additionally, a digital output may be considered to be more advanced due to its compatibility with computer programs. However, this will not be relevant to all applications as some data can be interpreted without the aid of a computer program. Further, with the aid of an analog to digital converter it is possible to convert an analog sensor’s output to a digital output.

Many control sensors use 4-20 mA current loops, where 4 mA represents low analogue signal and 20 mA represents high. This is often used for data transmission as current loops are insensitive to electrical noise.

6. Response time   

Most sensors have response times measured in milliseconds, although some (often sensors for gases and leaks) can be measured in seconds or minutes. The optimum response time will often be determined by the data being obtained.

7. Repeatability

It is important to consider if the variable that is being sensed can be consistently measured within the same environment. Repeatability refers to the consistency of a sensor against itself, determining whether it will provide the same result under the same circumstances again and again. This value is often associated with accuracy, however, a sensor can be inaccurate and yet able to produce repeat observations.

8. Cost  

Sensors typically operate within the confines of a larger infrastructure network. As such, when considering the potential for development and scalability, the cost of the sensor is an important factor. Whilst sensors are often an essential component of any infrastructure, they must be priced at a level that is proportional to the financial scope of the overall project that they support.

9. Special Requirements

There may be special requirements specific to a particular sensor or use of sensor that need to be taken into consideration. Such requirements may include excessively high temperatures, humidity or close proximity to welding processes which might render an otherwise suitable sensor inappropriate for a particular use.

The role of sensors in the fight against of Covid-19 

Sensors have been essential in the fight against Covid-19 in three key ways; by promoting social distancing, enabling efficient medical checks and by enhancing personal protective equipment (PPE) and wearables.

Sensor technology therefore enables workplaces to make their work environment as safe as possible for all employees by enabling contact tracing and offering medical checks prior to entry. Wearable technology has also been hugely beneficial during the pandemic thanks to the health data collected via sensors embedded within it. These technological advances have accelerated during Covid-19 and are likely to continue to do so into the future.

Sensors of the future  

Developments in the areas of microtechnology and nanotechnology are increasingly shaping the future of sensors, enabling them to be used in more creative and versatile ways. The demand for smaller sensors has risen as a result of their use when embedded into smartphones and wearable devices. A prime example of this is the use of accelerometers in mobile phones to ensure that the display on a phone is always the correct way up regardless of the device’s rotation. Further, when these micro sensors are combined with digital electronics and mechanical components (micro-electro-mechanical systems) they can be used to drive wider IoT applications and so have a broader use in this respect.

Additionally, technologies are being developed to include intelligent systems that operate increasingly like sentient beings. Such technologies are self-monitoring, self-correcting and self-modifying often with the ability to see, feel, smell, hear and communicate thanks to rapid progression in sensor development. Intelligent sensors that work in this way will inevitably be more efficient in fulfilling their role and will reduce the need for human intervention, resulting in a more convenient experience for consumers.

This focus on developing sensors to reduce labour is evident with the development of Radio Frequency Identification (RFID) sensors. RFID sensors have enabled the creation of a paperless bill system within smartphones and have improved security procedures through the use of automated door opening systems. RFID sensors have also had an impact on the global digital packaging market as they enable faster processing, transportation and delivery. This is hugely advantageous to industries that need to deliver goods within a tight timeframe, such as perishable goods within the food and beverage industry.

Another recent development in sensor technology is the increased use of sensor fusion. When a combination of sensors are used simultaneously, the strengths of each sensor compensate for the weaknesses of others. This leads to the creation of more efficient devices that benefit from the combined attributes of multiple sensors. A huge number of applications have derived benefits from sensor fusion, including health monitoring, transportation systems, the entertainment industry and weather forecasting to name a few. This ability to detect multiple qualities, obtain intelligent data and initiate action is revolutionary for a wide range of industries.

With these developments in mind, and in light of the considerations explored in this guide, choosing the best sensor for your required application should now be a less daunting task. The extensive range of sensors available on the market is exciting for the future of technology as the opportunity for greater innovative progress is possible.

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