BASIC KNOWLEDGE – SENSORS The relative merits of standard sensors and smart sensors

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For decades, sensors have been vital in providing process visibility to the equipment in our factories, offices, homes, and vehicles. Now, the rise of the IoT has driven the development of more highly functional, compact, and efficient smart sensors.

We receive information about the world around us, and react accordingly, through our five senses – hearing, vision, taste, smell, and touch. Control systems, computers, instruments, and other electronic devices can similarly understand their operating environment through using sensors; devices which convert physical variables like temperature and motion into electrical signals compatible with their input circuits.

Sensors are either analogue or digital. Very approximately, we could say that analogue sensors have been in use for decades in all types of industrial, commercial, and domestic equipment, while digital sensors are becoming increasingly important with the rise of the IoT. While this sets the scene for a discussion about the difference between standard and smart sensors, it is important to understand the situation in a little more detail.

An analogue sensor provides an electrical output which continuously changes in proportion to its measured variable. A typical example is a temperature sensor with an industry standard 4 – 20 mA current loop output. If it is calibrated for a 0 – 100 °C temperature range, it will output 4 mA at 0 °C, and 20 mA at 100 °C. Current levels between these limits will vary continuously according to the temperature measured. This analogue technology has been well-proven over decades of use, and is relatively simple and low cost to implement.

By contrast, a digital sensor output can only be OFF or ON. Digital sensors are used in three types of application:

• Switching applications like a central heating system thermostat: The output is either ON if the measured temperature is below the thermostat setpoint, or OFF if the temperature becomes higher than the setpoint.

• Technologies like motor shaft speed sensors, where magnets in a rotating shaft create detectable pulses with frequency proportional to the shaft RPM. The pulses can be counted by a digital device.

• Sensors which convert physical analogue variables such as temperature or pressure into digital values for computer processing or communication. This approach enables the development of smart sensors, which are discussed more below.

Accordingly, we will now look at how standard sensor technology is applied to measuring several common physical variables, and then summarize how such sensors work. Next, we will explore the differences between these sensors and the increasingly important smart sensor types.

1. Types of sensor

Motion sensor

A motion sensor, or motion detector, is a security device that detects movement from persons or objects through a specific area. Motion sensors are an essential part of any security system. When a sensor detects motion, it sends a notification to a security system and can be programmed to transmit an alert to a monitoring service or activate an alarm .

The two most prevalent motion sensor technologies are active ultrasonic sensors and passive infrared sensors, both of which are well-known for their accuracy and dependability.

Active ultrasonic sensors produce ultrasonic sound waves which reflect from nearby objects before returning to the motion sensor. A microphone within the sensor sends the pulse and receives the echo. The distance to the target is proportional to the duration between signal transmission and reception. Most motion sensors have sensitivity adjustment, which means they will not activate if the distance to the object is too great.

Passive infrared sensors (PIRs) detect fluctuations in infrared energy, which people, pets, and objects release as heat. A PIR device contains a pair of pyroelectric sensors. When no one is present, the PIR device detects ambient IR emitted by background objects such as walls and doors. When a new heat source walks past the device, the sensor detects the change and activates an alert, triggers an alarm, or sends a notification to the monitoring provider. PIR sensors, like active ultrasonic sensors, may be configured to ignore minor changes in IR, allowing building occupants to move around without continuously setting off alarms.

Temperature sonsor

Some applications, such as equipment used to create life-saving medications, require temperature sensors to be responsive and accurate for critical quality control; however, other applications, like car thermometers, do not require such accurate or responsive sensors.

The four most common types of temperature sensors, ranging in responsiveness and accuracy from high to low are:

• Negative Temperature Coefficient (NTC) Thermistors: sensors that measure the change in resistance of a ceramic or polymer as temperature changes. The output of an NTC thermistor is non-linear due to its exponential nature; however, it can be linearized based on its application. The effective operating range is -50 °C to 250 °C for glass encapsulated thermistors or 150 °C for standard thermistors.

• Resistance Temperature Detectors (RTDs): sensors that measure the change in resistance of a metal as temperature changes. Platinum RTDs offer a highly accurate linear output across -200 °C to 600 °C but are much more expensive than copper or nickel alternatives.

• Thermocouples measure the voltage difference between two dissimilar metals. Thermocouples are nonlinear and require conversion and compensation using tables when used for temperature control. Accuracy is low, from 0.5 °C to 5 °C but thermocouples operate across the widest temperature range, from -200 °C to 1750 °C.

• Semiconductor-Based Sensors: A semiconductor-based temperature sensor is usually incorporated into integrated circuits (ICs). These sensors utilize two identical diodes with temperature-sensitive voltage vs current characteristics that are used to monitor changes in temperature. They offer a linear response but have the lowest accuracy of the basic sensor types. These temperature sensors also have the slowest responsiveness across the narrowest temperature range (-70 °C to 150 °C).

Infrared sensor

This type of sensor measures the infrared radiation emitted by an object, and thermometers, which measure the temperature of a liquid or gas, are also available.

Current sensor

A current sensor detects and converts current to an easily measurable output voltage, which is proportional to the current through the measured path. There are many sensor types, each suitable for a specific current range and environmental condition. The technology used by the current sensor is important because different sensors can have different characteristics for a variety of applications.

A current sensing resistor is the most commonly used current sensor type. It can be considered as a current-to-voltage converter, where the voltage across a resistor in the current path is directly proportional to the current flowing through it.

Other current sensors are based on either open or closed-loop hall effect technology. A closed-loop sensor has a coil that is actively driven to produce a magnetic field that opposes the field produced by the current being sensed. The hall sensor is used as a null-detecting device; the output signal is proportional to the current being driven into the coil, and accordingly to the current being measured.

In an open loop current sensor, the magnetic flux created by the primary current is concentrated in a magnetic circuit and measured using a hall device. The output from the hall device is signal conditioned to provide an exact (instantaneous) representation of the primary current.

Pressure sensor

Pressure sensors are devices that measure the force exerted by fluid or gas on a surface. They work by using the piezo electric effect in which a material creates an electric charge in response to stress. The stress can be pressure, twisting, bending, or vibrations. The pressure sensor has a sensing element of constant area which is deflected by the pressure. The deflection is measured and converted into an electrical output which indicates the amount of pressure.

There are three common types of pressure sensor measurement :

• Gauge pressure, which is measured in reference to atmospheric pressure which is typically 14.7 PSI.

• Absolute pressure, which is measured against an absolute vacuum.

• Differential pressure, which is the difference between the pressure being measured and a reference pressure.

Other sensor types

Multiple other sensor types exist, to reflect the wide range of technologies used in today’s machines and equipment. Examples include but are not limited to :

• Vision and Imaging Sensors

• Radiation Sensors

• Proximity Sensors

• Position Sensors

• Photoelectric Sensors

• Particle Sensors

• Metal Sensors

• Level Sensors

• Leak Sensors

• Humidity Sensors

• Gas and Chemical Sensors

• Force Sensors

• Flow Sensors

• Flaw Sensors

• Flame Sensors

• Contact Sensors and Non-Contact Sensors

2. How does a sensor work?

Traditional sensors are either passive or active in operation.
Passive sensors require an external power source to operate, called an excitation signal. The signal is modulated by the sensor to produce an output. For example, a thermistor does not generate any electrical signal, but by passing an electric current through it, its resistance can be measured by detecting variations in the current or voltage across it.
Active sensors, by contrast, generate electric current in response to an external stimulus which serves as the output signal without needing an additional energy source. Examples include photodiodes, piezoelectric sensors, photovoltaics, and thermocouples.

The elements of a sensor depend on the type of physical variable it is measuring, and the technology it uses. However, most sensors comprise at least two core units:

• The sensor itself, which outputs a detectable electrical change, such as a variation in resistance or current flow, in response to its measured variable changing

• A signal conditioning stage, which converts the sensor output into an electrical signal which can be transmitted over a distance and/or read by a computer. This signal could be a 4 – 20 mA current loop, or a stream of digital pulses, for example

• The sensor may also have provisions for calibration and offset compensation

3. The differences between traditional sensors and smart sensors

Traditional sensors have been fulfilling vital roles within industrial, military, commercial, and domestic environments, as well as vehicles of every type, for many decades, and will no doubt continue to so for the foreseeable future. However, within the last decade, other factors have come to the fore which are changing the role of sensors as well as their technology – leading to the appearance of smart sensors in particular.

These emerging factors include increasingly powerful mobile phones, the growing penetration of electric vehicles, and, above all, the rise of the Internet of Things (IoT).

In enterprise settings, the IoT can bring the same efficiencies to manufacturing processes and distribution systems that the internet has long delivered to knowledge work. Billions of embedded internet-enabled sensors worldwide provide an incredibly rich set of data that companies can use to improve the safety of their operations, track assets and reduce manual processes.

Data from machines can be used to predict whether equipment will break down, giving manufacturers advance warning to prevent long stretches of downtime. Researchers can also use IoT devices to gather data about customer preferences and behavior, though that can have serious implications for privacy and security.

And the IoT has become enormous. According to Priceonomics , there were more than 50 billion IoT devices in 2020, and those devices generated 4.4 zettabytes of data (a zettabyte is a trillion gigabytes). By comparison, in 2013 IoT devices generated a mere 100 billion gigabytes.

Yet to install sensors in such large numbers, while still keeping their deployment economically, logistically, and even technically viable, they must offer new features not available on traditional sensor types. These include self-diagnostics, simplified wiring (or wireless operation) and communication, more efficient remote monitoring, and low or no power consumption.

These requirements can be met by using a smart sensor, which comprises one or more sensing elements, a microprocessor unit (MPU), and a communications module.

Multiple sensing elements save space, power, and simplify installation as one such sensor unit could replace several single-function types. An example would be ensuring business premises are kept comfortable for personnel. A smart sensor would be a vital part of the facilities systems by being able to measure air quality, temperature and humidity and alter heating and air conditioning as required.

The heart of a smart sensor is its MPU; the MPU’s intelligence can be used to perform signal conditioning and self-diagnosis. Signal processing functions include filtering and noise reduction before transferring data to a central controller, so reducing its load. Load on the central controller, and on the communication system, can be further reduced by using the sensor MPU to preprocess data before transmitting it. For example, instead of continuously transmitting measured temperature data to the central unit, the smart sensor could transmit only when a measured value exceeds preset limits.

In self-diagnosis, the sensor assesses its own reliability. It watches its performance and acts when it detects any performance deviation. Examples include self-calibration and auto-zero functions.

Self-diagnosis helps remove abnormalities occurring in the signal output before they are transferred to the main controller. This contrasts with standard sensors, where these abnormalities are removed manually (for example, manual inspection) or by implementing programming functions in the main controller.

MPUs should be low power types, to consume less energy. This maximizes intervals between smart sensor battery changes.

The communication module manages communication between the smart sensor and external devices. In addition to data, the smart sensor can communicate identification (ID), analyzing an individual sensor’s health and connection status. Another advantage is automatic sensor configuration sent from the main controller.

Sensors are added as a node in an intelligent sensor network. This helps expand the network without additional re-wiring in case of sensor addition.

Communication can be wired or wireless and is governed by communication protocols. There are many protocols for smart sensor communication, depending on the system application and application area. For example, smart sensors often utilize the Fieldbus protocol in industrial applications, or BACnet for building automation.

Wiring is reduced, simplified, and for wireless sensors, eliminated. Costs are reduced, while troubleshooting and locating faulty devices becomes easier.

Technicians can access the system remotely and check the system performance, fault statuses, and history.

Smart sensors also allow programming with the desired parameters from a remote location. This prevents human error and enables device identification and tracking.

EnOcean is an interesting company in this context, because they manufacture sustainable, low maintenance ‘energy harvesting’ sensors that gain their energy from movement, light, and temperature changes. For example, a tiny electro-mechanical converter inside their light switch derives energy from the switch movement to send a brief radio telegram.

4. A couple of sensor use cases

The two examples below show how advanced sensor technology is being used to solve some extremely demanding real-life problems.

Managing safety-critical power supplies: Medical device designers would like to use lithium-ion batteries instead of cables to achieve more flexible products, but current lithium-ion battery technology does not yet meet the required safety standards. Fraunhofer ISIT has addressed this issue and is developing sensor-assisted accumulators that meet the high standards of medical technology.

Their development involves Li-ion rechargeable batteries that can ensure greater safety through integrated reference electrodes, temperature, and pressure sensors. For example, a reference electrode in combination with an intelligent battery management system prevents overcharging and excessive discharging of the battery. Pressure and temperature sensors enable monitoring during operation and can switch off the battery rapidly in the event of a rise in temperature and/or pressure before damage occurs.

SmartMotion™, a high-performance, low-power balancedgyro™ technology in consumer applications: InvenSense's ICM-6xxx SmartMotion™ 45-axis MEMS motion sensor series introduces on-chip self-calibration, the industry's lowest power consumption, and BalancedGyro™ (BG) technology for the first time. BalancedGyro is the first gyroscope MEMS architecture that enables excellent vibration suppression and temperature stability – an enhancement that a consumer gyroscope has never received before. Applications such as robot vacuums and smartphones benefit from this technique as they require negligible gyro drift due to temperature and vibration fluctuations.

5. Which sensors are most common in which industries?

While there are many different types of sensors, we can usefully split the market into six product segments: pressure, temperature, proximity, flow, image, and level.

Pressure sensors are the largest sector; they typically use the piezo electric effect, and are found in many diverse applications, including drilling for oil, and industrial boilers.

Temperature sensors are also widely used, and are essential for many processes including pharmaceuticals and food production.

Proximity sensors are used to measure the distance to an object, typically by sending out a beam of infrared radiation, or an electromagnetic field, and looking at the returned signal. By measuring how distance varies, these sensors are also useful for vibration monitoring.

Flow sensors can be used with liquids or gases, with many different types available.

Image sensors may be implemented as simple light level detectors, or as sophisticated, high resolution cameras. Combined with image recognition programs, and artificial intelligence (AI), camera-based systems can provide sophisticated capabilities, such as recognizing defective products on a production line and taking appropriate action.

There are many different types of level sensor to detect liquid levels, as well as free-flowing solids like powders.

6. Future trends for sensors

Unsurprisingly, sensors are expected to become smarter, more accurate, quicker, wireless, safer, self-learning, smaller, standardized, and with other benefits. Many sensor developments covering all of these points are underway.

However, one particularly interesting trend is ‘sensor fusion’ which involves integrating different sensors into a single compact sensor application. The combination provides more new information to make applications more intelligent and more efficient.

In the future, sensors will be able to work autonomously in the same way we use all of our senses to understand the world around us, and react accordingly. A machine will then become self-learning. So, artificial intelligence with deep learning algorithms is the future.

The power of modern technology lies in using sensors to obtain large quantities of data, analyze it quickly, and then discover relationships between disparate areas of application. Smart devices will also be able to discover more possibilities and models than people can.

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