Sensor choices power industrial Internet innovation
The report "Markets for Sensors in the Industrial Internet" published by market analyst Nano Markets in November 2014 included factory automation, building automation, the smart grid and public transport as Industrial Internet applications, and predicted the sector would consume more than $20 billion of sensors per year by 2019.
Tomorrow’s technicians and maintenance staff will rely increasingly on rugged tablets to collect and process information from sensors integrated in industrial machinery, according to the VDC Research 2014 Strategic Insights for Industrial Automation and Sensors. VDC reckons the global market for position sensors alone will exceed $8 billion by 2018.
Industrial sensing applications
Position sensing is possible using technologies such as potentiometric, optical, magnetic, Hall-effect, magneto-resistive and inductive. Various types of inductive sensors are available, including linear and rotary variable differential transformers (LVDT/RVDT) capable of measuring displacement and position.
Inductive sensors can be very small and require no electronic circuitry, making them suitable for applications such as turbine valves, military and aerospace equipment, and industrial process controllers deployed in harsh environments.
Alternatively, LVDTs and RVDTs with built-in pre-calibrated signal conditioning help simplify external circuitry and system integration. Compatible control and instrumentation modules can further simplify design and accelerate time to market.
Photo-microsensors, on the other hand, are ideal for non-contact optical proximity or position detection. Various configurations are available, with or without an actuator, or with moulded mounting points for a custom actuator.
Various other options are available to satisfy differing requirements, such as sensors with raw phototransistor output or built-in amplifier, screw or push-fit mounting, electrical connection via PCB pins or miniature cable connector, and light-ON or dark-ON switching polarity. In addition, equipment designers must also choose the aperture size and sensing distance to meet the needs of the target application.
Turbidity measurement is another interesting application of optical sensing technology, for quantifying small particles of solid matter suspended in liquids. Applications include controlling industrial processes such as brewing, water bottling, pharmaceutical production, purification, or in aquaculture for monitoring marine water quality or studying the habitats of aquatic animals.
Turbidity can be quantified by measuring the depth of liquid needed to obscure a light source viewed through the liquid. In this case, turbidity is expressed in Jackson Turbidity Units (JTUs). The Nephelometric Turbidity Unit (NTU) is a more modern measure based on assessing the scattering of light by small particles in the liquid, using a detector placed alongside the light source.
A turbidity meter combines one or more sensors with data-acquisition and user-interface subsystems, and various types are available such as handheld or bench-top meters for laboratory use, or inline meters for plant and process applications.
The combined optical turbidity sensor and temperature sensing module shown in figure 1 uses a phototransistor and diode to measure the transmission of light through a sample of liquid. This basic sensor is ideal for use in domestic appliances, and can help to reduce energy consumption by detecting when clothes are clean allowing the wash cycle to be shortened.
For temperature sensing, thermistors are known to be robust and reliable, and are available with various temperature ranges suitable for industrial, HVAC, medical, military/aerospace and other applications. The type of housing and attachment are important criteria that can help to simplify equipment design. Pipe-clamp probes for example, can be fitted directly to the outside of a hot water pipe and enable equipment designers to eliminate costly processes such as boring and sealing needed when installing an intrusive probe. These probes are convenient to use, with no need to buy or design a separate fitting.Designers can also choose from screw-on probes or moulded probes for various applications, as well as screw-in probes for immersion monitoring in equipment such as boilers. Alternatively, board-mount temperature sensors in chip packages, or ceramic or glass-encapsulated styles, allow local temperature sensing to be implemented neatly inside the housing of a control module, and can help save connections to external sensors.
Maintenance teams inspecting factory equipment may be looking for evidence of abnormal vibration to detect excessive wear in items such as bearings. Vibration can be measured using piezo-capacitive sensors, which display a change in capacitance proportional to the acceleration applied. The nominal capacitance is typically several hundred picoFarads, and charge sensitivity can range from a few fC/ms-2 (femto coulombs per m/s2) to several tens of fC/ms-2. Often packaged as tiny, chip-size board-mounted devices, piezo-capacitive sensors can be supplied with detection angle of 0°, 25° or 90° relative to the board.
These sensors are also used for freefall protection in computer hard-disk drives. In addition to the freefall sensor two further sensors, positioned diagonally, can be used to generate a differential signal for monitoring motor rotational vibration. They can also be used in supply-chain applications such as recording impacts suffered by equipment or products when in transit.
As an alternative to chip-size sensors, panel-mount vibration sensors can be obtained in IP67-sealed units allowing use in safety-protection systems for sensitive equipment. These can be used for emergency shutdown of critical systems when an event such as an earthquake or other unexpected shock is detected. Panel-mount sensors are also ideal for tilt detection, and can be used in gaming equipment, vending machines or smart meters for tamper or vandalism prevention.
Safety systems in industry are likely to incorporate pressure sensors for various purposes such as pressure-limiting valves or gas leakage detection. Reliable networked pressure sensors are also valuable for gas supply utilities, for use in infrastructure equipment as well as smart meters.
Solid-state pressure switches utilising capacitive sensing are ideal for industrial applications and smart metering, and may also be used in medical devices such as Continuous Positive Air Pressure (CPAP) devices and respirators. Devices using MEMS-based sensing provide an alternative that delivers advantages such as a wide pressure-sensing range and low supply current.
Sensors in building automation
Smart buildings, as another aspect of the Industrial Internet, can help reduce greenhouse gas emissions and utility bills, while at the same time lowering maintenance costs and sustaining a more comfortable environment for occupants. The International Energy Authority estimates that electric lighting in residential and commercial buildings consumes around 20% of the total electricity generated worldwide, and wants to see savings of more than 2.4EJ per year by 2030.
As technical communities work to reduce the energy consumed, new building codes are evolving to promote the use of natural light and so minimise reliance on electric lighting. Automatic dimming of the lighting can help maximise the achievable energy savings by ensuring only the minimum quantity of electric light is used to “top up” the available daylight and maintain consistent illumination. An ambient-light sensor with spectral response similar to that of the human eye is ideal for controlling the electronic dimming, and can be positioned in a convenient unit such as a switch or the light fitting itself.
Turning lights off completely in unoccupied areas can deliver valuable energy savings, but is not a new idea. Notices beside light switches, advising users to switch the lights off whenever possible, are a familiar sight but occupancy sensors can now provide an economical and more effective alternative. A Passive Infrared (PIR) sensor can detect occupancy reliably by sensing body heat, and with a built-in amplifier and digital output can be connected directly to a host microcontroller allowing integration into the smart building network. Low power consumption is desirable in sensors designed for use in battery-powered wireless modules.
Occupancy detection also provides the opportunity to improve the control of heating and ventilation systems. Occupants may not be permitted to adjust HVAC settings manually, although real-time adjustment can deliver significant energy savings. Smart CO2 sensors in wall-mounted or duct-mounted locations can provide signals for adjusting ventilation, based on air-quality measurements from various zones within the building. Smart sensors are available in various form factors, including compact in-duct sensors and attractive wall-mount sensors.
Relative humidity measured at various points in the HVAC system allows further adjustment for optimum comfort and energy efficiency. Humidity sensors operate by measuring impedance change in a sensing element such as a thin-film polymer as water vapour is absorbed.
IoT applications, and particularly the Industrial Internet, are driving demand for smarter sensing throughout factories, buildings, and infrastructures such as the smart grid. The wide variety of sensors now available, using various technologies and in numerous form factors and package styles, can help simplify design-in, minimise integration challenges, and help engineers deliver attractive and easy-to-use smart sensors to market at competitive prices.
About the author:
Angelo Bosoni, Marketing Director Italy, Avnet Abacus – www.avnet-abacus.eu
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