By Bill Schweber for Mouser Electronics
Significant advances have been made in delivering high-performance, low-cost, easy-to-use sensors for a diverse range of physical phenomena, with progress often based on micro-electromechanical systems (MEMS) technology.
There’s always been a widespread need for electronic sensors that can handle the diverse range of physical phenomena such as light, pressure, sound, and temperature, the most-widely sensed parameter. The good news is that there have been significant advances in delivering high-performance, low-cost, easy-to-use sensors for many of these parameters, with progress often based on micro-electromechanical systems (MEMS) technology. For example, sensing of position and motion–two closely related physical attributes that are traditionally among the most challenging to measure–are now almost trivial, because of the widespread availability of tiny silicon-based accelerometers and gyroscopes that are widely used for navigation in drones and car safety.
Advances in solid-state sensing technologies have led to smaller, low-cost, low-power, efficient MEMS sensor solutions for measuring gas-flow rate, a critical parameter in many medical, industrial, and instrumentation applications.
One sensing area that has long eluded the development of high-performance, low-cost, low-power sensors: vapor and gas (odor) sensing. We are still a long way from the sensitive, universal odor-sniffing nose of a beagle or even a human. Sensors for gases, volatile organic compounds (VOCs), and even humidity have been especially challenging, and they have been harder to use, calibrate, and integrate than temperature sensors. Further, each gas sensor often has a different analog output format: scaling, voltage vs. current, timing, and other variations (Figure 1).
The problem is aggravated when a need arises to measure multiple different variables at a single location. Simply and successfully interfacing with the diverse variety of available sensing solutions required either limiting sensor choices, adding adapter circuits to yield a single output format, or interfacing with multiple output formats.
In the past decade, though, significant progress has been made, as MEMS and associated semiconductors are radically changing the situation and providing low-power, low-cost sensors for specific gases. These advances did not begin with gas sensing; instead, they began with application-specific impact pressure sensors for air-bag triggering. That research, development, and commercialization opened the way to progress in MEMS sensors for pressure, VOCs, and CO2 (carbon dioxide). The result has been widespread availability of microchip-based devices for basic sensing of gases and liquids, plus calibration, additional integrated features, and more embedded functions.
Trend to single-point, multi-factor sensing
As sensors advanced in terms of their basic performance, ease of interconnect, cost, and power requirements, there has been a corresponding trend to sensing multiple parameters at each location. This is a departure from previous architectures in which just sensed temperature (and perhaps humidity) is at one location and VOCs and CO2 is at another, for example. With the newer sensors, it is feasible and actually makes sense to integrate a full set of sensors at each site, with single-path (wired or wireless) connectivity.
Doing so serves diverse applications that need sensors data across multiple parameters. These include lighting control, building automation, security and motion/presence sensing, smart homes, connected home, air-quality monitoring, and energy management. Further, by combining the full roster of desired sensors into a single device, system designers, their projects, and even the end-users benefit from reduced time to market, fewer design “surprises,” and reduced installation costs, as designers no longer have to deal with individual sensors and interfaces (unless they need or want to do so). This enables low-cost, convenient placement of multifunction environmental-sensor nodes in industrial, commercial, institutional, and residential settings.
Leveraging this new multisensory opportunity, vendors have developed small boards and modules that offer a selection of commonly needed sensing functions as complete, highly integrated, calibrated, and easy-to-use solutions for these applications.
The examples below demonstrate how the change in integrated sensing of multiple physical variables, including gas-based ones, is greatly simplifying the challenge and also opening up new opportunities:
Example 1: TE Connectivity (TE) AmbiMate Sensor Module MS4 Series
This unit from TE Connectivity targets fixed-in-place installations such as buildings and is packaged on a PC board that measures a mere 16mm × 30mm (0.63” x 1.2”) (Figure 2). It provides sensing the well-known environmental and presence parameters of motion via a PIR (passive infrared) detector, light, temperature, and humidity. It also offers optional microphone-based detection of sound, VOC sensing for ambient air quality and hazardous conditions, and CO2 sensing (Figure 3).
The AmbiMate Sensor Module operates from a single 3.3VDC supply (nominal, with 3.1V minimum), and requires just 10mA (33mA with the VOC/CO2 sensing option) for low dissipation, high reliability, and long operating life from a local battery. Accuracy for the temperature reading is ±0.3°C from +5°C to +50°C, while the accuracy of its humidity reading over the 5 percent to 95 percent RH range is 2 percent. Both temperature and humidity sensors have one-second update rates.
For the gas-sensing functions, the unit can handle VOCs with concentrations from 0ppb to 1187ppb (parts per billion) and CO2 readings from 400ppm to 8192ppm (parts per million); both gas-related readings have 60-second acquisition periods.
The module can be queried at any time via its 100kbaud (kilobaud) I2C interface, and it also includes an interrupt-driven event pin for motion and sound-level detection, with response time under 0.5 of a second. The audio-alarm output has sensitivity between -25dBV and -19dBV over a frequency range of 100Hz to 10kHz. A brief product overview is available on this page, and a more detailed one is here.
Example 2: Bosch Sense Connect Detect (SCD) Module with Bluetooth Connectivity
The pairing of technology advances with trends in user requirements is providing significant impetus for what has been called Industry 4.0 or the Industrial Internet of Things (IIoT). For industrial sensors, it means incorporating multiple sensor types in a single easy-to-use package (thus eliminating the need to integrate them individually) and also including the necessary, seamless wireless connectivity. For example, the Bosch Sense Connect Detect (SCD) Module enables companies to get started easily with Industry 4.0. This plug-and-play sensor provides basic-level insight on machine performance and response, as well as raw data for further processing (Figure 4). No programming is needed to get it up and running.
By incorporating and integrating sensing of four key physical variables of industrial machines–acceleration, magnetic field, temperature, and light intensity—in a single package with built-in connectivity, the time and well-known frustration of setting up a unit, initializing it, and getting data is reduced to a trivial exercise. This cost-effective, reliable, easy-to-install sensor device attaches to almost any machine and component via a simple strip. Data acquisition can start immediately with a wireless Bluetooth Low Energy-based link for reporting on each of the four measured variables. The unit offers a wide dynamic range coupled with absolute accuracy commensurate with industrial applications (Table 1).
In addition, since many real-world applications are more concerned with reading resolution rather than accuracy, the Bosch SCD Module offers high resolution along with appropriate sampling rates to match the needs of each sensed variable (Table 2 and Table 3 respectively).
Of course, to complete Industry 4.0, sensing of multiple physical variables is only part of the story. Data-acquisition modules need to get the data to the “outside world” easily and with minimum connectivity issues, so the SCD BLE Low Power link has a range of up to 40 meters (dependent on external influences).
The final part of the story is the make use of the acquired data. To do this, the unit supports easy data visualization via a mobile app (Figure 5).
Packaging, power, and environmental specifications are also key factors for effective IoT placement and performance. The SCD is housed in a diminutive, IP67-rated package measuring just 70.9mm × 62mm × 11.3mm and weighing 21 grams. It is rated for operation from -20°C to +85°C and for relative humidity ranging from 0 percent rH to 100 percent rH. The low-power design supports operation for up to two years from a non-rechargeable CR2450 3V Li/MnO2 button cell (490mAh).
More information on this module is available on the overview page, as well as on the datasheet.
Example 3: Honeywell AWM700 Series Airflow Sensors
Despite our almost intuitive understanding of airflow, measuring it accurately over a useful range is not a simple challenge, except perhaps for wind speed where a rotating-cup anemometer is the well-known solution. For industrial or medical airflow in a pipe or tube, that’s clearly not an option.
Instead, a flow sensor such as the Honeywell AWM730B5 is a very attractive choice. This MEMS-based airflow sensor provides in-line flow measurement with specially designed bypass flow housing.
The device measures flow as high as 300 standard liters per minute (SLPM) (3.0mbar [millibars]/300Pa [Pascal]) while inducing a typical pressure drop of just 1” (H2O). The AWM730B5 has a high flow range capability in a small package, as well as a 10ms response time. The unit requires a 5VDC supply and consumes only 60mW of power.
This sensor is well suited for the many medical applications that require constant measurement and control of airflow, including oxygen concentrators, respirators and ventilators, nebulizers, continuous positive airway pressure (CPAP) systems, and anesthesia machines. On the instrumentation side, airflow measurement is needed for mass flow controllers, environmental climate controls, fuel cell controls, and even laser system controls.
Its analog output has a highly stable null and full-scale value, low hysteresis and repeatability errors (less than 0.35 percent of reading), and internal amplification and temperature compensation, all of which contribute to long-term accuracy and stability without a need for calibration (a difficult challenge for flow in many cases). Table 4 shows the relationship between the analog output versus flow as well as the tolerance range.
The compact package, with a flow tube approximately 3.25” (82.5mm) long and 0.9” (22mm) wide (plus mounting lugs and connector) means that these flow sensors occupy less space in the final product, so its enclosure size can be reduced for easier fit into ever-shrinking, space-constrained installations. Further, the physical wiring is simple as there are just thee active connections (V-, +5 VDC, and VOUT) and is easily accomplished via the standard 4-pin, snap-in, AMP 103956-3 provided with the sensor.
More details and specifications for this sensor are available here and on the datasheet.
There is no longer a need for original equipment manufacturers to identify, buy, integrate mechanically and electrically, and code multiple sensors from different sources. Highly integrated MEMS and semiconductor devices enable placement of multiple sensors for different physical variables in a single, tiny, low-power package. Even the challenge of sensing VOCs and CO2, as well as medical and instrumentation-related airflow, has been largely overcome because of these technological advances. Sensor and system performance is assured and known, as these devices come with detailed specifications. Circuit interfacing is greatly simplified, and system integration is streamlined via software drivers and tools.