Benchmarks for the IoT

Input Voltage

–Jeff Child, Editor-in-Chief

JeffHeadShot

I remember quite vividly back in 1997 when Marcus Levy founded the Embedded Microprocessor Benchmark Consortium, better known as EEMBC. It was big deal at the time because, while benchmarks where common in the consumer computing world of desktop/laptop processors, no one had ever crafted any serious benchmarks for embedded processors. I was an editor covering embedded systems technology at the time, and Marcus, as an editor with EDN Magazine back then, traveled in the same circles as I did. On both the editorial side and on the processor vendor side, he had enormous respect in the industry—making him an ideal person to spin up an effort like EEMBC.

Creating benchmarks for embedded processors was more complicated than for general purpose processors, but EEMBC was up the challenge. Fast forward to today, and EEEBC now boasts a rich list of performance benchmarks for the hardware and software used in a variety of applications including autonomous driving, mobile imaging, mobile devices and many others. In recent years, the group has taken on the complex challenge of developing benchmarks for the Internet-of-Things (IoT).

I recently had the chance to talk with EEMBC’s current president, Peter Torelli, about the consortium’s latest effort: its IoTMark-BLE benchmark. It’s part of the EEMBC’s IoTMark benchmarking suite for measuring the combined energy consumption of an edge node’s sensor interface, processor and radio interface. IoTMark-BLE focuses on Bluetooth Low Energy (BLE) devices. In late September, EEMBC announced that the IoTMark-BLE benchmark is available for licensing.

The IoTMark-BLE benchmark profile models a real IoT edge node consisting of an I²C sensor and a BLE radio through sleep, advertise and connected-mode operation. The benchmark measures the energy required to power the edge node platform and to run the tests fed by the benchmark. At the center of the benchmark is the IoTConnect framework, a low-cost benchmarking harness used by multiple EEMBC benchmarks. The framework provides an external sensor emulator (the I/O Manager), a BLE gateway (the radio manager) and an Energy Monitor.

Benchmark users interact with the DUT via an interface with which they can set a number of tightly defined parameters, such as connection interval, I²C speed, BLE transmission power and more. Default values are provided to enable direct comparisons between DUTs, or users can change them to analyze a design’s sensitivity to each parameter. IoTMark-BLE’s IoTConnect framework supports microcontrollers (MCUs) and radio modules from any vendor, and it is compatible with any embedded OS, software stack or OEM hardware.

It makes sense that IoT benchmarks focus on power and energy use. IoT edge devices need to work in remote locations near the sensors they’re linked with. With that in mind, Peter Torelli says that the benchmark measures everything inside an IoT system-on-chip (SoC)—including the peripheral I/O reading from the I2C sensor, the transmit and receive amplifiers in the BLE radio—everything except the sensor itself. Torelli says it was important to not use intelligent sensors for the benchmark, the idea being that its important that the MCU’s role performing communication be part of the measurement. Interestingly, in developing the benchmark, it was found that even the software stacks on IoT SoCs have a big impact on performance. “Some are very efficient when they’re in advertise mode or in active mode, and then go to sleep,” says Torelli, “And there are others that remain active for much longer times and burn a lot of power.”

Shifting gears, I want to take moment to praise long time columnist and member of the Circuit Cellar family, Ed Nisley. Over 30 years ago, Steve Ciarcia asked Ed to write a regular column for the brand-new Circuit Cellar INK magazine. After an even 200 articles, Ed decided to make his September column his last. Thank you, Ed, for your many years of insightful, quality work in the pages of this magazine. You’ll be missed. Readers can follow Ed’s continuing series of shop notes, projects and curiosities on his blog at softsolder.com.

Let me welcome Brian Millier as our newest Circuit Cellar columnist—his column Pickup Up Mixed Signals begins this issue. Brian is no stranger to the magazine, penning over 50 guest features in the magazine since the mid-90s on a variety of topics including guitar amplifier electronics, IoT system design, LCDs and many others. I’m thrilled to have Brian joining our team. With his help, we promise to continue fulfilling Circuit Cellar’s role as the leading media platform aimed at inspiring the evolution of embedded system design.

This appears in the November 340 issue of Circuit Cellar magazine

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MCU-Based Project Enhances Dance Game

Using Wavelet Transform

Microcontrollers are perfect for systems that need to process analog signals such as audio and do real-time digital control in conjunction with those signals. Along just those lines, learn how these two Cornell students recreated the classic arcade game “Dance Dance Revolution” using a Microchip PIC32 MCU. Their version performs wavelet transforms to detect beats from an audio signal to synthesize dance move instructions in real time without preprocessing.

By Michael Solomentsev and Drew Dunne

We designed a version of the traditional arcade game, “Dance Dance Revolution,” that synthesizes dance instructions from any audio source using the PIC32 MCU. Unlike the original game, in which users must choose from a pre-selected list of songs, our system allows users to plug in their audio device and play songs of their choice. The dance move instructions are then generated in real time by buffering the audio and processing it, using the discrete wavelet transform.

We were inspired by a mutual desire to work on a music-related project, and both of us had fond memories of playing this kind of game. We also wanted to add some sort of novel, interesting component, so we brainstormed the idea of allowing the player to play whatever song he or she wanted. The game is much more fun when the song playing is your favorite tune. All versions of the commercial game have pre-programmed song libraries, so replay value is limited. Our version has no such limitation. The discrete wavelet transform was selected as a processing method because we needed both time and frequency resolution. We also needed a computationally efficient algorithm.

The system requires two kinds of user input: an audio source and button presses from the dance mat floor tiles. The audio input must be processed, so it needs to be delayed or buffered until the processing is complete. Another reason for the delay of the audio output is to give ample time for the user to react to the instructions created from processing the audio. In contrast, the user input needs to be in real time. We use two PIC32s to do the input processing—one detects beats and reads the dance mat input, while the other buffers audio. We use a macOS application to display the beats and handle scoring.

Figure 1
Overview of our entire “Dance Dance Evolution” project fully set up

We built a custom dance mat for the game, consisting of five individual tiles that could each detect when players put their weight on it (Figure 1). They needed to be both durable and sensitive to pressure. To achieve this, we used force sensitive resistors wired in parallel. These were polled at approximately 20 Hz for changes in resistance. These resistors were placed directly between the tiles—which were made out of canvas covered boards—and the supports that raised them off the ground.

Hardware Design

We used a PIC32 development board designed by Sean Carroll, with an DAC socket and GPIO pins brought out, to provide flexibility for development [1]. We also used a second PIC32 on a smaller development board, with connections to the floor tiles and the Serial to USB cable. The floor tiles were wired underneath to a protoboard, and all-important signals were fed up to our main protoboard using a ribbon cable. Figure 2 shows the schematic of the system, incorporating Sean Carroll’s full-size and small PIC32 development boards.

Figure 2
Shown here is the schematic of the system, incorporating Sean Carroll’s full-size and small PIC32 development boards.

We soldered our audio circuitry on a protoboard to make it easier to debug and to reduce noise. Our audio input jack fed into a 500 µF capacitor to cut out any DC component, then we fed it into an offset circuit, such that the ADCs could read it with no clipping. The DAC output was fed directly to an audio jack and speakers.

Another PIC32 MCU handled audio buffering, because SPI communication with both a 128 KB serial SRAM and DAC took too many cycles to perform the necessary signal processing simultaneously. We used our professor Bruce Land’s code for the SRAM chip for reading and writing to the SRAM and writing to the DAC [2]. His code included some read/write methods, and handled the SPI setup and mode changes. We had to add code to read from the ADC in a timer interrupt at 40 kHz, write to a location in the SRAM, and finally read from a different location and write that value to the DAC. The locations written to and read from were incremented each time, to create a loop around the SRAM memory locations. To change how long we wanted to buffer the audio, we just needed to change the values of MAX_ADDR and MIN_ADDR. The closer together they were, the smaller the range of the SRAM we used. This was important, because using the entire SRAM gave us a buffer of about 3.3 seconds, and we wanted only about 2.5 seconds.

The major consideration that affected our tile construction was a desire for resiliency. Because users would probably stomp on each of the tiles fairly hard, we wanted to make sure that our press detection system could withstand a lot of force. We also wanted a simple, easy solution to mock-up and build.

Initially we looked into using strain gauges, but they would require mounting to a base plate and the tile to be pushed. Traditional buttons did not seem like a robust enough option. Instead, we decided to use force sensitive resistors (FSRs). Initial testing revealed that the unpressed FSRs had resistance of approximately 6 MΩ. When pressed, it was approximately 1 kΩ. This huge discrepancy made it easy to probe it for a press. We thank Interlink Electronics, who were gracious enough to donate 10 FSR402s for use in our project. ..

Read the full article in the November 340 issue of Circuit Cellar

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Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

Next Newsletter: Embedded Boards

Coming to your inbox tomorrow: Circuit Cellar’s Embedded Boards newsletter. Tomorrow’s newsletter content focuses on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

Bonus: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

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October has a 5th Tuesday, so we’re bringing you a bonus newsletter:
Digital Signage (10/30)  Digital signage ranks among the most dynamic areas of today’s embedded computing space. Makers of digital signage players, board-level products and other technologies continue to roll out new solutions for implementing powerful digital signage systems. This newsletter looks at the latest technology trends and product developments in digital signage.

Analog & Power. (11/6) This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op amps, batteries and more.

Microcontroller Watch (11/13) This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. (11/20) Covers what’s happening with Internet-of-Things (IoT) technology–-from devices to gateway networks to cloud architectures. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Fog-Detection Audio Project

Using Arduino UNO

Fog reduces visibility, sometimes down to a few feet. That’s why fog horns are so important. In this article, Jeff embarks on a project that makes use of humidity sensors to detect fog, and automatically plays an audio response when fog is detected. Aside from the sensors, his project also makes use of Arduino hardware and software.

By Jeff Bachiochi

Pea soup. Not one of my favorite dishes. I just can’t see green soup. All that said, as a consistency descriptor, “pea soup” is spot on for describing fog. You don’t have to live in San Francisco to appreciate the alien qualities of the scene in Figure 1. This phenomenon is commonplace to those who reside in London, Seattle, on any lake or in any valley. All we need is the right combination of temperature and humidity, and the clouds of heaven will visit us here on Earth.

Fog consists of visible water droplets suspended in the air at or near the Earth’s surface. This moisture is often generated locally from a nearby body of water, and forms when the difference between air temperature and dew point is less than about 4°F. The dew point is the temperature at which the water vapor in air (at constant barometric pressure) condenses into liquid water on tiny particles in the air, at the same rate at which it evaporates—forming fog. A change in temperature affects the relative humidity. As the dew point goes up, so does the relative humidity, creating a smaller differential between actual temperature and dew point temperature until fog forms.

The maximum amount of water vapor that can be held in a given volume of air (saturation) varies greatly by temperature. Cold air can hold less mass of water per unit volume than hot air. Relative humidity is the percentage of water found relative to the maximum possible, at a particular temperature.

Most of us have experienced the relationship between temperature and relative humidity firsthand. When the air temperature is high, our bodies use the evaporation of sweat to cool down. The cooling effect is directly related to how fast the perspiration evaporates. The rate of evaporation depends on how much moisture is already in the air and how much moisture the air can hold. If the air is already saturated with moisture (high humidity) perspiration will not evaporate, and we remain hot and uncomfortable. Discomfort can also exist when the humidity is low. The drier air can cause our skin to crack and tends to dry out the airways.

Measuring Relative Humidity

A hygrometer is an instrument used for measuring the humidity and water vapor content of the atmosphere, the soil and confined spaces. An instrument that measures humidity usually relies on the detection of some other quantity—such as temperature, pressure, mass or a mechanical or electrical change in a substance as moisture is absorbed. Today we use the electrical change of capacitance or resistance to calculate humidity.

Humidity measurement is among the more difficult problems in basic meteorology. Most hygrometers sense relative humidity rather than the absolute amount of water present. Because relative humidity is a function of both temperature and absolute moisture content, a small temperature change will translate into a change in relative humidity.

Some materials’ properties allow humidity levels to be determined based on a change in their capacitance, resistance, thermal conductivity or mass. Many humidity sensors include a temperature sensor, which allows them to approach 2-3% accuracy in a changing temperature environment. Table 1 shows several common humidity sensors. Many of them are available on some tiny PCB modules for easy interfacing.

Table 1
These are some popular humidity sensors with similar specifications.

I’ve used both the Honeywell HIH-5031 (with a Texas Instruments TMP102 temperature sensor) and a Silicon Labs Si7021 for measuring humidity in this project. The The Honeywell sensor has analog output, whereas the TI temperature sensor and Silicon Labs humidity/temperature sensor both use I2C to communicate. If you feel uneasy writing a function to perform the process, Arduino libraries are available for many sensors. This is an advantage for newbies, because you can get a program working with a library and often a sample program to get you started. Then you can go back and write the function yourself as a learning experience.

I don’t want to base the project totally on humidity, so I have added an ultrasonic distance measuring device to the project. Because fog forms as the air becomes supersaturated with water, this should mean that the humidity has reached 100% and water droplets in the air should begin to look solid. I’m hoping ultrasonics will be reflected by the droplets and cause a normal non-returned ping to be received. The combination of these two sensors exceeding some predetermined limits will satisfy my conditions to determine the presence of “fog.”

Avast, Ye Landlubber!

In a previous article (March 2011, Circuit Cellar 248), I presented the Microchip Technology (formerly Supertex) SR10, an inductorless switching power supply controller intended for operation directly from a rectified 120/240 VAC line. This was presented in support of creating a 5 V supply for a lighthouse fixture designed using LEDs. This month’s project will build upon that 5 V light, and will add audio to protect and guide boaters who find themsleves out on a lake under foggy conditions.

Figure 2
The schematic shows the connections made between the Arduino Uno headers and the sensor/module connectors mounted on an Arduino prototyping board. The finished board is shown in Figure 4.

I’m centering this project around the Arduino UNO. While I built the prototype interface (Figure 2) using the Arduino MEGA, it only uses the pins native to the UNO. The sensors/modules that are interfaced in this project are shown in Table 2.

Table 2
Each sensor/module will add some current draw to the project. Although current can be minimized in some cases while it is not active, this all becomes important if the project will run on batteries.

Because the UNO has only a single hardware serial port, I set up two additional software serial ports—one to talk with the DRPlayer and one to the optional LCD (output only). The main port and optional LCD don’t have to be used in the final project, but there is code written to display progress on each of these devices for seeing debugging information.  .  …

Read the full article in the October 339 issue of Circuit Cellar

Don’t miss out on upcoming issues of Circuit Cellar. Subscribe today!

Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

IoT Module Family Features Ultra-Compact Form Factor

Telit has announced the xE310 family of miniature IoT modules. With initial models planned in LTE-M, NB-IoT and European 2G, the new form factor will enable Telit to meet growing demand for ultra-small, high-performance modules for wearable medical devices, fitness trackers, industrial sensors, smart metering, and other mass-production, massive deployment applications. Telit will start shipping xE310 modules in Q4 this year.
Telit claims the xE310 family is one of the smallest LGA form factors available in the market with a flexible perimeter footprint supporting various sizes from compact to smaller than 200 mm2. The xE310’s 94 pads include spares to provide Telit the flexibility to quickly deliver support for additional features as technologies, applications and markets evolve. Spares can be used for modules supporting Bluetooth, Wi-Fi or enhanced location technologies—in addition to cellular—while maintaining compatibility with cellular only models. They can also be used for additional connections that may be required for new 5G-enabled features.

The new form factor also gives OEMs greater flexibility, efficiency and yield during design and manufacturing. The xE310 family provides easy PCB routing while minimizing manufacturing process issues such as planarity and bending. The unique circular pad facilitates correct package orientation for automated assembly.

To learn more about the new xE310 family, visit the Telit stand 431 at IoT Solutions World Congress in Barcelona, Spain on October 16-18.

For a look at how this new design is enabling smart metering applications, register for the Telit webinar on November 15: “From 2G to 5G: 5 things you need to know for smarter utilities”: https://www.smart-energy.com/industry-sectors/data_analytics/webinar-15-november-5-things-you-need-to-know-for-smarter-utilities/.

Telit | www.telit.com

Tuesday’s Newsletter: IoT Tech Focus

Coming to your inbox tomorrow: Circuit Cellar’s IoT Technology Focus newsletter. Tomorrow’s newsletter covers what’s happening with Internet-of-Things (IoT) technology–-from devices to gateway networks to cloud architectures. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

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Our weekly Circuit Cellar Newsletter will switch its theme each week, so look for these in upcoming weeks:

Embedded Boards.(10/23) The focus here is on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

October has a 5th Tuesday, so we’re bringing you a bonus newsletter:
Digital Signage (10/30)  Digital signage ranks among the most dynamic areas of today’s embedded computing space. Makers of digital signage players, board-level products and other technologies continue to roll out new solutions for implementing powerful digital signage systems. This newsletter looks at the latest technology trends and product developments in digital signage.

Analog & Power. (11/6) This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op amps, batteries and more.

Microcontroller Watch (11/13) This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

Semtech LoRa Technology Leveraged for Flood Sensor System

Semtech has announced that Green Stream has incorporated Semtech’s LoRa devices and wireless radio frequency technology (LoRa Technology) and Senet’s LoRaWAN-based network into its autonomous flood sensor systems for use in coastal areas, including towns and cities.

Green Stream’s solutions use LoRa Technology, a proven technology used in IoT environmental solutions. Green Stream’s end-to-end flood monitoring solutions are designed using commercial, off-the-shelf ultrasonic sensors and easy-to-deploy LoRa-enabled gateways. The data is communicated over a LoRaWAN-based network provided by Senet, a leading provider of Cloud-based LoRaWAN services platforms that enable the on-demand build out and management of IoT connectivity. The Green Stream LoRa-based flood sensors are autonomous, requiring no external power or wired network connection.

Each sensor is a self-contained, weather-proof, solar-powered unit that comes with a universal mounting bracket and extension arm. These sensors are small enough to be installed on top of crosswalks, light or electric poles, and bridges. The rugged sensor gateway is positioned above a body of water or over dry land.

Semtech | www.semtech.com

Next Newsletter: Embedded Boards

Coming to your inbox tomorrow: Circuit Cellar’s Embedded Boards newsletter. Tomorrow’s newsletter content focuses on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

Bonus: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

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Embedded Boards newsletter issue tomorrow.

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Our weekly Circuit Cellar Newsletter will switch its theme each week, so look for these in upcoming weeks:

October has a 5th Tuesday, so we’re bringing you a bonus newsletter:
Digital Signage (10/30)  Digital signage ranks among the most dynamic areas of today’s embedded computing space. Makers of digital signage players, board-level products and other technologies continue to roll out new solutions for implementing powerful digital signage systems. This newsletter looks at the latest technology trends and product developments in digital signage.

Analog & Power. (11/6) This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op amps, batteries and more.

Microcontroller Watch (11/13) This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. (11/20) Covers what’s happening with Internet-of-Things (IoT) technology–-from devices to gateway networks to cloud architectures. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Tuesday’s Newsletter: IoT Tech Focus

Coming to your inbox tomorrow: Circuit Cellar’s IoT Technology Focus newsletter. Tomorrow’s newsletter covers what’s happening with Internet-of-Things (IoT) technology–-from devices to gateway networks to cloud architectures. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Bonus: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

Already a Circuit Cellar Newsletter subscriber? Great!
You’ll get your IoT Technology Focus newsletter issue tomorrow.

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Our weekly Circuit Cellar Newsletter will switch its theme each week, so look for these in upcoming weeks:

Embedded Boards.(9/25) The focus here is on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

Analog & Power. (10/2) This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op amps, batteries and more.

Microcontroller Watch (10/9) This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

Velocity and Speed Sensors

Measuring Motion

Automatic systems require real-life physical attributes to be measured and converted to electrical quantities ready for electronic processing. Velocity is one such attribute. In this article, George steps through the math, science and technology behind measuring velocity and the sensors used for such measurements.

By George Novacek

Automatic systems respond to a multitude of inputs affecting their output. Real life physical attributes need to be measured and converted to electrical quantities ready for electronic processing. Velocity is one of such attributes. By its definition, velocity is a vector comprising speed and direction of a motion. The speed is the rate of change of an object’s position with respect to some reference point over time. In situations such as an automobile cruise control, the direction of the movement is not important. In other applications—typically a guidance system or aircraft autopilot—the velocity must be determined in three axes simultaneously. In those applications the velocity directions are often referred to as pitch, roll and yaw—although technically those are angular positions and their rates of change.

A common unit of speed in the SI (the metric system) is meters per second (m/s). Other units are derived, based on their magnitude, by multiplication or division, such as millimeters per second (mm/s), kilometers per hour (km/h) and so forth. The Imperial system expresses speed in miles, feet or inches per second, hour and such. We should not forget Mach, the unit of speed named after the Austrian physicist Ernst Walfried Mach. It is used primarily in aerospace applications. The Mach number is dimensionless, expressing the ratio of the local flow speed (v) over the speed of sound (c) in its particular medium, often air. Therefore M = v/c. Mach 1 equals to the speed of sound 331.46 m/s (1,193.26 km/h or 741.45 miles/h). The speed of sound, however, changes with the conditions of the medium, such as its temperature, so the Mach number may vary while the ground speed remains constant. Nautical and subsonic aviation in the English-speaking world commonly use a Knot (kn or kt). 1 kt = 1.151 miles/h = 1.852 km/h.

The angular—rotational velocity—of an object is the rate of change of the angular position with respect to time. It is a measure of how fast the object is turning. Angular velocity is also a vector with the direction being the axis about which the object rotates. In many control systems the axis is stationary, while our primary interest is the rotational speed. The common unit is rotations per minute, or RPM.

There aren’t very many transducers available for measuring linear speed directly. Such measurements can be performed by displacement sensors [1] combined with some method of time measurement. The speed “s” is expressed as:

The speed is defined as the change of position over time. When the amount of time is substantial with respect to the traveled distance, the resulting speed is likely to be considered average. For the time interval approaching zero the immediate speed is dx/dt.

LVDT and Doppler Methods

Low speed displacement measurement can be performed, for example, by a linear variable differential transformer (LVDT) [1], but the range is small due to the mechanical limitations of the transducer. At the opposite extreme, optical, ultrasound or radio displacement detection methods are used to establish the time for an object to pass detection points. Two basic methods are common. One is the measurement of the distance by the time of flight and calculating the speed per equation #1.

The second method is based on the Doppler effect, named after the Austrian physicist Christian Doppler who discovered it in 1842. If a periodic signal is emitted at some frequency ranging from acoustic to light, its reflection, frequency shifted up or down, depending on the object’ direction of travel, is received. The magnitude of the shift depends on the speed of the motion. Assuming the direction is straight towards the receiver, the measured frequency (Equation #2):

and the frequency shift (Equation #3):

where ∆f = f – f0 and ∆v = vr – vs. ∆v is the expression of the velocity of the receiver relative to the source.

Light interferometry methods, such as the VISAR system, are too specialized and too costly to be found in most embedded control systems. VISAR is an acronym that stands for “velocity interference system for any reflector.”

To measure speed, it is often convenient to convert a linear velocity into a rotary one, using a tachometer (essentially a dynamo), Hall Effect diodes or rotary encoders. Anemometers are a typical example where the fluid (or air) flow rotates a small propeller which rotates a tachometer.

Tachometers have been commonly recognized as automobile instruments, but the old mechanical devices are no longer ideal for modern electronic control. Today the automobile speed is often derived from counting the ignition firing. Some motors comprising an integral tachometer are often considered too bulky and too expensive by today’s standards. Instead, optical or magnetic sensors are preferred. The principle of optical speed detection is shown in Figure 1.

FIGURE 1
An optical rotational speed detector

Magnetic sensors use Hall Effect sensors to replace the light detector in Figure 1, one or more permanent magnets located on the rotating disc and the light source is omitted. The Hall effect was discovered by E. H. Hall in 1879. The Hall element is made from a thin sheet of a conductive material with the output connections perpendicular to the direction of current flow. When subjected to a magnetic field, the sensor generates voltage proportional to the magnetic field strength. The generated voltage is in the order of microvolts and therefore electronics are needed to amplify the output to a useful level. Hall sensors produce a voltage proportional to the magnetic field strength, while Hall switches output logic levels only. Hall switches are perfect for rotational speed measurement. They are found in brushless DC motors (BLDC) as speed feedback devices, in antilock braking systems and so forth. Permanent magnets attached to a rotating shaft cause the switch to generate a pulse every time a magnet moves past it. The rotational speed of electrical motors can also be measured by monitoring their back EMF (electromotive force) with no sensors needed.  …

Read the full article in the September 338 issue of Circuit Cellar

Don’t miss out on upcoming issues of Circuit Cellar. Subscribe today!
 

Note: We’ve made the October 2017 issue of Circuit Cellar available as a free sample issue. In it, you’ll find a rich variety of the kinds of articles and information that exemplify a typical issue of the current magazine.

Next Newsletter: Embedded Boards

Coming to your inbox tomorrow: Circuit Cellar’s Embedded Boards newsletter. Tomorrow’s newsletter content focuses on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

Bonus: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

Already a Circuit Cellar Newsletter subscriber? Great!
You’ll get your
Embedded Boards newsletter issue tomorrow.

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Our weekly Circuit Cellar Newsletter will switch its theme each week, so look for these in upcoming weeks:

Analog & Power. (9/4) This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op amps, batteries and more.

Microcontroller Watch (9/11) This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. (9/18) Covers what’s happening with Internet-of-Things (IoT) technology–-from devices to gateway networks to cloud architectures. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Tuesday’s Newsletter: IoT Tech Focus

Coming to your inbox tomorrow: Circuit Cellar’s IoT Technology Focus newsletter. Tomorrow’s newsletter covers what’s happening with Internet-of-Things (IoT) technology–-from devices to gateway networks to cloud architectures. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Bonus: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

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You’ll get your IoT Technology Focus newsletter issue tomorrow.

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Our weekly Circuit Cellar Newsletter will switch its theme each week, so look for these in upcoming weeks:

Embedded Boards.(9/25) The focus here is on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

Analog & Power. (10/2) This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op amps, batteries and more.

Microcontroller Watch (10/9) This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.