December Circuit Cellar: Sneak Preview

The December issue of Circuit Cellar magazine is coming soon. Don’t miss this last issue of Circuit Cellar in 2018. Pages and pages of great, in-depth embedded electronics articles prepared for you to enjoy.

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Here’s a sneak preview of December 2018 Circuit Cellar:

AI, FPGAs and EMBEDDED SUPERCOMPUTING

Embedded Supercomputing
Gone are the days when supercomputing levels of processing required a huge, rack-based systems in an air-conditioned room. Today, embedded processors, FPGAs and GPUs are able to do AI and machine learning kinds of operation, enable new types of local decision making in embedded systems. In this article, Circuit Cellar’s Editor-in-Chief, Jeff Child, looks at these technology and trends driving embedded supercomputing.

Convolutional Neural Networks in FPGAs
Deep learning using convolutional neural networks (CNNs) can offer a robust solution across a wide range of applications and market segments. In this article written for Microsemi, Ted Marena illustrates that, while GPUs can be used to implement CNNs, a better approach, especially in edge applications, is to use FPGAs that are aligned with the application’s specific accuracy and performance requirements as well as the available size, cost and power budget.

NOT-TO-BE-OVERLOOKED ENGINEERING ISSUES AND CHOICES

DC-DC Converters
DC-DC conversion products must juggle a lot of masters to push the limits in power density, voltage range and advanced filtering. Issues like the need to accommodate multi-voltage electronics, operate at wide temperature ranges and serve distributed system requirements all add up to some daunting design challenges. This Product Focus section updates readers on these technology trends and provides a product gallery of representative DC-DC converters.

Real Schematics (Part 1)
Our magazine readers know that each issue of Circuit Cellar has several circuit schematics replete with lots of resistors, capacitors, inductors and wiring. But those passive components don’t behave as expected under all circumstances. In this article, George Novacek takes a deep look at the way these components behave with respect to their operating frequency.

Do you speak JTAG?
While most engineers have heard of JTAG or have even used JTAG, there’s some interesting background and capabilities that are so well know. Robert Lacoste examines the history of JTAG and looks at clever ways to use it, for example, using a cheap JTAG probe to toggle pins on your design, or to read the status of a given I/O without writing a single line of code.

PUTTING THE INTERNET-OF-THINGS TO WORK

Industrial IoT Systems
The Industrial Internet-of-Things (IIoT) is a segment of IoT technology where more severe conditions change the game. Rugged gateways and IIoT edge modules comprise these systems where the extreme temperatures and high vibrations of the factory floor make for a demanding environment. Here, Circuit Cellar’s Editor-in-Chief, Jeff Child, looks at key technology and product drives in the IIoT space.

Internet of Things Security (Part 6)
Continuing on with his article series on IoT security, this time Bob Japenga returns to his efforts to craft a checklist to help us create more secure IoT devices. This time he looks at developing a checklist to evaluate the threats to an IoT device.

Applying WebRTC to the IoT
Web Real-time Communications (WebRTC) is an open-source project created by Google that facilitates peer-to-peer communication directly in the web browser and through mobile applications using application programming interfaces. In her article, Callstats.io’s Allie Mellen shows how IoT device communication can be made easy by using WebRTC. With WebRTC, developers can easily enable devices to communicate securely and reliably through video, audio or data transfer.

WI-FI AND BLUETOOTH IN ACTION

IoT Door Security System Uses Wi-Fi
Learn how three Cornell students, Norman Chen, Ram Vellanki and Giacomo Di Liberto, built an Internet connected door security system that grants the user wireless monitoring and control over the system through a web and mobile application. The article discusses the interfacing of a Microchip PIC32 MCU with the Internet and the application of IoT to a door security system.

Self-Navigating Robots Use BLE
Navigating indoors is a difficult but interesting problem. Learn how these two Cornell students, Jane Du and Jacob Glueck, used Received Signal Strength Indicator (RSSI) of Bluetooth Low Energy (BLE) 4.0 chips to enable wheeled, mobile robots to navigate towards a stationary base station. The robot detects its proximity to the station based on the strength of the signal and moves towards what it believes to be the signal source.

IN-DEPTH PROJECT ARTICLES WITH ALL THE DETAILS

Sun Tracking Project
Most solar panel arrays are either fixed-position, or have a limited field of movement. In this project article, Jeff Bachiochi set out to tackle the challenge of a sun tracking system that can move your solar array to wherever the sun is coming from. Jeff’s project is a closed-loop system using severs, opto encoders and the Microchip PIC18 microcontroller.

Designing a Display System for Embedded Use
In this project article, Aubrey Kagan takes us through the process of developing an embedded system user interface subsystem—including everything from display selection to GUI development to MCU control. For the project he chose a 7” Noritake GT800 LCD color display and a Cypress Semiconductor PSoC5LP MCU.

What’s the Role of 3D Printing in Embedded Systems?

Experts Weigh In

3D printing has gone from being a technology on the outskirts of embedded system design, to one that’s becoming a common tool for many design teams. On one hand people are crafting 3D printed enclosures of electronic systems—either for prototyping or end use. On the other hand, the idea of embedding electronic circuitry within 3D printed materials has gained momentum. To gather insights on these technology and design trends, I spoke with expert representatives from four innovative companies in the 3D printer business.

By Jeff Child, Editor-in-Chief

________________________________________________________

Mark Norfolk, President, Fabrisonic
www.fabrisonic.com

JEFF CHILD: 3D printing has evolved into a key technology for the design and development of embedded electronics-based systems. What do you see as the important trends today along those lines?

Mark Norfolk

MARK NORFOLK: Historically, electronics embedded using 3D printing has been relegated to embedding wires or 3D printed conductors in a 3D printed polymer. Recent advancement in solid state metal 3D printing has enabled engineers to now bury electronics into metal 3D printed components. Ultrasonic Additive Manufacturing (UAM) is a 3D metal printing technology that uses high frequency ultrasonic vibrations to scrub metal foils together layer by layer as opposed to using a directed energy heat source (for example, laser, e-beam and so on). Ultrasonic joining is a solid state (no melting) process, which enables direct integration of temperature sensitive components into the 3D metal part unlike fusion based processes. The low temperature nature allows sensors, communication circuits and actuators to be embedded into fully dense metallic structures for lasting security and reliability.

To embed electronics into a metal part, a channel or chamber is cut during the CNC stage of the UAM process. The electronic sensor or circuit is then placed into the void and consolidated with the additive stage. In the case of sensors, metal flow in the UAM process creates a strong mechanical joint between the matrix and sensor material, which in turn enables excellent strain transfer to the metal matrix for stress and temperature measurements (Figure 1).

Figure 1
(a) Fiber optic strain gauge embedded in an aluminum bracket using metal 3D printing. (b) CT scan of embedded fiber optic

A flat roof can be created over control circuitry allowing for a small air gap that can be potted or sealed. This allows high power electronics to be buried into a copper or aluminum box for high thermal conductivity. Furthermore, 3D printing allows for cooling channels to be printed surrounding the individual high-power components (Figure 2).

Figure 2
(a) Packaging concept using metal 3D printing and electronic 3D printing.
(b) Integrated electronics in a custom thermal shroud

J.C.: What have been some of the important trends and capabilities in 3D printing materials as they relate to electronic systems?

NORFOLK: For Fabrisonic, a significant portion of recent work has been in engineered materials for the interface between electronics and 3D printed metal. For instance, coefficient of thermal expansion (CTE) mismatch is an ever-present problem in traditional manufacturing. Ultrasonic welding allows printing of dissimilar metals in the same part. Thus, a gradient of CTEs can be printed through thickness in a cooling device. Fabrisonic has worked with materials such as molybdenum and invar to address the CTE gap. Similarly layers of heavy metals such as tantalum and tungsten have been integrated into 3D printed structures for radiation hardening (Figure 3).

Figure 3
Layers of tantalum printed in an aluminum laminate for radiation hardening

J.C.: How have 3D printers used by electronic system developers changed in the past couple years? What changes and advances do you see within the next couple years?

NORFOLK: Electronic system developers have a growing toolbox of 3D printing options. As any specific traditional manufacturing method cannot hope to make every electronics package, similarly no one 3D printing technology can meet every need. New tools are coming on the market as existing tools evolve to meet the needs of industry. Future improvements in 3D printed electronics will surely include:

• Better conductive inks that have lower resistance
• Integration of multiple 3D printing processes into a single production machine. For instance, technologies such as Aerosol Jet could be integrated into a UAM system to print electronics into a 3D printed metal part all in one integrated system
• Automated methods for inserting conventional electronics (wires, chips and such) into 3D printed builds live during the print job
• New electronic designs that take advantage of 3D printing’s ability to integrate in three dimensions

J.C.: We’ve talked in general so far about technology trends in 3D printing? What’s an example of a Fabrisonic system that exemplifies those trends in action?

NORFOLK: All of Fabrisonic systems are capable of embedding electronics. UAM is ultrasonic welding on a semi-continuous basis where solid metal objects are built up to a net three-dimensional shape through a succession of welded metal tapes. Through periodic machining operations, detailed features are milled into the object until a final geometry is created by removing excess material. Figure 4 shows a rolling ultrasonic welding system, consisting of two 20,000 Hz ultrasonic transducers and the welding sonotrode.

Figure 4
Shown here is a SonicLayer 4000 metal 3D printer based off a traditional 3-axis CNC mill. The ultrasonic “weld head” is another tool in the tool changer and can be swapped at any point for a traditional end mill. The additive “weld head” is used to print parts near net shape, while the CNC stage is used to mil to exact tolerances and to create internal voids for embedding electronics.

High-frequency ultrasonic vibrations are locally applied to metal foils, held together under pressure, to create a weld. The vibrations of the transducer are transmitted to the disk-shaped welding sonotrode, which in turn creates an ultrasonic solid-state weld between the thin metal tape and the substrate. The continuous rolling of the sonotrode over the plate welds the entire tape to the plate. Successive layers are welded together to build up height. This process is then repeated until a solid component has been created. CNC contour milling is then used to achieve required tolerances and surface finish.
______________________________________________________________

Clément Moreau, CEO and Co-Founder, Sculpteo
www.sculpteo.com

JEFF CHILD: What is your perspective on where 3D printing technology is today in terms of its application in electronic systems?

Clément Moreau

CLÉMENT MOREAU: 3D printing has been used to produce prototypes of enclosures of electronic systems for decades—and now longer and longer series of such enclosures. We have a growing number of customers using additive manufacturing to produce their final product up to tens of thousands of parts. This delays the costly and painful re-industrialization process of moving to mass manufacturing. Printing a full electronics circuit system—like a computer or a phone—is still really far away. But we see some application with 3D printed electronics for simple functions like powering LEDs, wiring a sensor and so on.

J.C.: When it comes to 3D printed electronics, what do you see as the most important aspect of that capability?

MOREAU: For 3D printed electronics, conductivity is key. The capability to print selectively using materials with high conductivity is progressing.

J.C.: Sounds like you’re optimistic about where the technology is heading. How do you see 3D printing advancing over the next couple years?

MOREAU: 3D printers are definitely evolving in terms of resolution and of versatility in materials. Still, the main use of 3D printing in this context is printing electronic devices enclosure. The ability to print in fire-resistant materials important is very important, for the electrical certification of devices.

______________________________________________________

Alexander Crease, Application Engineer, Markforged
www.markforged.com

JEFF CHILD: As an application engineer, what’s your perspective on the role 3D printing plays in the design and development of embedded electronics-based systems?

Alexander Crease

ALEXANDER CREASE: Overall, 3D printing has made it easier for anyone—whether you’re an engineer, designer, artist or manufacturer—to make things. Creating physical models used to be difficult. Either you’d have to pay thousands of dollars and wait weeks for parts to come in, or you’d need to piece something together with what you have on hand. Either way, manufacturing was a large roadblock—especially with multiple prototypes or iterations in the product development cycle. 3D printing has changed all that—serving as a catalyst for simplified production of parts.

With regard to electronic systems, 3D printing suddenly makes prototyping, testing and iteration much more efficient and makes it easier to create custom components. A large part of embedded electronics is its integration into its hardware—the system integration. Alan Rencher, CEO of Media Blackout, uses Markforged 3D printers to print custom TV and media equipment and sees high value in the printers. He says “Even on finished products that we used to have machined, if we need a part that is too expensive or physically not able to be manufactured, we can use the printer to make those parts.” The quick turnaround time and low cost of 3D printing means end-use parts are incredibly affordable to create. You can go through multiple iteration cycles in days, improving your product’s function and performance all while cutting costs. There’s also the design freedom inherent to additive manufacturing that allows you to incorporate your electronics into your product seamlessly. Both of those combined mean that—whether you’re working on a prototype or a custom end-use part—you can use 3D printing to create a professional, seamless and efficient integrated system.

J.C.: Everyone says that the capabilities of 3D printing are tied to the kinds of materials with which they can print. How do you see that aspect of 3D printing?

CREASE: 3D printing materials have only increased in strength and quality. With innovations like Continuous Fiber Fabrication (CFF), 3D printing has expanded from rough ”looks-like” mockups and prototypes to end-use applications, where durable, long lasting parts are needed. These types of high-strength composite materials mean that the critical parts for your electronics housings, fixtures and frames are strong, cost-effective and easy to make. Many electronics companies now turn to high-strength 3D printing to create strong, lightweight setups for their equipment. For example, Radiant Images developed a 360-camera rig (Figure 5) using a Markforged 3D printing system and saw 63% weight savings and 77% manufacturing time savings when compared to its previously machined counterpart. And it functions just the same.

Figure 5
Radiant Images developed this 360-camera rig using a Markforged 3D printing system and saw 63% weight savings and 77% manufacturing time savings when compared to its previously machined counterpart.

J.C.: It’s clear that the circle of people comfortable using 3D printers keeps getting wider. What’s behind that trend, and what advances in the future do you see attracting engineers to 3D printing?

CREASE: Until recently, 3D printing has been exclusive to mechanical engineers and technicians who know how to design for, operate and repair the machines. Today, a lot of the major improvements to printing we see are in a printer’s ease-of-use. You no longer have to be a trained professional to understand how it all works. It’s getting easier and easier for anyone to design the parts they need, load them into 3D printing software and hit go—then have a part ready in hours.

Looking forward, design optimization for 3D printing has been a growing trend. 3D design software can help engineers design parts that are optimized for the printing process. This not only makes printing even more accessible, but also allows for performance optimization of the parts you need. The introduction of powerful software tools that do the design thinking for you to make parts lighter, stronger, and more effective is something many engineers will be able to take advantage of to create high-performance designs right off the bat. That paves the way for more creativity and innovation in product design.

J.C.: Can you describe some of the details of your company’s Markforged X7 3D printing system?

Figure 6
The Markforged X7 3D printing system includes a sensor suite of that automatically calibrates the machine before each print—leveling the bed, calibrating the nozzles and more. That means there’s no need for a lot of the regular maintenance tasks required of typical 3D printers

CREASE: The Markforged X7 is the top-of-the-line model of our Industrial Series. Both the printer and the parts it delivers are reliable and robust (Figure 6). And the system itself is designed to be low-maintenance and easy to use. The X7 includes a sensor suite of that automatically calibrates the machine before each print—leveling the bed, calibrating the nozzles and more—meaning there’s no need for a lot of the regular maintenance tasks required of typical 3D printers. It prints in a broad range of high strength composite materials, including Kevlar, Fiberglass and Carbon Fiber. Customers can expect high-quality, metal strength parts produced on a low-maintenance workhorse.

 

 

__________________________________________________________

Simon Fried, President and Co-Founder, Nano Dimension
www.nano-di.com

JEFF CHILD: From your point of view, how do you see the state-of-the-technology when it comes 3D printed electronics?

Simon Fried

SIMON FRIED: The intersection of additive manufacturing and printed electronics offer several opportunities for new or improved ways of making things. The applications that lend themselves to this confluence of technologies cover a spectrum ranging from new ways of adding electronics to larger mechanical parts to—at the other end in terms of size—approaches to challenges confronting the component, semiconductor and electrical packaging industries. The larger scale applications include printing wiring and/or strain gauges into larger mechanical parts and so allow for the elimination of bulky wiring harnesses and connectors, as well enabling better preventative maintenance sensing.

Antennas can also be added to pre-existing parts to open the door to new ways of adding smarts to nose-cones in aircraft or missiles for example. At the other end of the scale spectrum are PCB, component or even wafer level applications. Additive manufacturing of multi-layer circuits or MIDs (molded interconnect devices) means these types of item can both be prototyped much more quickly, secretly and flexibly. They can also be designed differently given the novel non-planar geometries that an additive approach makes possible. At this higher resolution end of the additive electronics space, systems can also be found that can make the embedding of components within a 3D printed circuit an option.

J.C.: What do you see as some of the critical capabilities in 3D printing materials as they relate to electronic systems?

FRIED: Just as is the case in the traditional 3D printing space, it’s materials that set the boundaries of what can be made by way of additive manufacturing of electronics. The first key capability is the development of conductive materials that can be reliably deposited by means of extrusion, aerosol or jetting. Conductive polymers that may contain metals, graphene, carbon nano-tubes and other exotic materials offer lower levels of conductivity for FDM (fused deposition modeling) filaments. More conductive, often nanoparticle-based, inks can be deposited by aerosol or inkjet based additive systems. As these materials become easier to process, cheaper and more conductive, their application set continues to grow, including antennas for example.

For truly 100% additive printing of electronics, it is also necessary to deposit an insulating dielectric material. The traditional electronics industry has a dizzying array of such materials to choose from, each with specifications for a defined performance. While 3D printers don’t yet have materials matched to every need—whether mechanical, thermal or electrical— over the last few years more dielectric materials have become available. Specific inks for specific dielectric performances are now available, where before printers had to make do with whatever polymer was printable. As the set of materials expands so will the applications that an additive approach makes possible.

J.C.: What advances do you see with 3D printing in the next couple years? Is 3D printing as a mainstream, electronics manufacturing technology in sight?

FRIED: It is still early days in the evolution of this technology and as a result most of the work that we are aware of has been experimental and very much lab-based. Considering the amount of development in this space—being driven by the needs of industries as diverse as automotive, defense, medical, consumer electronics, contract manufacturing and many more—it’s highly likely that that such high definition functional 3D printing will start to deliver manufacturing solutions in addition to today’s prototyping and experimental work.

J.C.: Do you have an example of a Nano Dimension 3D printer product that illustrates the kinds of technology trends we’ve been discussing?

Figure 7 The DragonFly Pro can be used to print traditional planar circuits and antennas as well as to print non-planar designs.

FRIED: Nano Dimension’s focus is on delivering solutions to the challenges and opportunities of electrical and product designers. There are several benefits that additive manufacturing approaches can offer, including namely time compression, secrecy, customization and innovation acceleration in general. Our new DragonFly Pro 3D printer is a precision additive manufacturing tool that simultaneously deposits two very different materials, metal and polymer inks. The DragonFly Pro can be used to print traditional planar circuits and antennas as well as to print non-planar designs (Figure 7). It’s the beginning of an entirely new way of making things, as well as a route to making what is currently unmakeable by any other approach. …

See the article in the November 340 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.

November Circuit Cellar: Sneak Preview

The November issue of Circuit Cellar magazine is coming soon. Clear your decks for a new stack of in-depth embedded electronics articles prepared for you to enjoy.

Not a Circuit Cellar subscriber?  Don’t be left out! Sign up today:

 

Here’s a sneak preview of November 2018 Circuit Cellar:

SOLUTIONS FOR SYSTEM DESIGNS

3D Printing for Embedded Systems
Although 3D printing for prototyping has existed for decades, it’s only in recent years that it’s become a mainstream tool for embedded systems development. Today the ease of use of these systems has reached new levels and the types of materials that can be used continues to expand. This article by Circuit Cellar’s Editor-in-Chief, Jeff Child looks at the technology and products available today that enable 3D printing for embedded systems.

Add GPS to Your Embedded System
We certainly depend on GPS technology a lot these days, and technology advances have brought fairly powerful GPS functionally into our pockets. Today’s miniaturization of GPS receivers enables you to purchase an inexpensive but capable GPS module that you can add to your embedded system designs. In this article, Stuart Ball shows how to do this and take advantage of the GPS functionality.

FCL for Servo Drives
Servo drives are a key part of many factory automation systems. Improving their precision and speed requires attention to fast-current loops and related functions. In his article, Texas Instruments’ Ramesh Ramamoorthy gives an overview of the functional behavior of the servo loops using fast current loop algorithms in terms of bandwidth and phase margin.

FOCUS ON ANALOG AND POWER

Analog and Mixed-Signal ICs
Analog and mixed-signal ICs play important roles in a variety of applications. These applications depend heavily on all kinds of interfacing between real-world analog signals and the digital realm of processing and control. Circuit Cellar’s Editor-in-Chief, Jeff Child, dives into the latest technology trends and product developments in analog and mixed-signal chips.

Sleeping Electronics
Many of today’s electronic devices are never truly “off.” Even when a device is in sleep mode, it draws some amount of power—and drains batteries. Could this power drain be reduced? In this project article, Jeff Bachiochi addresses this question by looking at more efficient ways to for a system to “play dead” and regulate power.

BUILDING CONNECTED SYSTEMS FOR THE IoT EDGE

Easing into the IoT Cloud (Part 1)
There’s a lot of advantages for the control/monitoring of devices to communicate indirectly with the user interface for those devices—using some form of “always-on” server. When this server is something beyond one in your home, it’s called the “cloud.” Today it’s not that difficult to use an external cloud service to act as the “middleman” in your system design. In this article, Brian Millier looks at the technologies and services available today enabling you to ease in to the IoT cloud.

Sensors at the Intelligent IoT Edge
A new breed of intelligent sensors has emerged aimed squarely at IoT edge subsystems. In this article, Mentor Graphics’ Greg Lebsack explores what defines a sensor as intelligent and steps through the unique design flow issues that surround these kinds of devices.

FUN AND INTERESTING PROJECT ARTICLES

MCU-Based Project Enhances Dance Game
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 two Cornell students Michael Solomentsev and Drew Dunne recreated the classic arcade game “Dance Dance Revolution” using a Microchip Technology PIC32 MCU. Their version performs wavelet transforms to detect beats from an audio signal to synthesize dance move instructions in real-time without preprocessing.

Building an Autopilot Robot (Part 2)
In part 1 of this two-part article series, Pedro Bertoleti laid the groundwork for his autopiloted four-wheeled robot project by exploring the concept of speed estimation and speed control. In part 2, he dives into the actual building of the robot. The project provides insight to the control and sensing functions of autonomous electrical vehicles.

… AND MORE FROM OUR EXPERT COLUMNISTS

Embedded System Security: Live from Las Vegas
This month Colin O’Flynn summarizes a few interesting presentations from the Black Hat conference in Las Vegas. He walks you through some attacks on bitcoin wallets, x86 backdoors and side channel analysis work—these and other interesting presentations from Black Hat.

Highly Accelerated Product Testing
It’s a fact of life that every electronic system eventually fails. Manufacturers use various methods to weed out most of the initial failures before shipping their product. In this article, George Novacek discusses engineering attempts to bring some predictability into the reliability and life expectancy of electronic systems. In particular, he focuses on Highly Accelerated Lifetime Testing (HALT) and Highly Accelerated Stress Screening (HASS).

Stepper Motor Back EMF

Supply Voltage vs. Current Control

Continuing with the topic of stepper motors, this time Ed looks at back electromotive force (EMF) and its effects. He examines the relationship between running stepper motors at high speeds and power supply voltage requirements.

By Ed Nisley

Early 3D printers used ATX supplies from desktop PCs for their logic, heater and motor power. This worked well enough—although running high-wattage heaters from the 12 V supply tended to incinerate cheap connectors. More mysteriously, stepper motors tended to run roughly and stall at high printing speeds, even with microstepping controllers connected to the 12 V supply.

In this article, I’ll examine the effect of back EMF on stepper motor current control. I’ll begin with a motor at rest, then show why increasing speeds call for a much higher power supply voltage than you may expect.

Microstepping Current Control

As you saw in my March 2018 article (Circuit Cellar 332), microstepping motor drivers control the winding currents to move the rotor between its full-step positions. Chips similar to the A4988 on the Protoneer CNC Shield in my MPCNC sense each winding’s current through a series resistor, then set the H-bridge MOSFETs to increase, reduce or maintain the current as needed for each step. Photo 1 shows the Z-axis motor current during the first few steps as the motor begins turning, measured with my long-obsolete Tektronix Hall effect current probes, as shown in this article’s lead photo above.

Photo 1 Each pulse in the bottom trace triggers a single Z-axis microstep. The top two traces show the 32 kHz PWM ripple in the A and B winding currents at 200 mA/div. The Z-axis acceleration limit reduces the starting speed to 18 mm/s = 1,100 mm/min.

The upper trace (I’ll call it the “A” winding) comes from the black A6302 probe clamped around the blue wire, with the vertical scale at 200 mA/div. The current starts at 0 mA and increases after each Z-axis step pulse in the bottom trace. Unlike the situation in most scope images, the “ripple” on the trace isn’t noise. It’s a steady series of PWM pulses regulating the winding current.

The middle trace (the “B” winding) increases from -425 mA because it operates in quadrature with the A winding. The hulking pistol-shaped Tektronix A6303 current probe, rated for 100 A, isn’t well-suited to measure such tiny currents, as you can see from the tiny green stepper motor wire lying in the gaping opening through the probe’s ferrite core. Using it with the A6302 probe shows the correct relation between the currents in both windings, even if its absolute calibration isn’t quite right.

Photo 2 zooms in on the A winding current, with the vertical scale at 50 mA/div, to show the first PWM pulse in better detail. The current begins rising from 0 mA, at the rising edge of the step pulse, as the A4988 controller applies +24 V to the motor winding and reaches 110 mA after 18 µs. The controller then applies -24 V to the winding by swapping the H bridge connections. This causes the current to fall to 40 mA, whereupon it turns on both lower MOSFETs in the bridge to let the current circulate through the transistors with very little loss.

Photo 2
Zooming in on the first microstep pulse of Photo 1 shows the A4988 driver raising the stepper winding current from 0 mA as the motor starts turning. The applied voltage and motor inductance determine the slope of the current changes.

The next PWM cycle starts 15 µs later, in the rightmost division of the screen, where it rises from the 40 mA winding current set by the first pulse. It will also end at 110 mA, although that part of the cycle occurs far off-screen. You can read the details of the A4988 control algorithms and current levels in its datasheet, with the two-stage decreasing waveform known as “mixed decay” mode.

Although the H-bridge MOSFETs in the A4988 connect the motor windings directly between the supply voltage and circuit ground, the winding inductance prevents the current from changing instantaneously. The datasheet gives a nominal inductance of 4.8 mH, matching what I measured, but you can also estimate the value from the slope of the current changes.. . …

Read the full article in the May 334 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.