EEWeb Pulse - Volume 97 by EEWeb Magazines

Ed McMahon CEO of Epec

Electrical Engineering Community

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TABLE OF CONTENTS

Ed McMahon CEO OF EPEC ENGINEERED TECHNOLOGIES A conversation about how this PCB company branched out into different custom built products and why it was the key to their success.

Featured Products

Isolaâ&#x20AC;&#x2122;s Organic Synthesis Reactor Creates PCB Resin

A look into the process of developing a reactor in-house and how it enables Isola Group to be more efficient in all aspects of PCB development.

Research Into and Uses Of Terahertz Switching-Speed Transistors

BY ALEX TOOMBS WITH EEWEB With transistor switching speeds at an all-time high, researches are looking into how this could affect everything from home solar installations to consumer electronics.

Strain Gauge Sensors

BY UMANATH KAMATH WITH CYPRESS How to interface strain gauge sensors and how they can be implemented in a system design for strain gauge read-outs.

RTZ - Return to Zero Comic

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INTERVIEW

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EEWeb PULSE How did you get started in the PCB industry? Back in early 2001 I bought my first printed circuit board company, a company called Syrtek in Syracuse, New York. The beginning of 2001 wasn’t a great time in the electronics business. Of course, that’s when September 11th happened, which made it even worse. Our start in this business came at a really tough time in the electronics industry, but we have managed to stick around for this long. In 2003 we bought Epec, which is now basically a conglomeration of 15 different distressed printed circuit board companies that we acquired over the last 10 years. Back in 1999 there were over 1,200 printed circuit board shops in North America, and today there are fewer than 300. In today’s market, with Asia being able to come directly into North America a lot more easily, being in the printed circuit board business is not a very good long-term growth strategy. That is why, about five years ago, we branched outside of printed circuit boards into other custom build electronics -- battery packs, flexible circuits, membrane switches, fans, and motors.

While printed circuit boards are still a big part of our business, every year that part gets smaller. We focus a lot more on the high tech side of things now, things that can’t be done in Asia because it doesn’t have the US based engineering resources that allow boards built in Asia to be on the customer’s dock in 5 workings. That’s really how we built our business and how we are going to continue to grow. How big is Epec and how has the company grown over the years? As far as employees, we have 93. Thirty-two of them are located in our office and at factories in Asia. The balance is mostly in the U.S., but we’re kind of a virtual company. We’ve got design centers for all of our different products all over the country. Here in New Bedford, Mass., it’s printed circuit boards and user interfaces. In Denver, Colo. we have a tech center that has four engineers who just do our battery design, development, and testing. In the U.K., in Wales, we have our fans and motors design center -- they do all of our fan and motor design. Because we have acquired several different companies, we have several satellite

“A customer can come to us with just a concept, or come to us with an existing part they want to make changes to and we will do all the electrical and mechanical design, all of the prototyping, all of the software and hardware development.”

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offices across the U.S. where we have customer service and/or engineering. What kind of service do you provide around battery packs? We provide everything from a custom design all the way to full-production manufacturing. A customer can come to us with just a concept or come to us with an existing part they want to make changes to and we will do all the electrical and mechanical design, all of the prototyping, all of the software and hardware development. We will even do all of the regulatory testing. We do UN, DOT testing, UL—we manage the entire process. In the end, we can also do the production, either here in our factory in New Bedford, or in Asia. For medical and military applications, we do a lot of the manufacturing here, and for a lot of the higher volume industrial and handheld device applications, we do the manufacturing in Asia. Are the membrane switches and rubber keypads manufactured in the U.S. or overseas? For all of our products we do both. That is mainly because we have to be able to do prototypes and small quantities here. We are an ITAR Certified company so we must manufacture all of our ITAR part numbers in the U.S. Typically 80% of all of our manufacturing is done in Asia because we have developed a world class supply chain – but the fact that all of the engineering is done in the U.S. lets us produce a product in as little as 5 working days from Asia. That is one of the huge differentiators on all of our products; all of the engineering is done in the U.S. Our engineering department and our production planning groups both

INTERVIEW work 24-7. We have people working around the clock so if there are any issues in the middle of the night, we can go ahead and manage them and make sure that production keeps going. Do you still have PCB manufacturing that’s done here in the U.S.? We do. We have two operations in the U.S. - one in Detroit, Mich. and one in Dallas, Tex. - and we focus almost exclusively on high technology, quick turn delivery. So 12-18 layers in three days or less. We acquired a company called UPE at the beginning of the year that does EXTREME copper and we do up to 20 oz of copper on the outer layers and in holes. It’s a technology that’s really been accepted by the defense and aerospace industries, but very few people in the world can make printed circuit boards with that amount of copper. In terms of trace, width, and manufacturing minimum restrictions, what kind of technology does Epec offer? Right now we’re down to 2 ½ and 2 ½. We invested in laser direct imaging (LDI) equipment to be able to get down to those levels. A lot of folks have LDI for just the circuit imaging part of PCB manufacturing. We have LDI for imaging and solder masks, which allows us to get down to 3mil solder mask dams in a quad-pack design. This allows the customers much more ease-of-use when it comes to their assembly versus somebody that’s trying to use films to image the old-fashioned way, so to speak, where you can’t hold anything less than 5mils. Do your PCBs all run under the Epec brand? They do not. There are two reasons Visit www.eeweb.com

EEWeb PULSE why we don’t do that. One reason is that we used to have a printed circuit board manufacturing facility at our factory here in New Bedford as well as our other businesses. What we found is that you can’t be great at both under the same roof. We decided to break them apart and have supply chain, logistics, engineering and warehousing here, and our other two factories (which we have ownership in) separate and let each of them focus on what they do well. The one in Dallas does the ultra high-tech stuff, and the one in Detroit does a lot of lower-tech quick turns. We do a lot of one and two day turns there on a lot of lower technology parts. What types of services around fans and motors do you provide? We have two different types of services. From an industrial perspective we’re more of a custom-motor solution provider than a standard off-the-shelf product. If you got a small-axial fan that you’re buying from a distributor now, we’re probably not a good fit. But if you’ve got a solution where you need variable speeds, or you need it to be in a frost-free environment, or you need a specific IP requirement, that’s where we have a really good niche. Also, in the HVAC and the commercial refrigeration areas, we’ve developed two proprietary products, one called the ECplus™, and one called the EXR. They are far and away more energy efficient than any other product

on the market, especially when it comes to electronically commutated fans. When you look at the specific examples that we’re working on right now -- we’re working with the largest manufacturer of vending machines in the world -- what they’re finding by using both of our motors is that they’re reducing the operating costs of that vending machine by close to 35% using our motors. We’ve got applications for that and we’ve recently been approved by Coca-Cola to be used in their cabinets because of the energy-efficiency of our products. Is there a controller associated with the motor in that product? Absolutely. Almost every application in the HVAC and commercial refrigeration market is a custom motor. Our ECplus™ motor is one of the first motors and is the only ECAC motor on the market that actually has three speeds. The beautiful thing about our electronics in that motor is that we can customize the settings. It’s not just 3 speeds that are standard. We can put whatever RPMs you want. Does Epec have any online tools? We have a variety of online tools for rigid PCB’s and flexible circuits. Two of them are InstantPCBQuote and InstantFLEXQuote, which have been such a hit with engineers that business there has tripled over the past three years. You can put all your parameters in, get pricing, and place your order online. It’s really

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inexpensive, and we still do all of the DFM checks that we would typically do if you went through our regular customer service channels. It really allows engineers to play around with their design, get real-time pricing, place their order in the middle of the night, and have us get things rolling. It has been a real success with the engineering community across the country and we are starting to try and roll it out to colleges and the academic community, who are our future engineers. What’s on the horizon over the next two and three years? Everything we’ve gotten into, we did because a customer asked us to get involved. That’s how we got into batteries and that’s how we got into membranes, keypads, and fans. A customer came to us and said, “You’ve been doing a great job with the other stuff, can you help us out with this?” Right now, we’re about to roll out cable assemblies. We’ve been doing cable assemblies for the past two years, but we just haven’t made a big deal about it (or brought it to the marketplace officially), because our other products were taking a lot of our time and effort. But the customers still came to us and said, “We have all these cable assemblies and we want you to do them,” and so we’ve been doing them both here and in Asia. By the end of the second quarter we’re going to roll them out officially and update our literature on our website. ■

INTERVIEW

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MCUs with Tiny Series CPU Core The R8C/2A Group and R8C/2B Group of single-chip MCUs incorporates the R8C/Tiny Series CPU core, employing sophisticated instructions for a high level of efficiency. With 1 Mbyte of address space, and it is capable of executing instructions at high speed Power consumption is low, and the supported operating modes allow additional power control. These MCUs also use an anti-noise configuration to reduce emissions of electromagnetic noise and are designed to withstand EMI. For more information, please click here.

LIN RGB Slave for Ambient Light The Melexis MLX81106 LIN RGB slave provides a single chip solution for driving up to 4 LEDs via constant current sources in automotive ambient lighting applications. Every output can be programmed to a maximum current of 35 mA through the built-in Flash memory. All necessary components like physical layer LIN transceiver, LIN controller, voltage regulator, and 16-bit RISC-based microcontroller, as well as supporting functions like ADC, 16-bit current modulation, constant current high voltage capable outputs and LED color and aging compensation are integrated into the chip. For more information, please click here.

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Multi-Mode Wireless Power Receiver IC The IDTP9020 is a highly-integrated single-chip WPC-compliant wireless power receiver IC. The device receives an AC power signal from a compatible wireless transmitter and converts it into a regulated 5V output voltage which can be used to power devices or supply the charger input in mobile applications. The IDTP9020 integrates a high-efficiency synchronous full-bridge rectifier, high-efficiency synchronous buck converter, and control circuits used to modulate the load. For more information, please click here.

Integrated High-Side MOSFET The ISL78200 is a synchronous buck controller with a 90mΩhigh-side MOSFET and low-side driver integrated. The ISL78200 supports wide input range of 3V to 40V in buck mode. It supports 2.5A continuous load under conditions of 5V VOUT, VIN range of 8V to 36V, 500kHz and +85°C ambient temperature with still air. For any specific application, the actual maximum output current depends upon the die temperature not exceeding +125°C with the power dissipated in the IC, which is related to input voltage, output voltage, duty cycle, switching frequency, ambient temperature and board layout, etc. For more information, click here.

32-bit High Capacity RISC MCUs Fujitsu Semiconductor America introduced new 32-bit RISC microcontrollers based on the ARM® CortexTM-M3 processor. The 38 new MCUs include the MB9BF529TPMC, which is equipped with high-capacity memory, and the MB9BF121JPMC, which features low-pin-count packaging. The new devices, which are optimal for system control in household appliances, office automation systems and industrial equipment, bring the growing family of Fujitsu FM3 MCUs to a total of 570 products. Sample quantities of the latest devices will be available starting May 10, 2013. For more information, please click here.

Low Power Fully-Differential Amp The THS4531A is a low-power, fully-differential op amp with input common-mode range below the negative rail and rail-to-rail output. The device is designed for low-power data acquisition systems and high density applications where power consumption and dissipation is critical. The device features accurate output common-mode control that allows for dc coupling when driving analog-to-digital converters (ADCs). The THS4531A is also a valuable tool for general-purpose, low-power differential signal conditioning applications. For more information, please click here.

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ISOLA GROUP

is a global material sciences company that develops polymers for advanced multilayer printed circuit boards. Its R&D facility in Chandler, Arizona

recently expanded to include an internally designed, production-scale organic synthesis reactor that will allow the company to develop unique polymer resin systems. With the ability to create resins on site, Isola can distance itself from suppliers to enable completely proprietary products. However, building a reactor of this complexity is no easy task. We spoke with Stan Wilson, the manager of the Isola reactor project, about the process of developing with the reactor in-house and how it enables the company to be more efficient in all aspects of PCB development.

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ISOLA’S ORGANIC SYNTHESIS REACTOR CREATES PCB RESIN

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Design Concept of 800 Liter Reactor

Reactor Overview

From Prototype to Production-Size

When making printed circuit boards, you must begin with building blocks made of resin. In the past, Isola was tied to an outside resin manufacturer to supply them with the necessary materials for the building blocks. Once they received these resins from the manufacturers, the Isola team would add their own components in what they call their compounding area to bring the quality up to their standards and formulations.

Stan Wilson oversaw the reactor’s creation as project manager along with Charles Lehmann and Michael He, other lead engineers on the team. All of the production for the reactor and R&D facility was done in-house with Isola’s own design team. Initially, the team used a

According to Wilson, by having the reactor on-site, they are able to create a resin unique to the marketplace. “Our reactor synthesizes the resin and allows us to create a proprietary resin system that our otherwise competitors will be buying off the shelf.” With the base resin, the reactor basically heats it up and allows some of its molecules to separate in order to add components to change its structure. “It actually gives us good control of our own resin system,” Wilson explained to us, “especially in the high-performance market we’re targeting today.” This added control of the process removes the complexities of referring to an outside source, allowing more freedom to test and tailor it to the company’s high standards. This is a fairly new capability for Isola and, according to Wilson, is unique to their competition.

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Isola’s reactor allows them to synthesize resin to create a proprietary resin system that competitors will end up buying off the shelf.

relatively small, 100-litre prototype reactor that allowed them to come up with a unique resin. From there, the team saw the need for a bigger reactor that would be more cost-effective and convenient for customizing their products. “We went from a 100-litre reactor to a 800-litre, production-sized reactor,” Wilson told us. “This allows us to run projects at production levels to be able to supply the market.” The reactor took years to design and build after modeling from the prototype reactor. “It was a pretty intense project,” Wilson recalled, “The company put a lot of funds into making this happen.” Going from the prototype

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reactor to the production-sized reactor was a two-year process, with the reactor going into full production mode this past fall. It turns out it was worth the wait. “The feedback I’ve gotten has all been positive.” Wilson told us. “It has surpassed our requirements and is working better than we initially thought in terms of reaction time, batch size, and consistency.” Since time to market is very important to Isola’s customer base when designing PCB boards, the convenience of the on-site reactor has also proven to be a major benefit. Isola is now able to respond quickly to changing market conditions and environments to help their customers and OEMs get their product to market faster. ■

Above: (From left) Charles Lehmann, Michael He, Stan Wilson

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EE Making Wireless Truly Wireless: Need For Universal Wireless Power Solution

Dave Baarman Director Of Advanced Technologies

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Power Factor Correction Controllers ISL6730A, ISL6730B, ISL6730C, ISL6730D The ISL6730A, ISL6730B, ISL6730C, ISL6730D are active Features power factor correction (PFC) controller ICs that use a boost topology. (ISL6730B, ISL6730C, ISL6730D are Coming Soon.) The controllers are suitable for AC/DC power systems, up to 2kW and over the universal line input.

The ISL6730A, ISL6730B, ISL6730C, ISL6730D are operated in continuous current mode. Accurate input current shaping is achieved with a current error amplifier. A patent pending breakthrough negative capacitance technology minimizes zero crossing distortion and reduces the magnetic components size. The small external components result in a low cost design without sacrificing performance. The internally clamped 12.5V gate driver delivers 1.5A peak current to the external power MOSFET. The ISL6730A, ISL6730B, ISL6730C, ISL6730D provide a highly reliable system that is fully protected. Protection features include cycle-by-cycle overcurrent, over power limit, over-temperature, input brownout, output overvoltage and undervoltage protection.

• Reduce component size requirements - Enables smaller, thinner AC/DC adapters - Choke and cap size can be reduced by 66% - Lower cost of materials • Excellent power factor over line and load regulation - Internal current compensation - CCM Mode with Patent pending IP for smaller EMI filter • Better light load efficiency - Automatic pulse skipping - Programmable or automatic shutdown • High reliable design - Cycle-by-cycle current limit - Input average power limit - OVP and OTP protection - Input brownout protection

The ISL6730A, ISL6730B provide excellent power efficiency and transitions into a power saving skip mode during light load conditions, thus improving efficiency automatically. The ISL6730A, ISL6730B, ISL6730C, ISL6730D can be shut down by pulling the FB pin below 0.5V or grounding the BO pin. The ISL6730C, ISL6730D have no skip mode.

• Small 10 Ld MSOP package

Two switching frequency options are provided. The ISL6730B, ISL6730D switch at 62kHz, and the ISL6730A, ISL6730C switch at 124kHz.

• TV AC/DC power supply

• Desktop computer AC/DC adaptor • Laptop computer AC/DC adaptor • AC/DC brick converters

100

VLINE

Applications

VOUT

EFFICIENCY (%)

VCC ISEN

GATE

ICOMP

GND

ISL6730

VIN

ISL6730A, SKIP

80 ISL6730C

75 70

COMP BO

VREG

FIGURE 1. TYPICAL APPLICATION

40 60 OUTPUT POWER (W)

100

FIGURE 2. PFC EFFICIENCY

TABLE 1. KEY DIFFERENCES IN FAMILY OF ISL6730

February 26, 2013 FN8258.0

VERSION

ISL6730A

ISL6730B

ISL6730C

ISL6730D

Switching Frequency

124kHz

62kHz

124kHz

62kHz

Skip Mode

Yes-Fixed

Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2013 All Rights Reserved. All other trademarks mentioned are the property of their respective owners.

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Research Into and Uses of

Terahertz Switching

Alex Toombs

Electrical Engineering Student

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ecent adv concept o terahertz, are essentially s or voltage con lowest leve to flow th mathem in fa

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g-Speed Transistors

vances in semiconductor technology have pushed the of transistors with switching speeds greater than one , or one trillion cycles per second, closer to reality. Transistors semiconductor-constructed switches which can be either current ntrolled—depending upon the materials and design used. At its el, a signal on one terminal controls whether or not current is allowed hrough the “switch”, and how much of it is, depending upon a fixed matical relationship affected by the type of semiconductors used abrication.

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EEWeb PULSE Some lab specimens have reached over one terahertz already, though none are ready for commercial production. In 2001, Intel announced that they had fabricated several terahertz transistors, though none are available in their products today. The technology has great implications for communications, power distribution, home solar installations, computing, surveillance, and many other research and consumer applications. High-speed switching devices may even signify the continuation of Moore’s Law, the infamous observation that the number of transistors on an integrated circuit doubles every eighteen to twenty-four months. Above all else, terahertz speed transistors are desirable for their ability to increase speed and decrease power dissipation.

The Need for High-κ One of the largest barriers for terahertz transistors has been the need for high-κ materials in order to fabricate suitable gate oxides. The value κ stands for dielectric constant, which is directly proportional to the capacitance across the transistor terminals. Essentially, since the gate oxide layer is made from an insulating material, it holds charge on either side and prevents it from flowing across—acting like a parallel plate capacitor. This structure is critical to the operation of MOSFET transistors, and is shown below. High-κ dielectrics are necessary in order to increase that capacitance enough to block leakage. As transistor size has shrunk to accommodate more powerful integrated circuits, gate thickness has similarly decreased in order

to allow greater amounts of current. Gate thickness is inversely proportional to gate capacitance, though a smaller gate allows for more leakage current to pass through. Leakage current is that which inappropriately flows through the gate insulator, which it is more likely to occur when the gate layer is thin. This wastes energy in the same way that a leaky faucet wastes water, and produces undesirable results that may affect the behavior of the transistor. Thin gates also allow small amounts of current to flow when the device is turned off, changing operating characteristics and wasting even more power. Intel’s process involved using a material that increased the capacitance, and thus the current, without losing as much power due to leakage. While silicon dioxide is easily grown simply by oxidizing the existing silicon layers in a furnace, other technologies introduce more difficulties. Possible materials include hafnium silicate, zirconium silicate, and zirconium dioxide. The issues with these are that combining these different materials with a traditional polysilicate gate can introduce imperfections where the materials meet, slowing carriers down due to the effect known as phonon scattering. More recent announcements have stated that Intel is using hafnium-based insulators in addition to metallic gates in their 45nm gate processors released in 2007.

Source & Drain Resistance Additional problems include considerations involving the source/drain resistance of the device. With increasing transistor performance, the highest consideration is

Typical MOS Capacitor

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Carbon Nanotube Wire Structures decreasing device size. Unfortunately, the nature of resistance in devices is that smaller area essentially crowds carriers, meaning that smaller sources and drains will greatly increase the resistance of the carriers flowing through the device. In some cases, different silicon doping densities may help alleviate the problem, though not on a wide enough scale. One further problem with the source and drain channels are that they also act as a capacitor. The capacitance created across the junction increases the time the device takes to cycle, making transistors slower rather than faster. Essentially, each time the transistor needs to switch, some charge needs to build up or drain first before any current can flow.

Graphene-Based Transistors One promising alternative to hafnium-gates for commercially viable terahertz devices is that of graphenebased transistors. Graphene is a carbon-based molecule that has shown promise in a variety of fields, including semiconductor electronics. In 2010, a group at UCLA demonstrated transistors built from graphene nanowire that have achieved speeds of 300 gigahertz with a channel length of 140 nm. Channel length describes the distance between the gate and source in a device, across which the aforementioned capacitance exists. According to Xiangfeng Duan of the UCLA team, obtaining a channel Visit www.eeweb.com

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length of 50 nm or below would allow them to achieve terahertz graphene transistors. The advantage of this is that graphene is a cheap and plentiful material that is relatively easy to work with and mass produce. This is especially true when compared to expensive heterostructures that need expensive epitaxy processes to produce.

Reducing Gate Delay The goal of any terahertz transistor technology is to reduce the gate delay and increase the drive current. Gate delay is the time it takes to move current across the channel from source to drain, while drive current is the amount of current flowing through the device when it is in saturation mode. Aside from high-κ dielectrics, goals necessary to achieve these conditions include low operating voltages and a depleted substrate transistor structure with raised sources and drains. Low operating voltages reduce the overall power demands of the device, allowing it to operate more efficiently. This means that even if there is a leakage current associated with the small size of the transistor, it will be minimized by the amount of power available to the package. Depleted substrate transistors reduce the number of carriers available in the substrate, reducing leakage across the channel between the source and drain. Fabricating the device with raised drain and source would reduce the resistance across the device, reducing the overall power dissipation—one of the most important goals. Below as Figure 3 is a picture of a typical MOSFET transistor. The black regions labeled ‘S’ and ‘D’ are the source and drain, respectively. If the black region were to rise above the insulating light blue layer and become thicker compared to it, that would be a rough picture of what a device fabricated with a raised source and drain might look like.

High speed transistors will change many industries when they are finally viable. Home solar installations will be able to become affordable as inverters and other power electronics can use low cost, high speed, and low power consumption transistors to convert energy generated by solar installations for home consumption with much less lost to heat. Communications applications will be able to implement terahertz transistors to build high speed networking devices, increasing the rates at which information can be transferred locally and also wirelessly. The Intel Core i7 generation has remained for nearly four years now because home computing processors have been stagnating, as transistors become too lossy at smaller sizes to make competent integrated circuits. Further advances not only in terahertz transistor fabrication but also in high-κ dielectrics and manufacturing techniques will make these devices better, perhaps salvaging Moore’s law for the foreseeable future.

About the Author Alex Toombs is a senior electrical engineering at the University of Notre Dame, concentrating in semiconductor devices and nanotechnology. His academic, professional and research experiences have exposed him to a wide variety of fields; from financial analysis to semiconductor device design; from quantum mechanics to Android application development; and from low-cost biology tool design to audio technology. Following his graduation in May 2013, he will be joining the cloud startup Apcera as a Software Engineer.

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STRAIN-GAUGE BASED SENSORS AND THEIR COMPLETE ANALOG INTERFACE INTEGRATION FOR SYSTEM-ON-CHIP BASED DESIGN

Umanath Kamath Senior Design Engineer at Cypress Semiconductors

train gauges are useful sensors to measure the strain of an object. They are attached using adhesives to the object which is under application of force. The resulting change in electrical resistance contains information of the applied force. This has varied application. The most common application is weighing scale design and force measurement in various mechanical machines. Pneumatic valves and civil engineering applications, impact sensors, and medical sensors are a few additional applications. With so many applications, strain gauges are useful industrial sensor components. In order to make an accurate measurement of the force applied, understanding the strain gauge and itâ&#x20AC;&#x2122;s parameters is important. The sensor signal (change in resistance) needs to be efficiently conditioned so that the resulting signal-tonoise ratio from the system is enough for determining accurately the magnitude of stress. It includes an analog signal conditioning path followed by conversion to digital code. This article begins with an understanding of a strain gauge and proceeds to the various non-idealities one has to be aware while interfacing them. Finally an example of system design for read-out of such sensors is shown using Cypressâ&#x20AC;&#x2122; Programmable System-on-Chip where analog integration is shown to be possible which improves the overall realization of such an interface circuit for strain gauges.

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Understanding a Strain Gauge Strain is defined as deformation (within elastic limits) of a body under the application of force. It is a dimensionless quantity and can either be tensile or compressive depending on the surface to which the strain gauge is stuck to.

factor. In order to compensate, manufacturers usually compensate for the gauge depending on the temperature co-efficient of the material to which it is tied. This reduces the thermal sensitivity but does not completely eliminate it.

Resistive Sensor Readout A common method to readout the sensor information is by wheatstone bridge configuration, where in the imbalance caused by changes in the sensor resistance results in a voltage difference.

Figure 1: Concept of Strain

When a body is under application of a uniaxial force, there is an extension in its length along same direction as shown in Figure 1. While elongating, it also experiences a perpendicular contraction. This is defined by the poisson’s ratio, which is the negative ratio of strain in the perpendicular direction to the strain in the axial direction. Typically a strain gauge has a serpentine/wound pattern in order to maximize the area of the conducting material under the effect of the applied force. The strain gauge is tied to the object which is directly under the force. This is known as carrier and transfers the change in physical quantity to a linear change in electrical resistance of the strain gauge. Gauge Factor (G) is used to relate this transformation and is given by:

Figure 2: Bridge method to readout resistive sensors

The information from the strain gauge is contained in the differential signal(Vo).

(1) The strain-gauge has a nominal resistance which is usually between few tens of Ω to few kΩ. The first challenge in interfacing the strain-gauge to the electronics is the magnitude of the resistance change. For example, a maximum value of 1m strain together with a gauge factor of 2 gives a resistance change of only 0.2%. This change is very small when we consider the nominal resistance. The typical change can be few Ω or much lower. In addition to dealing with such low resistance, there can be many sources of errors during the transfer of force to the resistance change. Ideally the force applied should have a proportional increase in the resistance. But this is not the case, due to the parasitics. For example, lead wires from the strain gauge to the circuit can have a larger resistance than the actual change. Besides, temperature co-efficients of the strain-gauge resistance can be crucial as the measurement becomes dependent on another

(2) One of the simple method to measure is to have one resistance of the bridge (R1) replaced by the strain gauge, while other resistance of the arm is kept at the nominal resistance of the strain gauge (R3=Rn). This is called the quarter bridge method. R2 and R4 are kept the same magnitude resistance. (3) Using the above assumptions in (1) gives us:

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(4)

One additional modification to above quarter bridge circuit is to add a very accurate shunt resistance across the bridge, which can be used to calibrate (Finding voltage difference with shunt resistance and without shunt resistance will give a measure of voltage change per 立 change). As seen from (3), the output voltage depends on a nonlinear term. In order to remove this, we can modify the bridge to include instead of R3 a dummy strain gauge perpendicular to the applied force. This gauge is affected by temperature in same way while is not affected by the sensing quantity. Going one step forward, we can have an active gauge, which is affected in opposite direction (compress as against elongate), thus making the output linearly dependent with twice the sensitivity as given by (4). This is known as half-bridge method.

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in a modified gauge factor which should be used for correction when calculating the effective force applied. This additional resistance causes offset, gain error due to dependence of lead-wire resistance on temperature is an additional source of error.

(7) The bridge is strapped on to the surface of the object to monitor the applied force. The lead wire introduces additional resistance as well as acts as source of noise pick-up. Including these in a quarter bridge-configurations gives us the output voltage as:

(8) (5) By including all active elements, with each arm having gauges affected by increase and decrease of the applied force. This is also known as full-bridge method, which involves using four strain gauges and results in most sensitive response in terms of magnitude. (6) For using full bridge, maximizing circuit sensitivity is by including two gauges in transverse direction while two in the axial direction of measurement of force. It should be noted that using four bridges does not increase circuit sensitivity. Therefore it is not economical to use four gauges; instead two gauges should be connected in series for one quarter of bridge.

Where factor A is given by

(9) The lead-wire resistance is overcome by using three-wire connection. Consider the three wire bridge circuit shown in Figure 3. RL indicates the wire resistance while supply (VS) is the excitation voltage to the bridge. Considering R1, R3 to be equal we can determine the resulting strain gauge resistance as:

Sources of Error Firstly in all the previous analysis, we assumed ideal components. In real-world applications, tolerances on the resistances come into effect. These result in saturation of voltage as well as a large common mode offset since the actual resistance change in bridge is small. Techniques such as offset-cancellation using auto-zeroing or modification to include a balancing circuit to adjust for the tolerances by means of trimming or calibration can overcome this problem.

Figure 3: Three wire connection to the strain gauge

The second error is due to the wire resistance which connects the gauge to the bridge, which desensitizes the strain gauge measurements. As seen in (6), this results

(10)

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Figure 4: Example of an interface circuit for strain gauge sensor using Programmable System-on-Chip

The middle resistance through which no current flows, acts as a sense lead. The assumption in order to have complete compensation is that the lead resistances are the same, have same temperature coefficients and are maintained at same temperature. In reality this is not true and hence other complex schemes such as four wire or the offset compensation mechanism are used.

Interface Circuitry Design for Strain Gauge In this section, we discuss a possible interface circuit for strain-gauge. Emphasis will be on showing how the complete design outside of the bridge can be encapsulated by Cypress’ Programmable System-on-Chip (PSoC 3) The overall design is shown in Figure 4. The configuration we use here is a modulation scheme where in sinusoidal signal is used. The PWM generator is used to feed the source excitation signal through a bandpass filter with center frequency of 5kHz to the bridge. An active bandpass filter is realized using the available opamps within the PSoC. The C_hpf, R_hpf control the high pass filter response while the R_gain,R_gainf provides the gain while R_lpf and C_lpf provides the low pass filtering. A cascade of these provide the required bandpass behavior which filters the sinusoid from the PWM generator to be fed to the bridge as excitation source. The signal is then amplified by the instrumentation amplifier( which uses the three opamp topology) before being down converted using the mixer and the same sinusoidal signal source. The down converted signal which contains the sensor

information (difference in node voltages) is digitized by a ∑∆ converter. The ∑∆ converter can have upto 20-bits of resolution based on the sample rate required from the read-out channel.

Conclusion We have described the strain gauge as a useful sensor in industrial application. Its principle of operation was discussed and various forms of errors which affect the measurement were shown. Finally we showed a method of interface circuitry using carrier frequency excitation of bridge. The complete design can be implemented on-chip using Cypress’ PSoC which increases the performance and provides the required accuracy in such interface applications.

References 1. “PSoC3 Architecture TRM”, Cypress Semiconductors Corp. 2. “Strain gauge selection (TN-505)”, Vishay precision group. 3. Wendladt et al., “Signal conditioning for resistive strain gauge sensors in mobile applications”, SomSed Workshop, TU Hamburg,2009.

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