1. MEMS gyroscopes offer a simple way to measure angular rate of rotation, in packages that easily attach to printed circuit boards, so they are a popular choice to serve as the feedback sensing element in many different types of motion control systems. In this type of function, noise in the angular rate signals (MEMS gyroscope output) can have a direct influence over critical system behaviors, such as platform stability and is often the defining factor in the level of precision that a MEMS gyroscope can support.
       
    Therefore, “low-noise” is a natural, guiding value for system architects and developers as they define and develop new motion control systems.  Taking that value (low-noise) a step further, translating critical system-level criteria, such as pointing accuracy, into noise metrics that are commonly available in MEMS gyroscope datasheets, is a very important part of early conceptual and architectural work.  Understanding the system’s dependence on gyroscope noise behaviors has a number of rewards, such as being able to establish relevant requirements for the feedback sensing element or, conversely, analyzing the system-level response to noise in a particular gyroscope. 

    Once system designers have a good understanding of this relationship, they can focus on mastering the two key areas of influence that they have over the noise behaviors in their angular rate feedback loops: (1) developing the most appropriate criteria for MEMS gyroscope selection and (2) preserving the available noise performance throughout the sensor’s integration process. 

    Motion control basics
       
    Developing a useful relationship between the noise behaviors in a MEMS gyroscope and how it impacts key system behaviors often starts with a basic understanding of how the system works.  Figure 1 offers an example architecture for a motion control system, which breaks the key system elements down into functional blocks. The functional objective for this type of system is to create a stable platform for personnel or equipment that can be sensitive to inertial motion.  One example application is for a microwave antenna on an autonomous vehicle platform, which is maneuvering through rough conditions at a speed that causes abrupt changes in vehicle orientation.  Without some real-time control of the pointing angle, these highly-directional antennas may not be able to support continuous communication, while experiencing this type of inertial motion. 
    Figure 1: Example Motion Control System Architecture
      
    The system in Figure 1 uses a servo motor, which will rotate in a manner that is equal and opposite of the rotation that the rest of the system will experience.  The feedback loop starts with a MEMS gyroscope, which observes the rate of rotation (ωG) on the “stabilized platform.”  The gyroscope’s angular rate signals then feed into application-specific digital signal processing that includes filtering, calibration, alignment and integration to produce real-time, orientation feedback, (φE). The servo motor’s control signal (φCOR) comes from a comparison of this feedback signal, with the “commanded” orientation (φCMD), which may come from a central mission control system or simply represent the orientation that supports ideal operation of the equipment on the platform.
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  2. Editor's note: When I come across a really excellent tutorial for designers, I like to share it with the EDN audience. The following is one such lesson in designing a robust product and meeting customer goals at its best. Since it is also a medical product with all the added approvals and certifications that come along with such a design, this example will be all the more useful to designers. Plus, I really like Nuvation's talented design team and they can demonstrate how to execute a proper design methodology with professionalism.

       
    PERSONAL HEALTH MONITORING • REGULATORY COMPLIANCE • CONNECTIVITY STANDARDS • COMPONENT OBSOLESCENCE • NEW PRODUCT INTRODUCTION • DESIGN FOR MANUFACTURE • PROJECT RISK MITIGATION

     
      

     
    Project overview

    A Nuvation Engineering client in the tele-health industry was seeking assistance upgrading a health monitoring device used by patients who are managing their care at home. The device collects data from various personal health monitoring devices (PHM) and uploads it to a central monitoring station manned by live agents. The client was primarily a health monitoring services provider and developing electronic devices was not their core business.

    They needed the assistance of an engineering firm that could:
    • Work with an RFP that was based on functional requirements and not complex technical specifications
    • Provide up-front visibility of the entire project effort and costs from initial design to market-ready product
    • Possess the diverse skill sets needed to execute both software and hardware development
    • Manage all the complexities of medical and electronic device product testing and regulatory certification
    • Manage the project until ready-to-ship products were rolling off the production line.

     
    Personal health monitoring (PHM) devices being used in the home are connected to a device that collects and transmits the customer’s health status to a central monitoring station.

    Requirements

    The current device was several years old and some components had reached parts obsolescence. The device could also only support a single PHM device and needed to support multiple devices simultaneously. Support also needed to be added for newer communication technologies since the device was currently limited to plain old telephone service (POTS) as the only mode of data transfer to the cloud.

    The new device needed to: 

    • Collect health information via USB and Bluetooth from multiple PHM devices simultaneously (e.g. blood-glucose monitors, blood pressure monitors, pulse oximeters, etc.)
    • Upload PHM device data to the cloud via Internet, cellular networks, and home phone line
    • Be designed to meet North American and European EMC and Electrical Safety standards
    • Use components with lifecycles that exceeded the planned product life
    • Be manufactured at a price point that was well within the average home-care patient’s budget
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  3. These days, a lot of digital design work involves little more than patching together a few highly integrated blocks. It seems as if everything is VLSI (and CPLD/FPGA). A lot is, but not everything. "Discrete" logic ICs are still to be found.
    Let's briefly review some historical logic basics. SSI (small-scale integration) referred to ICs containing a few gates or flip-flops. MSI (medium-scale integration) was used for things like counters, comparators, decoders, encoders, and other more esoteric functions. LSI (large-sca... you know the rest) was used to denote devices such as memories, calculator chips, keyboard encoders, and eight-bit microprocessors. I'm not quite sure when VLSI (very large...) came into vogue – perhaps with 16-bit processors. (I find the claims in Wikipedia suspect.)
    In the discrete days, optimizing digital designs often meant finding le chip juste, and sometimes a bit of lateral thinking helped repurpose an IC to a function the designers may never have envisioned. There were a couple of standard series of logic parts:
    The 7400 series started out as TTL (transistor-transistor logic; an acronym still widely used to describe logic levels). Over the years, it spawned many sub-series, such as 74L, 74S, 74F, and 74LS. When CMOS processes became fast enough, we started seeing 74HC, 74AC, and so on. Many of these parts are still available, though less common parts in less common families have disappeared. Those who lived through the era can still recite a large number of parts by heart: 7400 is a quad NAND gate, 7404 is a hex inverter, 7432 is a quad OR, 7493 is a four-bit counter, 7474 is a dual flip-flop... OK, I'll stop.
    The other common series was metal-gate CMOS – the 4000 series. This is also still available, and though it's slow, it has the sometimes useful attribute of working from a very wide supply range (3-18V). There has also been some cross-fertilization. One may find a 74C00: 4000 technology in 7400 clothing (pinout), or a 74HC4060: 4000 functionality implemented with the faster 74HC silicon-gate technology.
    Of course, the introduction of programmable logic (PLAs and PALs) started a revolution in design. This story is entertainingly told in the award-winning book The Soul of a New Machine. But that's another story.
    Where is standard discrete logic still used? I would surmise that the most common parts are 8/16/32-bit buffers, transceivers, and registers, and the various forms of "tiny" logic – chips rooted in the 7400 past but containing only one or two gates or flip-flops, and coming in small six- or eight-pin packages. These parts minimize size and let you place logic exactly where you need it.
    To illustrate some uses of discrete logic, let me describe some examples from past designs.
    The first falls into the "lateral thinking" category. I was updating a small datacom protocol converter. The system address bus was 20 bits wide, but one of the new peripheral chips had a four-channel DMA controller that drove only 16 bits. The problem percolated in my head for a while, and then the answer appeared to me. A 74670. This chip is a four-word by four-bit dual-port register file. Very MSI. I hooked it up so the processor could write to the registers as part of DMA configuration. The outputs drove the top four bits of the address when the DMA controller had the bus. The controller had two outputs indicating the channel in use, which were used to address the register file. Talk about a perfect fit.
    The 74670 register file block diagram

    Does the 74670 still exist? Digikey and Jameco think so.
    A good example of 4000 series CMOS use is from an audio processor design, ca. 1999. Here, in an otherwise all-analog box, the low power and slow speed of the CMOS chips was an advantage. The main use of the parts was for volume control. I needed to control four signals, and I deemed the use of "LogDACs" preferable to an expensive, inaccurate, and custom four-section pot. The actual volume control was an incremental encoder, and a flip-flop plus a few gates converted those pulses to clock and up/down signals to drive an eight-bit counter. A few more gates and some RC networks generated the addressing and strobes required to write to the DACs. I guess a $1 PIC could have done the job too, but I find this kind of constraints-limited design challenging and rewarding.
    My last example is even more archaic – an analog music synthesizer module I designed for a high school project. Here we have counters, comparators, latches, multiplexers, a number of gates, and, at the heart, a pair of 2101 SRAMs (that's right – 256×4 each). All this logic & memory implemented a VCO and envelope generator with a programmable waveshape. What fun.

     
    So, the next time you need to brush a pretty bit of logic onto your PCB canvas, give the old discretes a try. There's life in them still.
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