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Motorola Offers Industryıs First Integrated Circuit for Three-Dimensional Electric Field Imaging

Device can replace more than 70 components, enables new class of low-level e-field imaging applications

The manufacturer says . . .

ChipCenter's Paul O'Shea says . . .

Motorola is offering the industryıs only integrated circuit (IC) that generates and detects a low-level electric field and powers and supports a microcontroller unit (MCU), all in a single chip. Now, design engineers creating embedded systems that require non-contact sensing and three-dimensional (3D) electric field (e-field) imaging may reduce system components and costs by employing the MC33794. This IC integrates support for a microcontroller and up to nine simple electrodes, which can be used independently to provide information on the size or location of an object in a weak electric field. The MC33794 is designed to replace numerous discrete devices previously needed to achieve the same functionality. Target applications of the IC include appliances, machine tools, automotive safety systems, and virtually any product or system that uses touch panel inputs as a user interface. "We chose Motorola's low-level e-field imaging IC because it is the first to offer such a high level of integration, which has allowed us to replace numerous ICs and discrete transistors and more than 70 passive components with one device. This has helped us to significantly reduce system size and increase reliability," said Phil Rittmueller, vice president of Elesys North America, Inc., a leading producer of automotive electronic safety systems. Elesys is now using the MC33794 in its SeatSentry occupant sensing system to stop an airbag from deploying on a child. The system generates low-level electric fields from multiple antennas mounted in the seatıs back cushion. The system can identify a child or small adult when the field detects and interprets a change in the current. By analyzing the changes in the various currents and by combining that information, the height of the passenger and his proximity to the side airbag is detected. If SeatSentry determines the occupant is a child or small adult, the airbag will not deploy. Currently, the occupant sensing system is in use on some Honda vehicles. The electric field imaging IC was developed through a collaborative effort between Motorolaıs Analog Products Division, Motorolaıs DigitalDNA Laboratory at the Massachusetts Institute of Technologyıs (MIT) Media Lab and Elesys North America (formerly NEC Technologies Automotive Electronics Division). "While simple capacitance measurement is one of the most mature kinds of sensing, working with Motorola and Elesys we found that tiny arrays of charging currents can be used to determine real three-dimensional geometries," said Professor Neil Gershenfeld, director of the MIT Media Labıs Center for Bits and Atoms. "Motorola and Elesys have brought this exciting capability to the market in a form that could help save lives today, and that promises to enable many new kinds of unobtrusive and responsive computer interfaces tomorrow." The MC33794 together with simple electrodes and an MCU can help designers readily implement 3D imaging and sensing. The ability of this combination to detect touch can be used to easily implement automatic on/off features in a wide variety of products. Other applications that may be simple to implement include; liquid level detection, spill detection or moisture sensing. For example, stoves with flat tops could be programmed such that when liquids boil over, the heat would be automatically reduced or turned off. More sophisticated applications include touch panel inputs. Here, an array of electrodes embedded in glass or behind a non-metallic surface could be sensed in a matrix fashion similar to contact switch inputs. This method eliminates membrane switches or expensive multi-layer resistive touch pads. Because the input is sensed without requiring a contact mechanism, problems of wear, dirt, and corrosion are virtually eliminated. This is especially beneficial in harsh environments. Another possibility is a virtual computer touch pad that acts as a mouse. With electrodes built in or under the desk, it could sense hand or finger positions on or above a desktop. The MC33794 includes: - High-purity tunable sine wave generator optimized for 120 kHz; - Supports up to nine electrodes and two reference capacitors; - Shield driver ı reduces capacitance effects caused by using coaxial cables connected to remote electrodes; - Lamp driver to show when a touch pad area is activated - MCU support ı includes watchdog and power-on-reset timers, a voltage regulator, and ISO 9141 physical layer communication interface. Manufactured with Motorolaıs SMARTMOS(tm) process, the MC33794 device combines high-density, high-speed logic with precision analog and high-voltage, high-current power circuitry. Motorola SPS, P.O. Box 17927, Denver, CO 80217 USA. Web Site

This product is pretty interesting because it offers some functions that allow customers to do things differently than before. It's an e-field sensor-imaging device that uses sensors in an electric field to sense position. Motorola didn't invent the electric field sensor; people have been using it for a long time. Yet, it is a complicated technique and many aspects of it stop designers from using it. To solve this issue Motorola put a solution in a single, easy to use chip. That allows you to think about the application and not worry about the design of the circuit. The design is inside the chip, so all you need to do is add some application software and add that to the microprocessor and you have a system. For example, it can be used anywhere you are trying to sense the presence of a person or intrusion into a space through non-contact sensing or monitoring. Additionally, you can do analog functions such as determining how full a glass is with liquid. This would be especially useful with toxic liquids, since you can also learn how full something is without putting a sensor down into the liquid. You can sense something on the inside from the outside or you can sense something on the outside coming in to a container. It all depends on where you put your electrodes. Most think that an electrode is just a piece of wire but it doesn't have to be. Unlike most sensors, this design doesn't need it to be an antenna. It could be carbon fiber or anything conductive, such as paint. For example, it uses an electrode at 100 pF at 120 kHz, therefore each of the nine electrodes has an impedance of 13 k ohms. So if you put something that is conductive with 100 ohms or even 1 k ohms it still makes an excellent conductor compared to the total impedance of the system. If you use a non-conductive material then the dielectric constant can be used as the sensor. The fact that it doesn't have to be a 100% conductor, like a wire, opens up a lot of possibilities. Motorola says they are just beginning to scratch the surface of where this could be used. Definitely it can be used where you want to do switching and since it can perform non-contact switching it could eliminate a relay. This product is an applications engineer's dream, allowing them to go into the lab and think of new ways to use the product. It is optimized for the Motorola processor but it will work with other processors. The only requirement of the microprocessor is that it must be able to do A-D converting. Motorola chose 5 V because the initial application was for 5 V and there are many popular 5 V processors. Though the company said they could easily change it to 3.3 V or less. The MC33794 has 9 electrodes and 2 reference pins so you can put a capacitor on the chip, providing a low and a high value to get an absolute measurement. Only one electrode is driven at a time. Each pin drives 120 kHz at 5 V. A signal goes through a resistor of 22 k ohms then passes through a multiplexor that selects which electrode to drive. The electric field looks like a capacitance to the electrodes and that forms a voltage divider between the 22 k ohm resistors and the external capacitance. The useable range is about 150 picoFarads. It also has a shield driver for coaxial cables. The shield driver has a multiplexer. Whatever electrode is being driven it goes into the buffer amplifier that has unity gain at 120 kHz and very near zero phase shift. It's called a shield driver because it's connected to the shield of the coaxial cable. If you have remote electrodes about 2 meters away from the circuit you can have a problem with the coaxial cable quickly driving the circuit up to the 100 pF limit. With this in mind the designers at Motorola decided to cancel out the effects of the coaxial cable. The shield driver is a very useful device but it is something that you have to work at to make sure you keep the gain and phase shift down to the very minimum, and of course in an IC you can do that. Nominally, it operates at 120 kHz but it can be adjusted to operate at 60 kHz and well above the 120 kHz. One of the problems that can occur is a result of harmonic frequency interference. To counter it there is a load distortion sine wave generator on board, which keeps the distortion and harmonic content down, preventing interference from AM radio reception localized around the electrodes. The MC33794 e-field imaging IC, housed in a 44-lead HSOP (heat sink outline package), is currently available in sample and production quantities. Unit pricing starts at $3.09 USD (suggested list price) for 10,000-piece quantities. Motorola offers an evaluation module to help reduce customers' system development cycle times. The module includes a pre-programmed MCU and necessary software for out-of-the-box measurements. The kit contains an MC33794, a Motorola 68HC908GR8 8-bit MCU, supporting components and RS232 communications port. The kit also includes documentation and software (on a CD-ROM) to assist in design evaluation. The evaluation module (part number KIT33794DHEVM) is available now for $70.00 USD (suggested list price). Data Sheet

General Backgrounder: Electric Field Imaging IC
Electric field (e-field) sensing provides a means for tracking electric current that can then be imaged to determine what is changing the current. It builds on the strengths of many alternative technologies, such as optical and acoustic sensing, while avoiding several of their weaknesses, such as not being able to detect objects that are not in the line of sight. The applications of low-level electric field imaging are vast, but, until recently, have been restricted by the difficulty in implementing practical electric field systems.

Evolution of E-Field Sensing and Imaging
Dr. Neil Gershenfeld is the director of the MIT Media Labıs Center for Bits and Atoms. From sensor development he originally did for a project with the cellist Yo-Yo Ma, Dr. Gershenfeldıs research group studied ways to measure charges on the human body. The resulting instrumentation and algorithms were then used by magicians Penn & Teller in a ısensor chairı that channeled fields from the body and transformed them into sounds. Phil Rittmueller, vice president of Elesys North America, saw this capability demonstrated, and quickly launched a joint project with the MIT Media Lab to develop and bring to market the low-level e-field imaging technique in order to meet an urgent industry need for ısmartı airbags. Elesys is a leading provider of automotive electronic safety systems.

Motorola was already a sponsor of Dr. Gershenfieldıs research through its DigitalDNA Laboratory at the MIT Media Lab. Engineers at Motorola realized that the 3D (three dimensional) electric charge measurement and inversion technique could be integrated onto a single chip that could enable a wide range of measurements that are contact-less, non-line-of-sight, fast, reliable and cost-effective. Elesys consequently worked with Motorola during the development of its MC33794 and is now using the chip in its occupant detection systems.

Motorolaıs MC33794
The MC33794 generates a high purity sign wave, steers it to one of nine electrodes and measures the resultant field coupled onto the non-driven electrode(s). A microcontroller (MCU) can be programmed to interpret the resulting data and make a determination of an object interfering in that field.

The MC33794 complements Motorolaıs line of microcontrollers, integrated microprocessors and digital signal processors with on-chip analog-to-digital converters, such as those found in the 68HC08, HCS12, 56800/E, MPC500, MPC5500 and ColdFire families. Motorola, according to analyst firm Gartner Dataquest, is the worldıs leading supplier of microcontrollers (June, 2002).

Example Target Applications
The MC33794 together with electrodes and a MCU can help designers readily implement 3D imaging and sensing. The ability of this combination to detect touch can be used in a wide range of applications from automotive to consumer electronics. For example, an automatic on/off feature can easily be implemented in home appliances. A few electrodes embedded in the body a hair dryer would allow the device to turn on when picked up and turn off when set down. A few more electrodes would allow the user to control the temperature or speed by merely touching a defined location on the barrel of the dryer.

Other applications that could be simple to implement include; liquid level detection, spill detection or moisture sensing. For example, stoves with flat tops could be programmed such that when liquids boil over, the heat would be automatically reduced or tuned off.

More sophisticated applications include touch panel inputs. Here, an array of electrodes embedded in glass or behind a non-metallic surface could be sensed in a matrix fashion similar to contact switch inputs. This method eliminates membrane switches or expensive multi-layer resistive touch pads. Because the input is sensed without requiring a contact mechanism, problems of wear, dirt, and corrosion are eliminated. This is especially beneficial in harsh environments.

Another possibility is a virtual computer touch pad that acts as a mouse. With electrodes built in or under the desk, it could sense hand or finger positions on or above a desktop. With advanced software techniques, the position of an object in 3D space can be determined. An example of this is the occupant detection system, such as the one offered by Elesys.

Elesys is now using the MC33794 in its SeatSentry occupant sensing system to stop an airbag from deploying on a child. The system generates low-level electric fields from multiple antennas mounted in the seatıs back cushion. The system can identify a child or small adult when the field detects and interprets a change in the current. By analyzing the changes in the various currents and by combining that information, the height of the passenger and his proximity to the side airbag is detected. If SeatSentry determines the occupant is a child or small adult, the airbag will not deploy. Currently, the occupant sensing system is in use on some Honda vehicles. For more information: www.motorola.com/analog

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