Controls: Direct and Digital (Feb. 2008)
January 31, 2008
The ongoing revolution in electronics technology has spurred a demand for advanced digital sensors. To help meet this demand, Inprox Technology Corp., Boston, has developed advanced inductive-based and capacitive-based digital sensor technology. The technology was originally, developed for advanced aerospace engine controls, but has since evolved from these specialized engine and flight-control systems into designs and products for a diversified set of critical applications involving measurements based on inductive and capacitive platforms including: position, pressure, vibration, torque, level, speed, deposition and temperature. Applications now include high-volume products in the home appliance and consumer electronics segments.
Branded as inGen Direct™, this technology suite provides a real time, continuously variable-frequency output in the form of a square wave. The ITC sensor circuit produces this square wave in one step, without the need for signal conditioning electronics, a digital simplification that often bridges the divide between analog devices and digital networks. The inGen Direct sensor output is a read-only signal that can be ported directly to the Generic I/O port (GIO) of any processor. (See Fig. 1.) In this methodology, the speed and accuracy of the processor is part of the sensor performance equation. The square-wave output is recognized as demonstrating stability to 6 significant figures, so accuracy of better than 0.25 percent of full scale is typically achievable. The output frequency is entirely scaleable and can be adjusted for the particulars of any system requirements.
Some of the key features to ITC’s digital sensor technology designs are: increased bandwidth and mean dynamic response rates (operating ranges and rates set between 10 – 700 kHz or 1-20 MHz); elimination of all signal conditioning electronics from design; the utilization of a distinct time-based measurement system; high accuracy; the use of novel targets and target materials; the acquisition of expanded sets of secondary data; high sensor operating temperatures of –100 DegC to 650 DegC (-76 DegF to 1,200 DegF); and a lower cost metric in production and accompanying systems-level reductions.
The widespread use of low-cost microprocessors allows digital sensor technology companies like ITC to offer attractive cost equations that are able to offload hardware and electronics burdens onto lower cost software implementations. This transition from hard solutions to soft solutions is a trend that continues in multiple markets, including those related to commercial products. For companies where software implementation and architecture changes are cumbersome, OEMs can choose the option of a “smart” integrated system with onboard or remotely located microprocessors for flexible application configurability and functionality.
There are different approaches in output and signal interface architectures; ITC takes particular advantage of modern electronics’ ability to measure time with great precision. In turn, this translates into greater sensor precision and, in many measurement applications, can provide a new set of capabilities not previously feasible in analog systems (using current cost metrics) with customers looking to improve the performance/cost impact.
The implementation of a variable-frequency, digital, square-wave output is achieved by counting the edges of the square wave or using the square wave as a counter enabler in order to realize high accuracy measurements. The speed of the processor determines the accuracy.
In terms of direct comparisons between digital and analog signals, inGen Direct can offer generally higher resolution, a reduction in errors, elimination of time delays, elimination of signal conditioning (A/D and D/A), better dynamic response, higher stability (where oscillation is separated from ground), fewer components (error terms engineered out), more immunity to interference, and easier transmission and signal processing.
The translation of analog signals through signal conditioning electronics to interface with digital networks causes delays, compounds errors and increases costs. ITC’s Direct Digital Transfer Signal Methodology can provide a read-only output using a method that is similar to pulse-width-modulation without the control feature. This is pulse-width-measurement in a sense, where designers can opt for the digital–to–digital interface they want without the need for protocols, handshakes, and carrier signals present in competing analog, or even more complicated digital sensor outputs.
ITC’s time-based system can provide measurement ranges that are designed in and span operating ranges anywhere between 10 kHz to 20 MHz, depending on specific application requirements, target materials, appropriate dynamic bandwidth, and microprocessor, with decentralized or centralized control requirements in mind.
Temperature measurementThe following demonstrates how an inGen Direct sensor can be used in a temperature measurement application. In this example, the temperature transducer is a reactive component in the frequency generating circuit. Change in temperature alters the reactance (capacitance/inductance) of the transducer, which changes the frequency of the sensor output. The inGen Direct sensor has an operating frequency range between 200 kHz to 220 kHz, for a bandwidth of 20 kHz over a measurement range of 600 DegC.
The sensor is used in conjunction with a processor that has a 400 MHz counter. Resolution of 1 percent of measurement range is required per the application. Making a measurement over 10 sensor periods will yield 20,000 counts at 200 kHz and 18,180 counts at 220 kHz. The mean count deviation per 6 DegC over the measurement range is 19.1 counts, thus resolution of 1 percent of the measurement range is attained.
The refresh rate with these conditions must also be examined. For example, the transducer can experience a change of temperature of 600 Deg C in 1 second. The same 1 percent resolution across the bandwidth requires a measurement every 10 ms. The longest sensor measurement time interval at 20 kHz is 500 us. So it yields 20 measurements in the 10 ms window.
Compare that to a typical, signal-conditioned temperature measurement scheme such as an RTD or thermocouple using an amplifier and A/D converter. The amplifier has gain error and temperature error. The A/D converter has input error (sample and hold, etc.) and resolution error (depending on whether it is 8, 12, 16 bits). The A/D also requires control signals from the controller to initiate a conversion. Then, at the end of a conversion, the controller must read the data. Many converters use a serial data bus, so the data rate is the baud rate times the number of bits. The controller now has a data word that represents a temperature measurement term with gain and A/D errors and time delays for conversion and data transmission.
Motor speed controlAnother potential application for inGen Direct sensors is in the realm of motor speed control. The obvious purpose of a closed-loop (feedback-based) motor speed controller is to receive a signal representing the actual speed, compare that to the demanded speed, and then adjust to drive the motor at the demanded speed. With a number of variables affecting motor speed (load, friction, temperature, etc.), sensing motion plays an integral part to the motor control system.
Feedback-based speed control offers better motor control and positioning over open-loop control (sensing motor drive parameters only), but typically has been a costly and complicated addition to the system assembly. ITC technology offers a cost-effective solution by removing the complexity on both the sensor-integration side and on the motor-control system side.
For speed-control applications where speed is directly proportional to the supply voltage (as with a DC motor), or for applications where speed is related to the switching frequency (as with an AC induction motor or 3-phase induction motors), the ITC sensor platform offers a high-resolution, fast-dynamic-response, high-accuracy approach over the range from zero to full speed.
While there are many non-contact optical and magnetic encoder systems available, many designers of motor-control systems have found the ITC technology to be an attractive option. For example, ITC has worked with DC motor manufacturers on control applications that involve rotary actuator position measurement using inGen Direct technology.
In one particular design, the change in the integral of dielectric in the field created by the sensor transducer changes reactance in the transducer and thus the frequency. In this case, a one-lobe cam was used for a target with a pair of sensors in order to produce a quadrature output. The target is fixed to the rotating shaft. The system gives absolute zero speed/position measurement, as well as direction of rotation and rotary speed. The same signal-processing methodology is used as in the previous temperature example above. Alternately, a single sensor can be used with a multi-toothed wheel to give an incremental indication of speed and position.
Benefits of using this approach include:
- Low measurement hysteresis.
- True zero speed output with one sensor.
- Not effected by dust, oil, moisture, etc.
- Lightweight targets can be used. (Aluminum, metal-coated plastic, etc.).
- Minimal effect from motor EMI.
- Fast dynamic response. Signal refresh to 500 kHz or more.
- Longer MTBF from fewer components.
- Digital output.
- Low power consumption.
- High resolution.
- Real time self-diagnostics. Continuous sensor output can be monitored for fault conditions/pre-warning.
- Non-contact design.
- Does not use permanent magnets. Exhibits no sensitivity to heat and no drift over time.
- Economical design.
- Can also be used to detect motor shaft run out/bearing run-out.
- Both absolute and incremental position output options.
- Temperature operating range of –110 DegC to 450/650 DegC operability.
- Rotational speed acceleration and de-acceleration.
- No induced magnetic field.
- Can be used as rev-limiters or governors.
- Ramp up speed and braking abilities are inherent in this sensor.
In addition to those benefits, the system integration of the digital sensor technology can provide more. When implementing the technology, the controller does not require a particular bus (PCI, Ethernet, USB, RS232, ISA, etc). The controller does not require any input filters, sample-and-hold circuitry, or translation hardware (A/D Converters, serial-to-parallel converters, level shifters, etc). No special interface IC’s, or amplifier boards, or required software tools or languages are needed.
Appliance project Another potential application for the technology can be realized in the home appliance industry, where the sensors can be used in ovens to detect the position of racks in the self-cleaning cycle. In this case, a change in the integral of dielectric in the field created by the sensor transducer changes reactance in the transducer and thus the frequency. ITC has developed a patent-pending sensor array for the cooking appliance market that can be embedded directly in most oven walls; providing a position measurement that operates up to 450 DegC and survives up to 600 DegC. The sensor array can be flush mounted within the oven’s inner cavity and detect an interior rack in any position for self-cleaning cycle monitoring. It is also possible to provide color matching between the sensors and the oven walls to make them invisible to the consumers.
ConclusionPrevious generations of analog devices and analog measurements have long been the standard of most industries. The costs of discrete components and signal conditioning electronics can be re-distributed in the digital approach on to the lower cost metrics of software.
Benefits on the component side include the ability to measure small physical changes very accurately; robust temperature operability ranges of –60 DegC to 650 DegC; the elimination of magnets and/or ferrites (in inductive designs); the ability to utilize novel targets and target materials, and the ability to gather secondary data for health monitoring and predictive maintenance.
Benefits on the systems level side include the Digital Signal Transfer Methodology itself; the associated hardware cost reductions (wire, cable, targets, etc.), possible manufacturing process changes and reduction in circuit board space; and positive interface and common bus options.
Digital sensor technology, therefore, represents a clear and fundamental shift forward on both the component and systems levels.
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