A multitude of devices, vehicles, tools and equipment possess rotating elements such as motors, gears and teethed targets. In many instances, the rotational speed of these elements needs to be monitored and/or controlled to ensure proper system performance. The need to quantify speed may be driven by safety, performance or environmental concerns. Space constraints, power availability and harsh environments often limit solution options.
Demands on the monitoring equipment include high operational air gaps (separation between the equipment and the monitoring electronics), consistent duty cycle and excellent overall reliability. Hall effecting sensing can usually meet these requirements, but is increasingly challenged by systems requiring high data-rate information through a limited packaging space. The resulting compact target geometry generates small magnetic signals that are often difficult to resolve with traditional sensor technology. Manufacturing variation and wear result in targets and gears that often have significant run-out and/or wobble. Moreover, severe operating temperature requirements and the need to provide stable operation over a wide temperature range push the limits of traditional IC fabrication technology.
Recent developments in BiCMOS Hall effect fabrication, circuit design and packaging technology have produced major advances towards meeting gear tooth speed sensing application requirements. The development of self-calibration techniques, threshold detection circuitry, greater on-chip protection circuitry and smaller packaging has dramatically increased usable air gaps, switching accuracy and overall reliability. These advances have been instrumental in meeting the needs of high data rate speed sensing applications.
The Hall Effect
In 1879 Sir Edwin Hall first discovered the principle that was to be given his name. The basic principle is that when a bias voltage is applied to the silicon plate via two current contacts, an electric field is created and a current is forced. If the plate is then exposed to a perpendicular magnetic induction , the Hall electric field gives rise to the appearance of the Hall voltage between the two sense contacts. This Hall voltage, Vh, is proportional to the amount of magnetic field applied normally to the plate. This basic principle is the foundation for all Hall effect sensors today.
Figure 1: The Hall Effect.
Back Biasing for Gear Tooth Sensing
Because Hall effect ICs detect the strength of a magnetic field, sensing is accomplished by changing the magnetic field passing through the IC. This is commonly accomplished through linear or rotary motion of a multi-pole magnet. However, it is often more practical and cost effective to use a simple target or gear made from a ferrous material such as low-carbon steel.
The position and rotation of ferrous gear teeth can be detected using the Hall effect by measuring the changes induced by the gear at the face of an opposing magnet. The presence of the ferrous gear alters the reluctance of the magnetic circuit and creates a concentration effect at the magnet surface. These changes can be measured by using a Hall effect sensing element located on the magnet face1. (Figure 2 shows the basic back-biased sensor configuration.)
Figure 2: Back-Biased Sensor Configuration, Facing a Tooth (top) and Facing a Valley (bottom).
Two basic methods of sensing are possible in the back-biased package by using either a single-element or differential sensor configuration. A single Hall sensor element supplies a voltage proportional to the absolute value of the magnetic field induced normal to the element. Amplification and conditioning of the single element signal can provide a digital representation of the target profile. Alternatively, a differential Hall element pair provides a signal proportional to the slope of the incident magnetic field. The differential sensor's output is typically zero when it is faced with either a tooth or valley; it produces a signal only when it is in the region of a tooth edge. Both of these solutions offer unique advantages but also have significant limitations when used in their basic configurations.2
Single Element Gear Tooth Sensing
When the single element Hall sensor is biased with a standard dipole magnet, a large field is induced through the Hall plate when no target is present. With traditional sintered magnets that possess high-energy products, this baseline magnetic field resides in the range of 1000 to 3000 gauss. Comparatively, the amplitude of the field induced by fine-pitched targets passing by the back-biased sensor can be less than 100 gauss.
Figure 3: Single Element Sensing.
The discrimination of this low amplitude signal is difficult and the necessary circuitry is typically AC coupled. Furthermore, the slope of the magnetic field varies tremendously with air gap making accurate edge detection difficult and limiting switch point accuracy. Additionally, the back-biased field values may change due to concentration effects caused by varying valley widths and target eccentricities resulting in a non-uniform baseline.
Differential Gear Tooth Sensing
The differential Hall element configuration eliminates the undesired effects of the back-biased field through the process of subtraction. Since each of the two Hall elements on the IC sees approximately the same back-biased field, the differential baseline field is close to zero gauss. As the target moves by the sensor, the resultant signal is still relatively small, as it was in the single element configuration. However since the background field is also very small and close to zero, classical threshold crossing techniques can be used to generate a digital representation of the target.4 Refer to Figure 4: Differential Element Sensing.
Figure 4: Differential Element Sensing.
Gear Tooth Speed Sensing
The challenges in developing a high performance differential gear tooth speed sensor include the elimination of the false switching and large duty cycle variations associated with classic threshold sensors and the reduction of the switch point drift over temperature. Though a traditional peak-detecting scheme could resolve these issues, it has an inherent liability of requiring an external capacitor for peak holding. This capacitor represents additional cost, reduced system robustness and high temperature performance limitations due to capacitance roll-off. To meet all cost and performance requirements, an advanced differential device is needed.
Sensor IC complexity has increased dramatically over the last decade. Component count in sensor ICs has risen from a count of 50 in 1980 to more than 7000 today. The semiconductor processes have emerged from Standard Bipolar and CMOS to BiCMOS and BCD (Bipolar/CMOS/DMOS) merged technology processes.5
Allegro Microsystems' merged semiconductor process, DABIC5 (Digital Analog BiCMOS version 5) provides for precision analog signal processing and complex logic functions in a fully integrated monolithic silicon IC. Bipolar components allow the design of low offset amplifiers while the CMOS components provide efficient A-D converter design. The increased component density provided by the DABIC5 process supports the development of sophisticated algorithms.
Using DABIC5, continuous operating temperatures of -40C to 150C can be accommodated. Surges beyond this range can be withstood up to a device junction temperature of 190C. DABIC5 also provides for reduced temperature induced switch point shift compared to previous technology, resulting in a more consistent device over the full operating temperature range.
DABIC5 also allows for design features like reverse power supply protection, transient protections, wide operating voltage range and output short circuit protection.
Digital Threshold Sensing
To preclude the poor vibration performance associated with fixed threshold or zero-crossing switching, the device utilizes dynamic thresholds. To ensure that the switch points always occur within the dynamic range of the normalized signal, the thresholds are established as a percentage of the peak-to-peak signal. Since the highest degree of accuracy is realized at the zero-crossing point of the amplified magnetic signal, the thresholds are established very close to this level. (Figure 5 shows how thresholds are used to generate the output signal.)
Figure 5: Threshold Switching.
Traditional threshold detection provides limitations in the presence of significant system offset. Large teeth or valleys can produce large magnetic offsets due to installation tilt or non-uniform back biasing magnets. In these undesirable scenarios, a threshold can be crossed and create a timing shift in the position of an edge by as much as one tooth.
Since threshold sensing is less immune to system offsets than is peak detecting, threshold sensing is best suited for targets that produce sinusoidal signals. Since most speed applications meet this criterion, threshold sensing with gain adjustment is the optimum solution.
Self Calibration: AGC
Timing accuracy and duty cycle performance of typical sensors is greatly influenced by the variation in the slope of the magnetic signal with air gap. A large gain is typically required to generate a suitable signal at large air gaps. At close air gaps this gain makes the signal exceed the dynamic range of the internal operational amplifier and results in signal clipping. The resulting variation in the slopes of the signals over the operational air gap range causes large timing accuracy and duty cycle shifts.
Improved timing accuracy and duty cycle performance is achieved through self-calibration circuitry. The self-calibration is accomplished through an Allegro patented automatic gain control (AGC) technique. This circuitry is engaged at device power up and measures signal amplitude to normalize the device gain. (Figure 6a shows peak-to-peak signals over air gap for a device without AGC while figure 6b shows similar data for a device with AGC. Note the consistent switch point that would result over air gap with the signals in the AGC circuit.)
Figure 6: Differential Output, without AGC (top) and with AGC (bottom).
Since sequential targets do not have a signature region, they can be "learned" by the sensor within a single set of output transitions. This rapid learning allows AGC to be disabled just after start-up and provides an accurate and consistent output signal almost immediately.
Self Calibration: Update
In addition to quick start-up, optimum performance requires adaptation to changing conditions. These changes can be short in duration (transient) or continuous; they can have a very small effect or a very large impact on the magnetic signal. For example, a small amount of target run-out, or wobble, may have a small effect on the magnetic signal, but it continuously effects the sensor operation. Alternatively, a physical impact on a system component that momentarily changes air gap could have a very large effect that might occur only once in the life of the system. The optimum compensation circuit must handle these extremes and a multitude of cases in between.
The update algorithm is enabled just after start-up and continuously monitors the magnetic signal to ensure that switch points are established in the most accurate manner. Accuracy is optimized through the establishment of switch points on the most recently acquired peaks. The update circuit is never disabled and is re-initialized when power is reset. (Figures 7 shows how the update algorithm establishes switch points.)
Figure 7: Update Establishing Operate Point (top) and Release Point (bottom).
In addition to providing vibration immunity, threshold sensing close to the zero crossing also provides improved timing accuracy, and therefore, duty cycle, over the installation air gap.
Figure 8: Duty Cycle Performance.
Recent innovations in packaging help the device achieve high air gap performance and meet restrictive space requirements. The patented Allegro SG package allows the magnet to sit closer to the IC than it does in traditional packaging approaches. This geometric advantage allows the device to meet high air gap performance with a smaller magnet than that found in standard packages. The resultant small package very easily fits in the tight spacing dictated by ABS applications.
Figure 9: SG Package Isometric.
Additional benefits are realized in the SG package through the single step molding operation. Typical sensors are manufactured with successive mechanical assembly steps. The clearances required for assembly result in voids throughout the interior of the sensor assembly. Since the SG has no such air gaps, heat dissipation is improved and air entrapment that could occur during subsequent potting operations is eliminated.
A further improvement in heat dissipation is realized through the reduced heat conduction path of the SG package. Because of the single step molding process, the layer of plastic normally seen between the IC's lead frame and the magnet is eliminated. The increased thermal conductivity signifies greater heat sinking capacity by the magnet, allowing for operation at higher ambient temperatures. The lead configuration of the SG allows for easy PWB surface mounting and the simple attachment of a bypass capacitor. With the spacing provided between the two leads, an axial leaded capacitor can be welded across them. Attachment to the sensor can be made with a lead frame to preclude the use of a costly PWB.
Figure 10: Exploded View of SB Package.
Figure 11: SB Package Cross Section.
Figure 12: SG Package Cross-Section.
Large Operational Air Gap
AGC provides for accurate switching over a wide air gap range.
Immunity to Target Run-out and Wobble
The continuous update algorithm provides excellent immunity to run-out and wobble allowing the device to retain excellent accuracy and duty cycle performance.
AGC and the dynamically established thresholds provide an air gap independent switching hysteresis, which results in significant vibration immunity.
Improved Timing Accuracy and Duty Cycle
Signals normalized by AGC over air gap, low hysteresis thresholds and continuous update all contribute to optimized switching consistency.
High Temperature Operation
DABIC5 process and single thermoset molding step for improved heat dissipation provide operation at elevated temperatures. Inherent offset cancellation, optimized circuitry and updating algorithm provide for stable operation over temperature.
Operation Independent of Supply Voltage
Internal regulator provides stable operation over the full operating voltage range. DABIC5 supports supply voltages up to 26.5 V.
Operation Independent of Speed
High bandwidth provides high-speed operation. Zero speed capability is provided.
Simple System Implementation
Integrated back-biased package with optimized magnet in a small, robust package requires minimal expertise in magnetic circuit design
Insensitivity to Tilt
The differential sensor, which possesses an inherent ability to reject offsets, when integrated into the package with a back-biased magnet, provides insensitivity to tilt.
New advances in Hall effect sensor technology are providing solutions to the increasing demands of speed sensing applications. Advances in IC fabrication allow for increased component density, which supports the needs of sophisticated algorithms. These circuits address the problems created by applications with increased air gaps, mounting-induced magnetic offsets and target run-out.
The advanced circuitry and differential sensor configuration avoid the problems associated with traditional sensors. Additionally, the resulting sensor output signal is independent of both speed and supply voltage and is very insensitive to changes in the mechanical and magnetic systems.
IC advances also produce more robust components that provide operation at higher temperatures and voltages. Packaging advances reduce sensor size without compromising performance and contribute to an expanded operating temperature range.
The advancements in Hall effect speed sensing technology can be evaluated with the Allegro Microsystems ATS665LSG.
1 Milano, Shaun, and Ravi Vig. "Self-Calibrating Hall Effect Gear Tooth Sensing Technology for Digital Powertrain Speed and Position Measurement." 30th Annual International Symposium on Automotive Technology & Automation, Volume 2; ISATA (1997).