Speed sensor interface including differential comparator

A sensor interface circuit for a vehicle includes a signal conditioning module having at least one raw sensor signal input, and at least one conditioned sensor signal output, and a differential comparator module including a differential comparator and an adaptable hysteresis module. The adaptable hysteresis module provides a first hysteresis magnitude to the differential comparator when a sensor signal is below a threshold and a second hysteresis magnitude to the differential comparator when the sensor signal is above the threshold.

TECHNICAL FIELD

The present disclosure relates generally to vehicle sensor arrangements, and more specifically to a speed sensor interface circuit including a differential comparator.

BACKGROUND

Vehicles, such as commercial and industrial vehicles, utilize speed sensors to detect the rotational speed of one or more components within an engine, or elsewhere on the vehicle during operation of the vehicle. The output of the speed sensor is, in some examples, provided to a differential comparator and the differential comparator provides a readable output to a microprocessor indicating when the speed has exceeded a pre-determined threshold. Based on the readable output, the microprocessor generates controls, thereby controlling the rotating component or any other system within the vehicle.

In existing interface circuits for connecting the output of a speed sensor to a microprocessor, the magnitude of the hysteresis used in the processing of the sensor signal is increased in correspondence with a speed increase. Variable reluctance speed sensors, and sensors that operate in a similar fashion to variable reluctance speed sensors, have an output signal with a magnitude that increases in correspondence with an increase in speed. As a result, at zero or low speeds, the output of a variable reluctance speed sensor can be difficult to distinguish from noise on the output signal line, and a greater hysteresis is required. In contrast, at high speeds, the magnitude of the output signal is significantly larger than the noise, and minimal hysteresis is required to interpret the signal.

SUMMARY OF THE INVENTION

Disclosed is a sensor interface circuit including a signal conditioning module including at least one raw sensor signal input, and at least one conditioned sensor signal output, and a differential comparator module including a differential comparator and an adaptable hysteresis module, wherein the adaptable hysteresis module provides a first hysteresis magnitude to the differential comparator when a sensor signal is below a threshold and a second hysteresis magnitude to the differential comparator when the sensor signal is above the threshold, and wherein the first hysteresis magnitude is greater than the second hysteresis magnitude.

Also disclosed is a method for operating a sensor interface circuit including receiving a sensor signal from a sensor, comparing the sensor signal to at least one threshold using a hysteresis comparator, wherein a magnitude of hysteresis applied by the hysteresis comparator is a first hysteresis magnitude when the sensor signal is below a threshold, and wherein the magnitude of hysteresis applied by the hysteresis comparator is a second hysteresis magnitude when the sensor signal is above the threshold, and outputting a high signal to a controller when the sensor signal exceeds the threshold.

Also disclosed is a vehicle including a speed sensor, a signal interface module operable to receive and condition an output of the speed sensor, a hysteresis comparator module operable to compare the sensor against a threshold and output high when the sensor signal exceeds the threshold and output low when the sensor signal does not exceed the threshold, and wherein the hysteresis comparator module has a first hysteresis magnitude when the output of the speed sensor does not exceed the threshold, a second hysteresis magnitude when the output of the speed sensor does exceed the threshold, and the first hysteresis magnitude is greater than the second hysteresis magnitude, and a controller operable to receive an output of the hysteresis comparator module.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1schematically illustrates a vehicle10. The vehicle10includes multiple rotating components12and a speed sensor20measuring the speed of at least one of the rotating components12. In one example the speed sensor20is a variable reluctance speed sensor and the output magnitude of the speed sensor20increases as the speed of the rotating component12increases. A signal conditioning circuit30connects the output of the speed sensor20to an input of a differential comparator40. The differential comparator40includes an open collector output that is provided to a controller50. In some examples the controller50is a microprocessor. In other examples, the controller50is a general system controller including one or more microprocessors, as well as other control system components, and provides multiple vehicle systems controls. The signal conditioning circuit50and the differential comparator40are collectively referred to as an interface circuit60.

Due to the correspondence between the magnitude of the sensor signal and the speed of the sensed component, at low or zero speed, the magnitude of the output of the sensor20is low relative to the magnitude of noise present on the output signal. This condition is referred to as a low signal to noise ratio. If the signal to noise ratio is too low, a significant application of hysteresis in the signal conditioning circuit30is required in order to prevent the noise from inadvertently tripping the differential comparator module40and to prevent unstable oscillations. Hysteresis is the utilization of previous states of a signal to filter the current signal. In other words, hysteresis is the application of a positive feedback loop to the input terminal of a comparator. A larger hysteresis results in a greater accuracy despite a low signal to noise ratio. The utilization of a large hysteresis, however, increases a delay in response times.

When paired with standard speed sensors, existing interface circuits increase the hysteresis as the speed of the rotating component12increases or maintain the hysteresis at the same level independent of the speed of the rotating component12. Because of the low signal to noise ratio of variable reluctance sensors at low speeds, a large hysteresis is desired at zero and low speeds, while a low hysteresis is desirable at high speeds.

FIG. 2schematically illustrates a more detailed interface circuit200that encompasses the sensor20, the signal conditioning circuit30and the differential comparator40ofFIG. 1. A sensor110, such as a variable reluctance sensor, includes a positive output112and a negative output114. Each of the outputs112,114is provided to a signal conditioning circuit120. The signal conditioning circuit120processes the outputs112,114from the sensor110, and places the signal in a condition that is usable by a differential comparator module130.

By way of example, the signal conditioning circuit120can provide terminal dampening reflections, a filter or a voltage drop for high voltage sensor signals, filter noise from the sensor signal, and clamp the inputs to a maximum voltage, thereby preventing damage to the overall circuit200. In alternate examples, the signal conditioning circuit120can process and prepare the outputs112,114in other ways as needed by the corresponding differential comparator module130.

The signal conditioning module120provides two outputs, a positive output122and a negative output124. The positive output122is provided to a negative terminal of a differential comparator module130. Similarly, the negative output124is provided to a positive terminal of the comparator module130. The comparator module130compares the outputs122,124against two thresholds. The comparator module130output switches from low (zero volts) to high (positive voltage) when a high threshold is exceeded. The comparator130output switches from high to low when the sensed speed falls below a low threshold. In alternative examples, the low output of the comparator module130can be a non-zero voltage that is lower than the voltage of the high output. In one example, the differential comparator in the comparator module130is an open collector output differential comparator.

The output132of the differential comparator module130is provided to a switching module140, and to a microprocessor output134. The microprocessor output134provides the output of the differential comparator module130to a microprocessor in a controller50, such as the controller50illustrated inFIG. 1, thereby allowing the controller50to utilize the sensed speed in control operations.

The switching module140receives the output132of the comparator module as a switch control signal. The switching module140includes an input142connected to a voltage supply (not illustrated). In the illustrated example, the output132provided to the switching module140causes the switching module140to switch on when the output of the differential comparator module130is high. In alternate examples, the switching module140can be replaced with a current mirror circuit, and operate in a functionally similar manner.

The adaptable hysteresis module150includes a hysteresis circuit that provides a first, higher, hysteresis level to the comparator module130when the sensed speed is below a speed threshold (when the comparator output is low). The adaptable hysteresis module150then switches to a lower hysteresis level when the sensed speed exceeds a predetermined threshold (when the comparator output is high). The predetermined threshold is set based on the physical qualities of components, such as resistors and capacitors, within the adaptable hysteresis module150.

In operation, the on time of the switching module140controls whether the adaptable hysteresis module150is in a high hysteresis mode or a low hysteresis mode. As the on time of the switching module140is increased, the magnitude of voltage provided to the adaptable hysteresis module150through the switching module140in a given time period is increased. As a result, at least one capacitor, or similar charging component, within the adaptable hysteresis module150begins to charge at a faster rate than it discharges. Once the capacitor, or similar charging component, is fully charged, the adaptable hysteresis module150switches into the low hysteresis mode. As long as the capacitor, or similar charging component is charged, the adaptable hysteresis module150remains in the low hysteresis mode.

Once the speed of the sensed component falls below a threshold, the switching module140will no longer be on long enough in a given time period to charge the adaptable hysteresis module150faster than the adaptable hysteresis module150discharges, and the adaptable hysteresis module150reverts to the high hysteresis mode. A detailed example of the adaptable hysteresis module150is illustrated inFIG. 5and is discussed below.

With continued reference toFIG. 2, and with like numerals indicating like elements,FIG. 3illustrates an example conditioning circuit120for interfacing the raw sensor outputs112,114with a comparator module130. The signal conditioning circuit120includes a first filter210having a resistor212and a capacitor214. The first filter210provides an initial filtering of the raw sensor signal received from the speed sensor110. The filtered signal is then provided to a terminal block220having a pair of resistors222,224. The terminal block220dampens sensor reflections on the sensor outputs112,114and provides the dampened sensor signal to a second filter230and a voltage clamp240.

The second filter230operates in a similar fashion to the first filter210, and reduces noise on the sensor output. The voltage clamp240utilizes diodes242to clamp the sensor signal output at a maximum voltage, prior to outputting the sensor signals from the signal conditioning module120. The signal conditioning circuit120further includes a bias voltage block250, that provides a bias voltage from a voltage source (not pictured, connected to node252). The bias voltage biases the differential comparator module130to a desired voltage.

In alternate examples, the signal interfacing module120can include additional signal processing elements, or less signal processing blocks as warranted by the specific application.

With continued reference toFIGS. 1-3,FIG. 4schematically illustrates a differential comparator module130. Once the raw signal from the sensor110has been processed by the signal conditioning module120, the sensor signal is provided to a positive input terminal410of a differential comparator420, and a reference signal is provided to a negative input terminal412of the differential comparator420. The illustrated differential comparator420is a standard open ended differential comparator configured with an open collector output, and provides an output signal via a comparator output430. A feedback resistor420connects the output of the differential comparator420to the positive input410of the differential comparator420. As is understood by one of skill in the art having the benefit of this disclosure, the resistance of the feedback resistor440sets the zero crossings and the thresholds of the differential comparator420, thereby determining when the differential comparator420outputs high and when the differential comparator420outputs low according to known differential comparator principles.

A bias voltage450is provided through a bias resistor452to the output signal430, and the combined bias voltage450and output signal330is provided as an output432,434from the differential comparator module130. The two outputs432,434are identical, and one of the outputs432is provided to a controller or microprocessor to facilitate controls, while the other output434is provided to the switching module140.

As described above, the switching module140can be either a transistor bused switch module, such as a Field Effect Transistor (FET) circuit, or a current mirror circuit. In each of the examples, the switching module140on time depends on the input received from the differential comparator module130. In other words, the percentage of time during which the switching module140is on, alternately referred to as closed, during a total period of time increases as the sensed speed (and thus, the output of the differential comparator) increases.

The switching module140connects the bias voltage source to the adaptable hysteresis module150when the switching module140is on. When the switching rate of the switching module140exceeds a threshold (e.g. when the sensed speed exceeds a speed threshold), the rate at which the adaptable hysteresis module150is charged is faster than the rate at which the adaptable hysteresis module150is discharged. Once this condition begins occurring, the adaptable hysteresis module150switches into a low hysteresis mode corresponding to a speed exceeding the speed threshold. The adaptable hysteresis module150provides a hysteresis to the comparator module130, with the magnitude of the hysteresis depending on the on time of the switching module140, as described above.

In alternate examples, the adaptable hysteresis module150can be functionally replaced by a digital logic circuitry, which applies hysteresis to the signal using a pre-established logic circuit within a microprocessor. In the alternate examples, the output334is provided directly to the hysteresis microprocessor or logic circuit, the hysteresis microprocessor or logic circuit determines the correct hysteresis to apply, and applies the hysteresis. The microprocessor or logic circuit then provides an output to the negative input312of the differential comparator320, as in the solid state example adaptable hysteresis module150. One of skill in the art, having the benefit of this disclosure will be able to generate the necessary digital logic sequence to perform the above described function using known digital logic protocols.

With continued reference toFIG. 2,FIG. 5illustrates an example solid state circuit300for the adaptable hysteresis module150. The solid state circuit300includes an input310that is connected to the output of the switching module140ofFIG. 2. The input310receives a positive voltage from the switching module140when the switching module140is turned on, and no voltage when the switching module140is turned off.

The charge from the switching module140is passed through a conditioning element320, including resistors322,324and a diode326. The conditioning element320is connected to a neutral302, alternatively referred to as a ground. Also connected to the conditioning element320is a charge element330. In the illustrated example the charge element330is a capacitor302. One of skill in the art will recognize that alternative charge elements functioning in a similar capacity will provide functionally similar operations and can be substituted for the illustrated capacitor with minimal alterations.

Connected to the high side of the charge element330is a gate of a field effect transistor340. As a result of this connection, the charge element330controls the open/closed state of the FET340. While the charge element330is charging (e.g. not at full charge), the FET340is maintained in an open state. Once the charge element330has become fully charged, however, voltage provided from the input310is provided to the gate of the FET340, and the FET340is closed.

Also included in the adaptable hysteresis module,150is a pull up circuit350connected to a bias voltage at a bias voltage input352. The pull up circuit350includes two resistors354,356, and is connected to a gate of a hysteresis control transistor370. The pull up circuit350ensures that the gate of the hysteresis control transistor370remains high, thereby turning the hysteresis control transistor370on, as long as the FET340is open. Once the FET340becomes closed, a direct path to neutral302is provided for the bias voltage, and the gate of the hysteresis control transistor370is pulled down. When the FET340re-opens, the gate of the hysteresis control transistor370is brought back up by the pull up circuit350and the hysteresis control transistor370is turned on.

The hysteresis control transistor370controls the resistance in a hysteresis resistor network380by switching a resistor384into and out of the hysteresis resistor network380. When the hysteresis control transistor370is on (closed), the second resistor384in the hysteresis resistor network380is switched in, parallel to a first resistor382and provides an alternative path to neutral302. The inclusion of the parallel resistor384in turn decreases the overall resistance of the hysteresis resistor network380, thereby decreasing the amount of hysteresis applied to the signal being received by the differential comparator module130.

While each branch of the hysteresis resistor network380is symbolically illustrated as identical resistors382,284, one of skill in the art, having the benefit of this disclosure will understand that multiple different resistors can be included in each branch as needed, and thereby control the magnitude of the applied hysteresis in each condition.

Also included within the adaptable hysteresis control module300is an output360. The output360provides a binary output to a controller indicating what mode the adaptable hysteresis module300is in at a given time.

Furthermore, while the above system is described with two modes, high hysteresis and low hysteresis, one of skill in the art, having the benefit of this disclosure will understand that additional iterations of the adaptive hysteresis module can be utilized in a single system to provide additional levels of hysteresis control with minimal adaption to the circuits and systems described herein.

With continued reference toFIG. 1-5,FIG. 6is a flowchart illustrating a method500for operating the above described speed sensor interface circuit. When the vehicle is first started, the speed sensor outputs a low/zero speed signal in an “Output Low/Zero Speed Signal” step510. The interface circuit processes the sensor output in a processes sensor signal using the signal conditioning module in a “Condition Sensor Signal” step512.

The conditioned sensor signal is provided to a hysteresis comparator that applies hysteresis to the signal and compares the signal against a reference voltage in a “Compare Signal Against Thresholds” step520. At the startup, or when the previous speed outputs were low the hysteresis applied during the comparison are a higher level of hysteresis and enable a microprocessor to distinguish the sensor signal from a noise level.

When the detected speed of the speed sensor exceeds the high threshold of the hysteresis comparator, the hysteresis of the system is adjusted to a lower hysteresis value, and the comparator is switched to outputting a high value in an “Adjust Hysteresis When Speed Exceeds High Threshold” step530. Once the hysteresis has been set to a lower value, the hysteresis is maintained until the output of the speed sensor falls below the low threshold of the hysteresis comparator. When the speed sensor output falls below the low threshold, the hysteresis is adjusted again to return to the high hysteresis value corresponding to low/zero speed output of the speed sensor in an “Adjust Hysteresis When Speed Falls Below Low Threshold” step540. The hysteresis adjustments of steps530and540are continued throughout the course of vehicle operation, thereby ensuring that a low hysteresis is applied when the speed sensor detects a high speed and a high hysteresis is applied when the speed sensor detects a low or zero speed.

One of skill in the art, having the benefit of the above disclosure will be able to modify the system and method described above to incorporate additional hysteresis levels beyond a binary high/low using a similar circuit with only minor modifications to the above described circuit.