The present invention relates to non-destructive testing and non-destructive instruments (NDT/NDI) and more particularly to a Hall Effect probe and measurement device with compensation of measurement drift caused by temperature change.
Hall Effect sensors have been used in measurement devices such as thickness gages (e.g. Olympus NDT Magna Mike 8500) to accurately measure thickness of nonferrous materials. One of the most often seen applications is thickness measurement on plastic bottles. A Hall Effect sensor typically comprises a probe that has magnet(s) generating a primary magnetic field. Measurements are performed by holding the device's magnetic probe to one surface of the test material and placing a small steel target ball on the opposite surface. The target ball, in responding to the primary magnetic field, generates a secondary magnetic field, which varies according to the distance between the probe and the steel target ball. A Hall-effect sensor, which measures the strength of the secondary magnetic field, built into the probe measures the distance between the probe tip and target ball. Typically measurements are instantly displayed as easy-to-read digital readings on the device display panel.
One unique challenge encountered and overcome by the present disclosure involves a hall sensor that is not part of an integrated circuit on board the instrument. As required by many Hall Effect instruments or applications, a major portion of the circuitry is assembled on the main body of the instrument, which is coupled to the Hall Effect sensor or probe via wires or cables with a length that meets the operator's needs, e.g. 1 meter. This physical distance between the Hall Effect sensor and the instrument presents an unknown wiring and connector resistance. As the Hall Effect sensor is located in the probe and sensitive to temperature changes, it presents a unique challenge for the instrument to compensate the temperature of the probe assembly including both magnetic parts and the Hall effect sensor.
It also presents more unique challenges when the operation of a Hall effect sensor based instrument involves interchange of Hall sensor probes and gage and maintaining an accurate and temperature compensated system.
However, it's been widely observed that the accuracy of a measurement from a Hall effect thickness gage drifts with temperature quite noticeably. It is also known that the resistance of the Hall Effect sensor varies with temperature. Because the measurement is directly related to the resistance of the Hall Effect sensor, a change in temperature would result in a change in the Hall Effect resistance and a change in the result of the magnetic measurement. This is also called measurement drift due to temperature.
An existing effort made in an attempt to reduce this effect was to re-calibrate the instrument whenever the instrument is in a condition called “Ball-Off condition”, i.e. whenever there are no targets. By re-calibrating, adjustment is made so that the sensor is calibrated to the current testing conditions, including temperature. However, since this Ball-Off condition does not always occur, or occur frequently enough, the measurement could drift with temperature change without the knowledge of measurement taker or operator.
Another existing effort has been seen in U.S. Pat. No. 5,055,768 in which a temperature sensitive current source is deployed to solve the problem of Hall effect sensor sensitivity to temperature. This current source is intended to be part of the Hall effect sensor. However, the circuit as disclosed is limited to compensating temperature effects inside the Hall sensor residing on the same chip.
Yet another existing effort seen in U.S. Pat. No. 6,281,679 involves a system that uses a magnet and a Hall Effect sensor to measure distance. However, the magnets and the Hall sensor move in relation to each other. It teaches a method by which two Hall sensors are matched so that temperature is not a factor. It also addresses methods of regulating the temperature of the magnet and Hall sensors by auxiliary temperature control, including using circulated air. Yet, it failed to mention the challenge brought by and hence the solution to the issue of temperature variation between the locals of Hall probe and the processing circuit, which is located in the instrument.
U.S. Pat. No. 8,274,287 uses a magnet and a Hall sensor to detect disturbances in the field. It also employs a temperature sensor to control the temperature compensation of its measurement on quantity of metallic debris. However, the patent did not make use of the unique property and the subsequent advantages presented by Hall sensors' sensitivity to temperature. It did not make any effort in measuring changes of Hall sensors circuitry reading attributed to temperature change. In addition, it explicitly regards the temperature response as linear, which is not an accurate representation of this line of Hall sensor devices.
U.S. Pat. No. 6,154,027A uses a temperature reference circuit to control the current flow from a temperature-variable current source to a Hall-effect element according to sensed temperature conditions. It in a way compensates the temperature drift reflected in the current source. However, it does not, in Applicant's opinion, directly compensate the temperature effect within the Hall Effect sensor.
In U.S. Pat. No. 4,327,416 ('416), the temperature of the Hall element is measured by a thermistor to develop a temperature dependent voltage. In one embodiment, the Hall element voltage and temperature dependent voltage are digitalized and supplied to address the ROM to generate a temperature compensated Hall voltage. In another embodiment, the ROM is addressed by only the temperature dependent voltage to generate a correction voltage that is added to the Hall element voltage for temperature compensation. However, in Applicant's opinion, '416 does not take into account that the source of temperature change is from both the temperature drift within the Hall Effect sensor, and within the magnetic source, noting that it only relies on the thermistor to gauge the instant temperature. In addition, this effort does not use a second pair of voltage measurement from the Hall sensor, which could lead to temperature compensation with higher fidelity.
US Patent 2008/0074108 also relies reading from a temperature sensor to compensate on Hall Effect measurement. It does not use the voltage measurement of a second pair from the Hall Sensor as the basis to the calculation of temperature effect directly on the Hall Sensor itself, not just on the magnetic fields.
It is therefore a primary objective of the present disclosure to provide a Hall Effect instrument with the capability of compensating for temperature drift consistently, accurately and in real time of operation.
It is another objective of the present disclosure to accurately measure the Hall Effect sensor resistance via a four-point ohmmeter circuit to track the effect of temperature on the Hall Effect sensor.
It is yet another objective of the present disclosure to provide a Hall Effect instrument configured to constantly measure the change in Hall sensor resistance due to change in temperature and to derive a relationship between the temperature and the compensation index on a per probe basis, which has exhibited a deterministic difference observed by the present inventor.
It is yet another objective of the present disclosure to provide a Hall Effect instrument configured to make compensation of the measurement result based on system-wide temperature changes, including temperature changes caused by locale distance between the Hall sensor and the magnets, the Hall sensor (Hall probe) the processing circuit (the instrument), etc.
a and 3b are schematic circuit diagrams depicting the two-wire or four-wire ohmmeter circuit, respectively, used in the present disclosure to accurately measure the Hall Effect sensor resistance, which is highly sensitive to temperature change.
It should also be noted that some terms commonly used in the industry are interchangeably used in the present disclosure to denote the Hall Effect sensor. For example, “Hall Effect sensor”, “Hall probe” and “Hall sensor”, etc., all denote to the Hall Effect sensor shown as 203 in
Referring to
According to
As can be seen in
It should be appreciated that the temperature compensation function as novel aspect of the Hall instrument is largely carried out and executed concurrently with other normal operational functions of the Hall instrument, and can be built within the same components that otherwise serve other functions of the Hall sensor instrument. For instance, Hall sensor probe 101 both serves for Hall Effect measurement and temperature measurement with a temperature sensor 207. Data acquisition and processing module mainly serves the processing need for the Hall Effect measurement, and also provides the data processing need for temperature compensation as described in the present disclosure. In other words, the steps and or modules embodied in the present disclosure can largely co-use hardware components of the Hall effect instrument that are designed for the main purpose of the Hall Effect measurement, i.e., thickness measurement.
Optionally, the Hall Effect measurement system can be coupled with two types of probes in separate measurement sessions, the first type of probes includes temperature sensor 207, the second probe does not include any temperature sensor. Accordingly, data acquisition and processing module 103 optionally includes a mode selection module 1100, selecting modes between the first type of probes is coupled or the second type of probes is coupled. The mode selection can be done either automatically based on probe identification, which is an existing practice, or manually by means of operator input. The embodiment of the measurement system to be without temperature sensor 207 is described in details later associated with
It should also be appreciated that any of the steps or modules shown in
Reference now is turned to
A thickness measurement is taken by placing Hall sensor probe 101 between a nonferrous material to be measured and a target ball 201. Hall Effect sensor 203 measures the magnetic field between target ball 201 and Hall sensor probe 101. Magnets 206 encased in probe casing 209 generate a magnetic field between the probe and the target ball. This magnetic field is detected by Hall Effect sensor 203. It then sends the Hall Effect sensor measurement signals, the Probe Slope (described later) from the EEPROM 208 and the temperature from temperature sensor 207 into the data processing circuitry of the measurement system residing on the instrument for further processing.
In addition, temperature sensor 207 provides the temperature of the magnets Tmag for temperature compensation module 104. Lastly, Hall sensor probe 101 uses memory EEPROM 208 to record the probe specific information, such as the Probe Slope (described later) used in the temperature compensation module 104 and other probe identification parameters common to existing practice. How the Probe Slope was derived is subsequently explained in relation to
Referring to
According to
The three differential pairs of Hall Effect Sensor signals are defined as:
Continuing to the right-hand side of
It should be noted that the Hall Effect measurement circuit includes a sub-circuit that happens to be the same as that used in the existing practice involving a four-wire ohmmeter. One of the novel aspects of the present disclosure is to repurpose the four-wire circuit for Hall Effect measurement. The temperature compensation aspect of the operation also uses the signals retrieved from the four-wire circuit. It can therefore be understood that the four-wire ohmmeter itself had existed in prior practice. However, the use of such circuit for Hall Effect measurement, thickness measurement and for further making temperature compensation of such measurements are considered novel by the present disclosure.
Still referring to
With the accurate measurement of the Hall Effect sensor resistance by measuring the voltage across Points 1 and 2, we have VMON
Continuing with
Similarly, the Hall Effect sensor voltage, VSNS
And the constant current, ISRC, via VIMOD
Lastly VMON
Reference is now made to
It should be noted that voltage source 309 can be an AC or DC source, bearing in mind that drive current monitoring circuit 310 is effective when AC is used.
As voltage source 309 does not need to be extremely stable for excellent instrument performance, VIMOD is used to compensate or performance limitations of circuits inside the gage only in this preferred embodiment shown in
As an example, referring to
Reference is now turned to
Three signals, VMON
The three Temperature Uncompensated Filtered Data outputs are defined as:
Reference is now made to
Probe parameter temperature compensation module 501 receives four inputs: VMON
Probe slope temperature compensation module 502 receives two inputs: temperature (Tmag) from temperature sensor 207, and Probe Slope from EEPROM 208. This module 502 then produces a second compensation factor VCOMP
Probe temperature compensation index calculation module 503 receives two inputs: VCOMP
Referring now to
V
COMP
P
=V
SNS
F
+V
MON
F*(α+VSNS
wherein, Tmag is the temperature from temperature sensor 207;
α, β, γ and δ are constants based upon the manufacturing tolerances of Hall sensor probe 101. They can be obtained by those skilled in the art according to Eq. 1, and empirical data from conducting experiments on the probe of each probe type, yielding readings of the VSNS
Once VCOMP
As can be seen, the temperature compensation calculation according to Eq. 1 reflects temperature changes both in the Hall sensor, through reading VMON
It should be noted that in Eq. 1, it is assumed that ISRC is a constant and the factor represented by VIMON
It should be noted in connection to
For the embodiment of the measurement circuit 310 with voltage source monitoring (
V
COMP
P=(((VSNS
wherein there are six major contributing parts to VCOMP
It should be noted that reference temperature herein used in the equation (22° C.) is an exemplary ambient temperature. Different values can be used when calibration is done differently.
A, B, C and D are constants based upon the manufacturing tolerances of Hall sensor probe 101. They can be obtained by those skilled in the art according to Eq. 2, and empirical data from conducting experiments on the probe of each probe type, yielding readings of the VSNS
Once the VCOMP
Again it can be noted that Eq. 1 compensates measurement inaccuracies due to temperature drift quite well without using the factor related to VIMON
Eq. 2 provides better temperature compensation taking into account when ISCR varies. In addition, measurement variation due to interdependencies between the four inputs (VSNS
Reference is now made to
The Probe Slope derived from the experimental data graph similar to
In parallel to the calculation of VCOMP
V
COMP
S=Probe Slope*(Tmag−Reference Temperature) Eq. 3
Once VCOMP
The VCOMP
Temperature Compensation Index=VCOMP
Once the Temperature Compensated Index is calculated, it goes through a temperature compensated index output 534, and is sent to measurement conversion module 105.
Temperature Compensated Index, or compensated Hall Effect reading Vcomp, is then fed into measurement conversion module 105 and converted by a probe-target specific conversion, such as shown in
Reference now is made to
The alternative embodiment herein presented shares the same principle as presented in the parent co-pending U.S. application Ser. No. 14/077,322 of providing accurately measure the Hall Effect sensor resistance via a four-point ohmmeter circuit to track the effect of temperature on the Hall Effect sensor. However, as shown in
Therefore, as shown in
In this alternative embodiment, Hall Effect measurement circuit 102 is the same as that of in the co-pending U.S. application Ser. No. 14/077,322. For temperature compensation module 104, there is no signal feed from a temperature sensor. Therefore, for this alternative embodiment, there is no temperature sensor 207 and its associated temperature signal feed in
Subsequently, for this alternative embodiment without magnet temperature sensor, Eq. 1 is changed to:
V
COMP
P
=V
SNS
F
+V
MON
F*(α+VSNS
wherein, α, β, γ and δ are constants based upon the manufacturing tolerances of probe 101. They can be obtained by those skilled in the art according to Eq. 3, and empirical data from conducting experiments on the probe of each probe type, yielding readings of the VSNS
Further for this alternative embodiment without magnet temperature sensor, for circuit 310 in
It should be appreciated by those skilled in the art that various implementations can be achieve for embodiments with or without the usage of magnet temperature sensor. All of such implementations are within the scope of the present disclosure.
For example, one can design the Hall Effect measurement instrument by coding data acquisition and processing module 103 with only one set of equations Eq. 1 to 4 for both embodiments suiting for probes with or without the magnet temperature sensor, providing an option-toggle key of either a virtue or physical button allowing users to choose between the embodiments. The toggle function can also be automatically triggered once the probe is plugged in based on the probe identification, which is an existing practice to those skilled in the art. When option is chosen to be without the magnet temperature sensor, the coefficient C and D are set to zero in Eq. 2, and Tmag is automatically set to be 22° C. in Eq. 3. The toggle function can be implemented by mode selection module 1100 described in association with
It should be noted that usually when Hall sensor probes are relatively small with smaller magnets for applications of making thickness gages on thinner material, the temperature difference between the end of magnets and the tip of the Hall sensor can be negated. In this case, a Hall sensor probe without magnet temperature sensor can be used.
An alternative example would be for one to design the Hall Effect measurement instrument only for using probes without magnet temperature sensor. In this case, Eqs. 1-4 are correspondingly modified as follows and coded to data acquisition and processing module 103.
V
COMP
P
=V
SNS
F
+V
MON
F*(α+VSNS
V
COMP
P=(((VSNS
Temperature Compensation Index=VCOMP
Subsequently, there is no input or consideration for Vcomp
Further, for the instrument designed only for probes without magnet temperature sensor, the following elements are removed from data acquisition and processing module 103 in the following manner:
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure herein.
This application is a Continuation-in-Part of co-pending U.S. application Ser. No. 14/077,322 entitled A HALL EFFECT MEASUREMENT DEVICE WITH TEMPERATURE COMPENSATION and filed Nov. 12, 2013, which is herein incorporated by reference.
Number | Date | Country | |
---|---|---|---|
Parent | 14077322 | Nov 2013 | US |
Child | 14601961 | US |