WO2019083807A1 - Pcb-based gauge system - Google Patents

Abstract

A force sensor integrated directly into a printed circuit board (PCB) is provided that utilizes standard PCB manufacturing techniques. Traces on the PCB are arranged such that as a force is exerted on or imparted to the board, the inductance of the circuit changes, thereby causing a shift in the resonant frequency. The shift in resonant frequency may be correlated to the force such that the PCB functions as a force gauge. Signal conditioning and processing circuitry, including non- volatile memory for storage of calibration data, may be located on the same PCB as the sensor functionality. The disclosed force sensor is low cost, requires no connector, has a low profile, reduces variation due to assembly of separate components, is easy to calibrate, and is easily modified, prototyped, and manufactured to meet a variety of design criteria and commercial applications.

Description

PCB-BASED GAUGE SYSTEM

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority benefit to a provisional patent application filed with the U.S. Patent Office on October 26, 2017, entitled "PCB-Based Gauge System," which was assigned Serial No. 62/577,416. The entire content of the foregoing provisional patent application is incorporated herein by reference.

BACKGROUND

[0002] A traditional strain gauge generally includes resistive foil whose resistance varies with applied force. (j¾t£Bs; w^ A connector is attached to the strain gauge, which is then wired to a printed circuit board ("PCB"), that is outfitted with the necessary electrical components to calculate the applied force. As force is applied, a strain is produced, which causes a change in the resistance of the strain gauge, which can then be used in an arrangement to result in a direct current "DC" voltage signal, wherein an electrical component, located on the PCB, calculates the amount of force applied.

[0003] This technology, although widely used in the industry, has several drawbacks. As a result of the small signal produced by the strain gauge, significant signal amplification is generally required before the electrical component can process the signal. A combination of the connector and wires, and the signal amplification creates noise that requires significant signal conditioning. Furthermore, as a result of the many components, strain gauge assemblies are relatively large. The additional components, e.g., connector and wires, as well as the hand installed sensing elements, e.g., foil strain gauge, semiconductor, or piezoelectric element, create variations between each strain gauge. Therefore, calibration is required to "zero-out" each strain gauge, a time-consuming and potentially unreliable task.

[0004] Commercially available load cells that are well calibrated are often suited to high accuracy measurements and often cost in the hundreds of dollars. On the other end of the spectrum, inexpensive, mass produced, load cells are not calibrated and the designs are too bulky to be used in low profile wearable devices.

[0005] A PCB typically includes a non-conductive base, e.g., fiberglass, with conductive lines, e.g., copper, printed or etched on it. The non-conductive material further generally includes various electrical components, e.g., capacitors, resistors, microprocessors, among others, that are connected to each other by the conductive lines.

[0006] A sensor that is smaller in size, low cost, precise and is easily adaptable for new sensor applications is needed. Further, a sensor that requires little calibration is also desired, and more preferably, a sensor that requires no calibration. Additionally, the ability to calibrate several sensors at one time would be of commercial benefit.

SUMMARY

[0007] The present disclosure relates to a force sensor integrated directly into a printed circuit board (PCB) that utilizes standard PCB manufacturing techniques. Although the force sensor is described using tensile force measurement as one example, the methodologies discussed herein can be applied to other loading conditions, e.g., compression and bending. Traces on a PCB are arranged such that as a force is exerted on or imparted to the board, the inductance of the circuit changes, thereby causing a shift in the resonant frequency. The shift in resonant frequency may be advantageously correlated to the force such that the PCB functions as a force gauge.

[0008] Further, all signal conditioning and processing circuitry, including non- volatile memory for storage of calibration data, can be located on the same PCB as the sensor functionality. This results in a low-cost force sensor that requires no connector, has a low profile, reduces variation due to assembly of separate components, is easy to calibrate, and is easily modified, prototyped, and manufactured to meet a variety of design criteria and commercial applications.

[0009] Additional features, functions and benefits of the disclosed force sensor will be apparent from the description which follows, particularly when read in conjunction with the appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] To assist those of skill in the art in making and using the disclosed PCB force sensor, reference is made to the appended figures wherein:

[0011] FIG. 1 depicts a PCB with a force sensor integrated within.

[0012] FIGS. 2A-2E depict various trace layouts for use on a PCB. [0013] FIG. 3 depicts a PCB with a metal core substrate and an insulating layer.

[0014] FIG. 4 depicts a stand-alone force sensor.

[0015] FIGS. 5A-5B depict a PCB calibration fixture.

[0016] FIG. 6 depicts a reference design generator for creating force sensors.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

[0017] An exemplary embodiment of the present disclosure includes a force sensor integrated directly into a printed circuit board (PCB) and utilizes standard PCB manufacturing techniques. The disclosed force sensor advantageously includes traces that function to sense a force applied to the PCB. The traces of the PCB are arranged such that as a force is exerted on the board, the inductance of the circuit changes, thereby causing a shift in the resonant frequency. Although the force sensor is described using tensile force measurement as an example, the methodologies discussed herein are applicable to other loading conditions, e.g., compression and bending.

PCB Assembly

[0018] With reference to Fig. 1 , an exemplary PCB assembly 10 includes a PCB substrate 12, which is the primary structure of a circuit board. The PCB substrate 12 can be made from various materials, e.g., fiberglass, metal core substrates, and polymers, each offering their own advantages for use in potential force sensor applications. Located on at least one face of the PCB substrate 12 is the force sensor 14, which includes trace layers 16 and mounting holes 18. Trace designs can vary depending on the trace material and the desired output, examples of such include, but are not limited to, square, spiral, zig zag, coil, etc.

[0019] Specifically, FIGS. 2A-2E depict several exemplary trace 16 design options, illustrated with force vectors for optimum orientation of the traces in relation to the force vector. The sensor traces may be arranged on one or more layers of the circuit board.

[0020] Further included on the PCB substrate 12 are electrical components 20 for processing the sensor data, e.g., signal conditioning and processing circuitry, and no n- volatile memory for storage of calibration data. Additional components can be included on the PCB substrate 12, as will be apparent to one skilled in the art. Integrating the processing components with the force sensing technology 14 results in a low-cost force sensor that requires no connector, has a low profile, reduces variation due to assembly of separate components, is easy to calibrate, and is easily modified, prototyped, and manufactured to meet a variety of design criteria.

[0021] The most common substrate material is fiberglass, with the most common type being FR-4. However, the present invention is not limited to such and additional fiberglass types can be used. Since fiberglass is the most common substrate material, manufacturing techniques are standardized and inexpensive. Further, fiberglass offers a wider range of trace materials as opposed to other substrate materials. However, a disadvantage of fiberglass for use in a force sensor is the mechanical response to the load. Fiberglass is typically very stiff, resulting in a smaller strain, which leads to a comparably smaller signal. Since fiberglass is very stiff, it is more susceptible to cracking and once it begins to break, the entire structure weakens. To compensate for these weaknesses in material properties, the sensor must be designed to operate within a safe operating range to prevent permanent mechanical deformation of the substrate and/or traces.

[0022] With reference to FIG. 3, an additional substrate material is a metal core substrate 100, wherein a metal substrate 102, e.g., aluminum, copper, or the like, is insulated from the traces 16 by at least one insulating layer 54. Metal core substrate boards are commonly used in high heat applications where the high thermal conductivity of the metal is used to facilitate heat dissipation. Metal is better suited for loading over fiberglass because metal is more ductile and may strain harden to prevent a weakened structure. However, metal substrates are typically more expensive and have manufacturing limitations as compared to fiberglass substrates.

[0023] Another class of substrate materials are polymers, for example, Teflon, however, other materials can be used. Polymers produce a larger strain for a given geometry than fiberglass boards, and are less susceptible to cracks that propagate over time. However, polymers are more susceptible to creep and permanent plastic deformation over time than metal substrates. Such deformation would produce a signal output error.

[0024] The substrate material choice may be dependent on the type of trace being used, as certain traces may not be available with certain substrates due to etching chemistries. In any of the material choices, the thickness of the substrate can be varied to allow the board to be loaded to different values. A thicker substrate will be able to handle larger forces, but will produce a smaller signal for the same force. A thinner substrate will produce a larger signal but will be limited to lower forces. This feature is beneficial in producing a variety of sensor designs with differing sensing ranges and sensitivities while maintaining the same trace geometry and other circuit elements. Initial tests have shown that a 0.063" fiberglass FR-4 sensor with a width of 0.25" and copper traces arranged in a single layer coil design can be repeatedly loaded to 100 Newtons in tension to produce a reliable output signal.

[0025] PCB traces 16 are the patterned material that electrically connects the various components on the circuit board. As a force is applied to the circuit board load cell, the trace layer experiences a strain comparable to the strain of the substrate material, as long as the two remain laminated together. To ensure the layers stay laminated together, a safe force operating range for each sensor must be established through modeling and testing. The most common trace material is copper, due in part to its low resistance and ease of patterning into traces. To produce a trace design that has the inductance necessary to produce a signal, a long, narrow trace design must be utilized. This can be accomplished by routing the traces back and forth and/or by having multiple layers of traces. The traces then become a planar inductor with the self-inductance of the copper traces changing as force is applied to the circuit board. Advantageously, copper traces are inexpensive, easily patterned, and available on a wide range of substrate materials.

[0026] The planar inductor formed by the pcb traces will be connected in parallel with a capacitor to form an inductive-capacitive tank ("LC Tank"). The LC Tank will have a characteristic resonant frequency. The resonant frequency of the sensor will be measured by an integrated circuit, such as the LDC1612 by Texas Instruments, or by equivalent circuitry. The chip drives the sensor circuit and reference circuit at their resonant AC frequencies and outputs the frequencies on two 28-bit resolution channels. Further, by placing non-volatile memory on the circuit board, the sensor calibration constants can be written directly to the memory for later reading by the microcontroller, discussed in more detail in the calibration section.

Force Measurement

[0027] The present invention utilizes an alternating current ("AC") circuit, wherein the resonant frequency is determined by the inductance and capacitance of the PCB traces and a capacitor in parallel. Significantly, the shift of the self-inductance of the PCB trace design can be used without an additional conductive target. As a load is exerted on an active sensor, the inductance changes, which causes a shift in the resonant frequency of the sensor. The change in inductance can be caused by a variety of factors, including, but not limited to, one or a combination of, variation in trace length, trace width, and/or the distance between the traces.

[0028] A constant capacitor, with a value larger than the self-capacitance of the traces, was added in parallel with the trace coil. As a result, the primary shift in resonant frequency then becomes the shift of the inductance of the traces, and eliminates capacitive coupling to external sources. The shift in resonant frequency will be caused by the changing inductance and capacitance of the circuit as follows, where L is the inductance, and C is the capacitance: i

t fsenssr ¾s _^rr,

[0029] In an exemplary embodiment, the active force sensor is oriented on the PCB to experience an axial load, which is when the load is applied in the plane of the PCB. Axial loading is most suited to measure tensile forces, as opposed to compressive forces. When the desired axis of measurement is axial, it is necessary to either fix the direction of the load or compensate for off-axis loading or bending. For example, in a bending configuration, traces can be placed on both sides of the PCB which will cancel out the effects of bending, as one will be compressed and the other will be stretched. This technique of two active force sensors is easily implemented on PCBs because many boards already have a top and bottom trace layer.

[0030] In another embodiment, the bending configuration enables the two active sensors to measure the compressive forces and the tensile forces during bending. One of the sensors will exhibit an increased inductance, and the other will show a decreased inductance. By comparing the ratio of the two, the force can then be determined. The bending configuration will typically result in a larger strain and corresponding signal. In this configuration, two separate measurement traces may be used and located on the top and bottom of the circuit board to compensate for temperature or other external noise sources. To prevent electrical interference between the two sensors, it may be advantageous to turn the sensors on and off in rapid succession, so that only one sensor is active at a time. Without a proper adhesion method, the internal stresses of the PCB as a result of the bending may cause the layers to delaminate. To prevent delamination, an operating force range will be determined for each sensor design. Furthermore, without a proper PCB substrate material, bending may result in a higher likelihood of material failure. Additionally, depending on the size of the sensor and the size of the displacement due to bending, the signal may not be linear.

[0031] The systems of the present invention may be sensitive to temperature variation, which can cause the electrical and mechanical characteristics of the traces to change, thereby resulting in a shift in inductance and resonant frequency. To compensate for temperature variation, a second force sensor, designed identical to the first, may be advantageously integrated into the PCB. In contrast to the first force sensor, the second force sensor will not experience any load, but rather will be utilized as a reference sensor. The second force sensor will be in either an area of the PCB that does not experience load or the traces will be oriented in a direction such that the traces will not measure the load, for example, perpendicular to the primary loading axis. The effects of the temperature will be determined from the frequency shift of the second force sensor, compared to the first force sensor, which will isolate the shift of the first force sensor due to the tensile load.

[0032] In a situation where there are two active sensors, temperature variation can be accomplished by assessing the ratio of the two outputs. In doing so, the temperature effects would cancel and the output would be the applied force.

Calibration of PCB

[0033] Traditionally, because strain gauges are assembled by hand, there is a potential for an increased variation between different gauges. By integrating the force sensor into the PCB, and utilizing PCB manufacturing, the present invention should experience less of a variation between PCBs, thereby possibly avoiding calibrating each sensor individually. However, if a greater variation is found and/or if increased accuracy is required, calibration constants can be recorded by gathering data when there is no load on the sensor and when a predetermined load is applied, collectively referred to as a known load. These constants can be written onto the no n- volatile memory located on the PCB. To calibrate the reference sensor, if necessary, a constant would be gathered when no force is applied. A constant when force is applied is unnecessary because the reference sensor will not see any load. [0034] Calibration can occur after the PCB is assembled or after the PCB has been installed into a final product. Alternatively, if the sensor takes the form of a stand-alone sensor module 14, as illustrated in FIG. 4, i.e., an off-the-shelf pre-calibrated sensor, the calibration can occur as part of the manufacturing process.

[0035] If calibration of the PCB 10 occurs before assembly into the final product, or if the PCB is a stand-alone sensor module, a calibration fixture 200 can be utilized (see FIG. 5A and FIG. 5B). Such a fixture is not the only means of calibration, but can make calibration more repeatable and much quicker. In one embodiment, the calibration fixture 200 can hold the PCB 10 in place on a calibration base 202 and electrical contacts 204, e.g., spring pins, would make electrical contact with the circuit board 12. Through the electrical contacts, the calibration fixture 200 would read the sensor output at a known force, calculate the calibration constant using a processor on the calibration fixture 200, and then write the calibration constant(s) to the non-volatile memory located on the PCB 10. Utilizing the calibration fixture 200 allows for calibration of processor- less PCB. The known force, as mentioned above, will be at least two points, a zero load and a predetermined load. The predetermined load can be applied by a spring 206, a hanging weight on a pulley 208, or other methods to produce consistent force, as will be apparent to one skilled in the art.

[0036] If calibration occurs after the final product has been assembled, a fixture could be used to load known forces onto the sensor. Raw sensor data could be collected when no load is applied and when the predetermined load is applied. The final product could be triggered wirelessly, e.g., Bluetooth^®, or physically, e.g., pressing a button, to collect the raw data and internally calculate the calibration constants. These constants could be used when the product is in the field.

Electrical Shielding

[0037] Electromagnetic interference ("EMI") often impacts the output or performance of electronic devices. In one example, EMI from a running motor, located approximately one foot away from the inductive PCB sensors, created significant noise in the sensor's output. As a result, shielding the inductive coil from electromagnetic radiation is vital to obtain clean sensor data. The alternating magnetic field produced by the sensor can also induce an eddy current in nearby conductive materials, resulting in a coupled inductance that will affect the sensor output. [0038] One shielding technique requires integrating the shield into the PCB. In an exemplary embodiment, the sensor technology, located on the middle layer of the PCB, is encased between a top and bottom shielding layer. The shielding can be, but is not limited to, a solid copper pour or a hatched shielding pattern.

[0039] In another embodiment, ferrite sheets can be used to shield the PCB. The ferrite can be applied directly to the PCB using an adhesive. Ferrite is a desirable material to use with inductive sensors because it can concentrate and redirect magnetic flux, increase the dynamic range of the sensor, and shield the sensor from undesired metal near the sensing coil. The ferrite sheets help prevent the effects of induced eddy current in nearby conductors.

[0040] In another embodiment, shields can be integrated into the mechanical device either locally, directly enclosing/attached to specific components on the PCB, or globally, enclosing the entire sensor. At the local level, in one example, EMI cans are attached to the board around those components that require shielding. For example, with regards to the inductive PCB sensor, the can would enclose the sensor coil. EMI cans prevent electromagnetic fields from entering and exiting by creating a Faraday cage around the component by utilizing the EMI cans and a ground plane of the PCB.

[0041] At the global level, EMI shields can enclose the entire sensor or device to either reflect or absorb the electromagnetic radiation. In one embodiment, the entire case of the device is made out of a conductive material, e.g., copper or aluminum, which is ideal for reflection because of its high conductivity. However, additional and/or alternative materials can be utilized, as will be apparent to one skilled in the art. Alternatively, materials with a high magnetic permeability are ideal for absorption such as an adhesive ferrite sheet. As a result, whether the EMI shield is reflective or absorptive, electromagnetic radiation is prevented from impacting any part of the PCB.

[0042] Furthermore, in another embodiment, a similar outcome to the above shields can be accomplished by lining, either internally or externally, a non-metal housing with a conductive material. For example, materials such as aluminum foil or conductive fabrics that are coated with highly conductive metals can be used. However, additional lining materials are available and the present invention is not limited to the above examples. Software Interface for Generating Coil Designs

[0043] To assist potential customers in designing a custom sensor for their PCBs, without spending the development time needed to optimize the sensor design, a user can input desired qualities of the sensor into a software and the software would auto generate sensor designs in accordance with the user's criteria. With reference to FIG. 6, an input screen for a reference design generator 300 to assist the user's development, as described above. Examples of some of the input criteria, include: force sensing range; desired resolution; desired dimensions; max coil size; minimum trace width; and number of layers of board. However, the above input criteria is not an exhaustive list and additional criteria can be included.

[0044] After the software generates at least one potential design, based on the user's input criteria, users can sort resulting coils, if there is more than one resulting coil, by power consumption, size, material, or any criteria that was not specified during the input criteria step. Next, the software will output the design file per the user's specifications. In a preferable embodiment, the software would output the file in standard PCB design software formats with the required parallel capacitor value. Additionally, the software can output the configuration parameters for the inductive sensor integrated circuit, for example, the drive current, gain, or any other configuration register in a chip, e.g., LDC chip.

[0045] Although the present disclosure has been provided with reference to exemplary embodiments thereof, the present disclosure is not limited by or to such exemplary embodiments. Rather, the present disclosure is susceptible to various modifications, refinements and/or variations without departing from the spirit or scope of the present disclosure.

Claims

1. A force sensing device, comprising:

a PCB substrate; and

at least one trace defined thereon;

wherein a load imparted to the PCB substrate translates to an inductance and a resonant frequency generated by the trace that may be correlated to such load.

2. A force sensing device of claim 1 , wherein the change in resonant frequency correlates to the amount of load applied.

3. A force sensing device of claim 1, wherein the PCB substrate is fiberglass.

4. A force sensing device of claim 1, wherein the PCB substrate is a metal core substrate.

5. A force sensing device of claim 1, wherein the PCB substrate is a polymer.

6. A force sensing device of claim 1, wherein the trace defines, at least in part, a coil.

7. A force sensing device of claim 1, wherein the trace defines, at least in part, a zigzag.

8. A force sensing device of claim 1, wherein the trace defines, at least in part, a square.

9. A force sensing device of claim 1 , wherein the trace is made of copper.

10. A force sensing device of claim 1, wherein electrical components are affixed to the PCB substrate.

11. A force sensing device of claim 1, wherein a second trace is defined opposite the first trace.

12. A force sensing device of claim 11, wherein the second trace functions as a reference.

13. A force sensing device of claim 1, wherein at least a portion of the substrate is electromagnetically shielded.

14. A force sensing device of claim 13, wherein at least a portion of the substrate is encased between a top and bottom shielding layer.

15. A force sensing device of claim 13, further comprising one or more EMI cans that define a Faraday cage around at least a portion of the substrate.

16. A method of calibrating a PCB force sensing device, the method comprising:

mounting the PCB force sensing device on a calibration fixture, the PCB force sensing device including a substrate and at least one trace defined thereon ;

connecting at least one electrical contact to the substrate; and

applying a known force to the substrate;

wherein the calibration fixture reads a resonant frequency generated by the at least one trace at the known force, calculates a calibration constant, and then writes the calibration constant to a non- volatile memory associated with the PCB sensing device.

17. The method according to claim 16, wherein the known force includes a measurement at a zero-load and a predetermined load.

18. The method according to claim 17, wherein the predetermined load is applied by a spring.

19. The method according to claim 17, wherein the predetermined load is applied by a weight attached to a pulley.

20. The method according to claim 16, wherein the electrical contact is a spring pin.

The method according to claim 16, wherein the PCB utilizes an alternating current.