GB2372825A

GB2372825A - Brake force measurement using a load cell

Info

Publication number: GB2372825A
Application number: GB0105070A
Authority: GB
Inventors: Andy Pitcher; Simon Roberts
Original assignee: NOVATECH MEASUREMENTS Ltd
Current assignee: NOVATECH MEASUREMENTS Ltd
Priority date: 2001-03-01
Filing date: 2001-03-01
Publication date: 2002-09-04
Also published as: GB0105070D0; WO2002070917A1

Abstract

A load cell 1 for use in a braking system (e.g. an anti-lock braking system), between the vehicle structure and a brake caliper 2, comprises a spring element having a strain sensing section 4 on which a shear stress sensing element 5 is mounted. An end fixing adjoins and is coaxial with the spring element. The strain sensing section 4 may be a T or an I section. The load cell may have a mass concentration surrounding the strain sensing section 4 to help remove extraneous axial loads.

Description

Braking Force Measurement The present invention relates to a load cell for measuring a braking force by measurement of a resultant force exerted by a brake caliper. In addition the invention relates to a device comprising a load cell for accurate measurement of a braking force, an anti-lock braking system comprising the device.

Within the context of this specification the word"comprises"is taken to mean "includes, among other things". It is not intended to be construed as"consists of only".

It is well known that the measurement of a braking force can provide important diagnostic information and/or influence safety of a vehicle. The results of this measurement can be used for fault diagnosis and/or as the basis for providing an improved braking system, for example an anti-lock braking system.

Known devices for measurement of braking force detect pressure of brake fluid in hydraulic brake systems or rotation of road wheels, neither of which feed back information on the real braking force achieved by a wheel. Ie, the known detection devices are not capable of distinguishing the braking force provided by a skidding wheel on a dry road surface from the force provided by a similar skidding wheel on an ice surface. However, the resulting braking effort achieved by both skidding wheels is considerably different. This is because the known devices do not measure direct forces relative to the retarding frictional forces (or braking torque) of a wheel and a road surface.

In the light of this, it is clear there is a need for a new device which can be used to measure the resultant force on a brake caliper. However, until now this has presented difficulties in view of the large number of forces that must be taken into account.

Remarkably, a new sensor in the form of a load cell has now been produced which is capable of overcoming a wealth of extraneous force inputs which would ordinarily affect the accuracy of a standard sensor.

A load cell produces an electrical output signal which changes in magnitude depending on the magnitude of a force or weight applied. Load cells can be used as sensors for weighing the contents of vessels, bins, hoppers or other similar applications. Since a load cell measures a resultant force vector, usually arranged to be in a single direction, eg vertical forces acting on a container, in order to achieve an accurate measurement it is important that all restraints, (ie forces other than a force in the direction to be measured) eg forces other than the weight of material being weighed, are eliminated or kept to a minimum.

Typically, a load cell includes a spring element, which may be in one of a number of forms including a hollow or solid column, cantilever, diaphragm, or shear beam, to which gauges are bonded to measure a strain generated. The spring element extension changes with respect to a force applied to the load cell and the resistance of a gauge is related to the extension. Compensation components are generally included to compensate for zero output and zero drift with environmental conditions including temperature.

Generally, a load cell includes a protective case sealed to exclude the external environment, but capable of allowing deformation of the spring element to occur

when a force is applied. In some cases restraining diaphragms minimize the effects of side loading.

In general, the structure of a vessel, bin, hopper or platform to be weighed is a factor which must be carefully considered in the arrangement of load cells. The supporting structure must also to be considered since it must carry the full weight of the vessel and its contents via point loading of a load cell. In the light of this, care must be taken to ensure that the supporting structure presents a load acting in line with the axis of a load cell and that there is a minimum of side loading.

Furthermore, a number of conditions must be taken into account in order to achieve accurate measurements. Reasons for poor performance or unreliability of a load cell fall into three main categories: (a) problems presented by a non-axial load; (b) side forces which affect the load cell; and (c) free vertical movement of the load being impaired. These could be caused by non-axial loading, side loading, shock, stay rods for holding a load in a horizontal plane, pipework attached to the load. In addition, environmental considerations must be taken into account including high temperatures, temperature changes, moisture, wind, vibration, and electrical considerations.

Clearly, all of these factors must be taken into account in the measurement of a braking force and this has lead to difficulties in development of an appropriate sensor.

Therefore, a need exists for a new sensor and a new device comprising a sensor which can be used for measurement of a braking force.

The present invention addresses the problems set out above.

Remarkably, it has now been found that it is possible to provide a sensor in the form of a new load cell specifically adapted for measurement of a braking force. This has

not been considered previously. This has allowed braking systems employing electrical control and electromechanical callipers to be developed whereby the integration of feed back from the brake calliper sensor provides the advantage of enhancing the braking control to achieve anti-locking or higher performance braking overall compared to known braking systems. An embodiment of the new load cell can be employed in a device for accurate measurement of a braking force. Surprisingly, this can be used to address the problems presented by a wealth of extraneous forces which need to be eliminated from the measurements taken.

It will be apparent that the invention extends to measurement of a braking force on any caliper applied to a disc brake arrangement.

Consequently, in a first aspect the present invention provides a load cell for measuring a braking force by measurement of a resultant force exerted by a brake caliper wherein the load cell comprises a spring element comprising a strain sensing section having a shear stress sensing element and an end fixing which adjoins the spring element and which is coaxial with the spring element.

In contrast to the invention, a conventional shear beam load cell has a spring element in the form of a beam fixed to an integral mounting heel wherein the heel is generally mounted on a surface and the beam projects outwardly from a side of the heal above the plane of the surface. Fixings are typically provided inclined at a normal to the axis of the beam which extend through the heel and anchor it to the surface. A load point is positioned on the beam distal to the mounting heel and the surface.

Therefore, the load is applied to the load cell inclined at a normal to the longitudinal axis of the beam.

Preferably the end fixing comprises an axial flange or axial threaded stud fixing. Advantageously, this facilitates avoidance of stress concentration effects in the load cell.

Preferably, the shear stress sensing element senses stress due to a shear load and provides the advantage of obviating any effects of a varying force centre position.

This can be visualised by considering a shear load applied to a cantilever having an anchored end and a free end. The shear load, and hence shear stress, at a position A on the cantilever remains constant for a load applied to the cantilever regardless of the position where the load is applied between the free end and position A.

Preferably the load cell is inherently immune to the effects of a varying force centre position. Preferably this is achieved by surrounding the I section of the strain section of the load cell by at least one mass concentration which removes extraneous axial loads by diffusion and/or diversion. Preferably the mass concentration is an integral homogenous solid region of the load cell body positioned between the spring element and the gauge which diverts or diffuses stress applied.

Preferably, a mass concentration has a geometry which ensures a uniform extraneous load stress vector distribution over the spring element, greatly reduced in magnitude by the diffusion of stress within geometrical constraints. A small value of extraneous force influence is thus experienced and by careful arrangement of the spring element and strain gauge pair the effect can be almost entirely negated Preferably the load cell comprises a spring element of T or, more preferably, I beam cross section which serves to reduce unwanted strain due to extraneous forces by virtue of the spring element being inherently stiff due to its perpendicularly arranged structure.

Preferably, the spring element comprises a rod having a first end and a second end.

The first end of the rod is positioned adjacent an end fixing and the second end is distal to the end fixing. Preferably, the radius of the rod between the first end and the second end is about 7mm to about 9mm, more preferably about 8mm. Preferably, the radius of the rod adjacent the second end, for a length axially along the rod of from about 10mm to about 15mm, more preferably about 12mm, is about 9mm to about 1 lmm, more preferably about 10mm. Preferably, the radius of the rod adjacent the first end, for a length axially along the rod of from about 5mm to about 10mm, more preferably about 7mm, is about 9mm to about Ilmm, more preferably about 10mm.

Preferably the length of the rod is about 30mm to about 40mm, more preferably about 36mm.

Preferably, the load cell comprises an end fixing which comprises at least part of a mass concentration. Preferably it is geometrically proportioned to avoid stress concentration effects and the influence of theses effects on the strain gauge.

Preferably, the invention has the advantage of having component design with ease of installation and force transmission via axial fixings. Advantageously, if any single component need to be replaced, this can be carried out without the need to replace an entire load cell. The component design enables the load cell to be built into a confined space, for example the space available within a typical calliper brake assembly.

Preferably, structural failure of an embodiment of the invention is achieved at at least about 400% of rated load. This provides the advantage of a high safety factor.

An embodiment of the invention has the advantage that it is capable of sensing shear loads and is capable of providing accurate load measurement under varying load point

position. This is particularly relevant to brake calliper use whereby the position of the centre of pressure within the brake pad/disk contact foot print may vary throughout application of a brake.

Advantageously, an embodiment of the invention is capable of operating accurately at about 250oC. This is achieved by providing a means for compensating for sensor changes induced by temperature changes wherein the load cell comprises a temperature sensitive potential divider at the output of the sensor.

Until now, the problems presented by temperature fluctuations has been addressed by compensating for errors generated by temperature induced span changes by use of nickel or Balco foil resistors in series with the input sensor excitation connections.

However, it has now been found that this method suffers from the problem that it is limited in accuracy and provides a source of error due to the fact that resistance of the foil resistors changes non-linearly with respect to temperature. This problem is exacerbated in view of the fact that the extent of non-linearity of resistance with respect to temperature changes increases at temperatures above 100OC. Therefore, the greater the variation in temperature, the greater the potential error.

Furthermore, the known foil resistors suffer from the problem that they are not suitable for use at high temperatures above about 200 C. Indeed, the known sensors provide errors due to temperature induced span changes at temperatures lower than

2000 C. In addition, at high temperatures the backing material of the known sensors deforms. This adversely affects the accuracy of the sensor.

In addition, the known sensors have been found to suffer from the problem that their performance is degraded when span adjustment resistors are added to the excitation input of a bridge circuit.

In addition, it has been found that the known sensors can only be compensated for temperature induced errors over a small temperature range of about 500 C. In the light of this, the known devices are not suitable for use where a greater variation in temperature could occur.

Furthermore, the degree of non-linearity of temperature induced errors is increased with high temperature. In the light of this, the known sensors are not suitable for use in conditions where there is a high temperature of more than about 150oC.

Furthermore, in view of the fact that until now it has been necessary to select particular specialised sensors for particular uses ie-it has been necessary to match the compensation required to the intended use of a sensor. This has resulted in an increased cost.

Therefore, a need exists for a new device and method for facilitating elimination of errors in measurements caused by changes in temperature.

The present invention addresses the problems set out above.

Remarkably, it has now been found that it is possible to compensate for sensor changes induced by changes in temperature using a thermometer element in a temperature sensitive potential divider circuit at the output of the sensor.

Surprisingly, this can be used to address the problems presented by changes in temperature.

Remarkably, an embodiment of the present invention can be used at temperatures of more than 200oC, preferably even more than 250oC, and a surprisingly high level of

accuracy can be maintained. This is due to the fact that the components of an embodiment of the invention do not degrade at temperatures below 500oC. Furthermore, in contrast to the known sensors, compensation for temperature induced span changes is not limited to changes of temperature within only a small range of a

mere 500 C. Instead, compensation can now be achieved over a greater temperature range of at least 50oC, more preferably at least 100 C, even more preferably at least 200oC.

In addition, even at high temperatures of about 2500 C an embodiment of the invention provides accurate linear compensation for temperature induced changes in span. The degree of compensation does not become increasingly non-linear with increased temperature. Clearly, this is critical for optimum performance.

Remarkably, an embodiment of a load cell according to the invention provides accurate linear compensation for changes in temperature over a temperature range of absout -500C to about 500oC.

Preferably, an embodiment of a load cell according to the present invention comprises a potential divider having a temperature coefficient which is the inverse of the signal temperature coefficient provided by a sensor span change induced by a change in temperature. This provides the advantage of compensating for any change in the span of the sensor due to a change of temperature. Therefore, only a change in force applied to the sensor is measured.

Preferably, an embodiment of a load cell according to the present invention comprises a temperature sensitive potential divider which has at least one platinum resistance

thermometer element. This provides the advantage that accurate linear compensation for temperature induced sensor span changes can be achieved.

In addition, this provides the advantage that very small sizes of platinum resistance thermometer elements are available allowing the invention to be applied to small load cells. This provides a relative reduction in weight and provides greater flexibility with regard to positioning of a load cell.

Preferably, an embodiment of a load cell according to the invention comprises a body having a sensor which detects a physical parameter and a temperature sensitive potential divider with a low temperature coefficient resistor being positioned external to the body. This embodiment is suitable for use at temperatures up to at least about 250 C, more preferably at least about 500 C. This provides the advantage that the potential divider is separated from the body of the load cell, which is subject to changes of temperature, and ensures that compensation for temperature induced span changes is accurate.

Preferably, an alternative embodiment of a load cell according to the invention comprises a body having a sensor which detects a physical parameter and a temperature sensitive potential divider positioned inside the body. This embodiment

is suitable for use at temperatures up to at least about 100 C, more preferably at least about 150 C. This provides the advantage that the potential divider is protected by the body of the load cell, however it is subject to changes of temperature experienced by the body.

In a second aspect the invention provides a device for accurate measurement of a braking force comprising at least one load cell according to a first aspect of the invention.

Preferably, an embodiment of the device comprises two load cells according to a first aspect of the invention and a brake caliper wherein the load cells are capable of being mounted on a vehicle and the caliper is mounted on the load cells.

This preferred feature provides the advantage that differential expansion force reaction is absorbed equally by the load cells and in view of the fact that this can be assumed to be equal and opposite this force can be eliminated from consideration.

Preferably, the load cells are mounted on a vehicle adjacent a disk which turns with a road wheel and the load cells are an equal distance from a radius of the disk.

In a third aspect the invention provides an anti-lock braking system comprising a load cell according to a first aspect of the invention or a device according to a second aspect of the invention.

Additional features and advantages of the present invention are described in, and will be apparent from, the description of the presently preferred embodiments which are set out below with reference to the drawings in which: Figure 1 shows a force diagram of the forces acting on a load cell of the invention; Figure 2 shows a plan view of a load cell according to the invention; and Figure 3 shows a side view of a load cell according to the invention.

Figure 4 shows an outline of a load cell according to the invention.

Figure 5 shows an outline and a section through a load cell according to the invention.

Figure 6 shows an outline of a load cell according to the invention indicating gauge alignment.

For the purposes of clarity and a concise description features are described herein as part of the same or separate embodiments, however it will be appreciated that the

scope of the invention may include embodiments having combinations of all or some of the features described.

As seen in Figure 1, a brake calliper (2) can be mounted on two load cells (1) or force sensors (1); these sensors are specifically produced to overcome a wealth of extraneous force inputs which would ordinarily affect a standard sensor accuracy.

The predominant resultant reaction is vertical and the sensors (1) are specially produced and arranged to measure this predominant vertical component only.

Force Model and Enabling Solutions by production and arrangement of the force sensors Referring to Figures 1,4, 5 and 6: Nomenclature R = Resultant brake calliper reaction force, Vertical component (Y AXIS) F = Brake Calliper/Sensor reaction, Horizontal Component (X AXIS) P = Calliper/Sensor vertical reaction component due to combined R and differential expansion force E.

M = Z axis component sensor/calliper reaction.

1. Force Model The following assumptions have been made: 1. The differential expansion force reaction is equally absorbed by the two load cells, Sensor 1 and Sensor 2, being equal and opposite.

2. The brake calliper (2) imposes a symmetrical brake pad pressure distribution over a contact area with a brake disk (3), the implication being the horizontal components F and F2 imparted to sensor 1 and sensor 2 are equal and opposite, also the Z components M, and M2 are equal and opposite for the same reason.

3. The brake calliper (2) vertical reaction is divided equally by the vertical reactions on sensor 1 and sensor 2.

For equilibrium: R + P1 + P2 # and P1 + R/2 + E # P2=R/2-E &commat; Substitute # and) in (D : R = (R/2 + E) + (R/2-E) = R QED Therefore, by summing the vertical reactions of the two sensors the expansion forces are cancelled.

From assumption 2, since the sensor (1) outputs are summate any small extraneous effects due to M or F forces are always equal and opposite and cancel out by the summation process. i. e.

#P1 and #P2 are extraneous output effects due to M and F forces.

Therefore from equation (D R = P1 + #P1 + P2 + #P2 # And by definition 8P ; = 8F ; + #M1 8 denotes extraneous small sensor output effect and #P2=#F2 - #M2

Since 18F, =) 8F 1 and l8M, j-) 8M21 Substituting in &commat; R = P, + 5F + 8M + P2-8F-8M Therefore R=P, + P2 Thus illustrating that all extraneous effects are cancelled within the summation by virtue of the arrangement of the two load cells.

2. Sensor Features The force sensor (1) uses a shear stress sensing element and is inherently immune to the effects of a varying force centre position.

Referring to Figure 2, the mutually perpendicular components of forces in the X and Z axes, which are unwanted'extraneous'forces are minimised by: 1. Symmetrical arrangement of the strain gauges (5) about the longitudinal X axis so that strain effects caused by bending moments about all axes are self-cancelling.

2. The mechanical structure of the strain section (4) being an'I'beam cross section serves to reduce unwanted strain due to extraneous forces by virtue of the sections presented perpendicularly being inherently stiff.

3. The inert mass concentrations (6) and end fixing (7) details are produced to minimise the effects of fixing forces and extraneous forces, the analysis being made using FEA (Finite Element Analysis) techniques.

3. Detailed Production The production principle and proportional dimensions are suitable for sensors (1) of any load range. Key dimensional/load ratios are detailed and relate to a comparative error of 2% maximum between a theoretical value of strain due to the required load (with no extraneous force influences) and the resulting strain found by FEA (Finite Element Analysis).

Example 1

An example load cell (1) according to the invention is rated at 10 kN and is designed for use from about ambient temperatures (-200C) to about 250oC.

DIMENSIONAL RELATIONSHIP FOR 2% ACCURACY CRITERIA : Referring to Figure 3, the relationship between load P and D is as follows : 0 < P < 500 D2 The relationship between dimensions H and B is as follows : H < 2. 4 B It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art.

Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims.

Claims

Claims 1. A load cell for measuring a braking force by measurement of a resultant force exerted by a brake caliper wherein the load cell comprises a spring element comprising a strain sensing section having a shear stress sensing element and an end fixing which adjoins the spring element and which is coaxial with the spring element.
2. A load cell according to claim 1 wherein the end fixing comprises an axial flange or axial threaded stud fixing.
3. A load cell according to claim 1 or 2 wherein the shear stress sensing element senses stress due to a shear load and provides the advantage of obviating any effects of a varying force centre position.
4. A load cell according to any preceding claim wherein the strain sensing section of the load cell is surrounded by at least one mass concentration which removes extraneous axial loads by diffusion and/or diversion.
5. A load cell according to claim 4 wherein a mass concentration has a geometry which ensures a uniform extraneous load stress vector distribution over the spring element
6. A load cell according to any preceding claim wherein the spring element is of T or I beam cross section.
7. A load cell according to any preceding claim wherein the spring element comprises a rod having a first end and a second end, the first end of the rod is positioned adjacent an end fixing and the second end is distal to the end fixing; the radius of the rod between the first end and the second end is about 7mm to about 9mm ; the radius of the rod adjacent the second end, for a length axially along the rod of from about 10mm to about 15mm, is about 9mm to about Ilmm ; the radius of the rod adjacent the first end, for a length axially along the

rod of from about 5mm to about 10mm, is about 9mm to about Ilmm.
8. A load cell according to claim 7 wherein the length of the rod is about 30mm to about 40mm.
9. A load cell according to any preceding claim which comprises an end fixing which comprises at least part of at least one mass concentration.
10. A load cell according to any preceding claim which comprises separable components.
11. A load cell according to any preceding claim wherein structural failure is achieved at at least about 400% of rated load.
12. A load cell according to any preceding claim capable of operating accurately at about 2500C which comprises a means for compensating for sensor changes induced by temperature changes wherein the load cell comprises a temperature sensitive potential divider at the output of the load cell.
13. A load cell according to any preceding claim which comprises a potential divider having a temperature coefficient which is the inverse of the signal temperature coefficient provided by a sensor span change induced by a change in temperature.
14. A load cell according to any preceding claim which comprises a temperature sensitive potential divider which has at least one platinum resistance thermometer element.
15. A load cell according to any preceding claim which comprises a body having a sensor which detects a physical parameter and a temperature sensitive potential divider with a low temperature coefficient resistor being positioned external to the body.
16. A load cell according to any one of claims 1 to 14 which comprises a body having a sensor which detects a physical parameter and a temperature sensitive potential divider positioned inside the body.
17. A device for accurate measurement of a braking force comprising at least one load cell according to any preceding claim.
18. A device according to claim 17 which comprises two load cells according to any one of claims 1 to 16 and a brake caliper wherein the load cells are capable of being mounted on a vehicle and the caliper is mounted on the load cells.
19. A device according to claim 17 or 18 wherein two load cells are positioned on a vehicle adjacent a disk which turns with a road wheel and the load cells are an equal distance from a radius of the disk.
20. An anti-lock braking system comprising a load cell according to any one of claims 1 to 16 or a device according to any one of claims 17 to 19.
21. A device or a load cell as hereinbefore described with reference to or as shown in the accompanying drawings.