WO1993006442A1

WO1993006442A1 - Axle loading measuring device for trucks

Info

Publication number: WO1993006442A1
Application number: PCT/CA1992/000438
Authority: WO
Inventors: Alan Robb
Original assignee: Innov-Tronics Technology Inc.
Priority date: 1991-09-24
Filing date: 1992-09-24
Publication date: 1993-04-01
Also published as: GB9120310D0; AU2654192A

Abstract

An axle load measuring system for use in vehicles such as trucks, comprising a plurality of load sensors (52) mounted between respective suspension means (53) and axles (55) of the truck, for generating respective output signals proportional to pressure load applied thereto. A pair of front and rear sensor interface units (5, 11) are connected to front and rear ones of the plurality of sensors (1, 3, 7, 9) for receiving analog output signals therefrom, and applying power thereto. A sensor signal conditioning and measuring circuit (15) receives the analog signals and converts them to digital signals indicative of wheel load on respective ones of the truck wheels. An on-board controller (17) is provided including data entry means for inputting a user command and in response selecting predetermined ones of the output signals from among the plurality of digital load signals for visually displaying the wheel loads on the aforementioned predetermined ones of the wheels.

Description

AXLE LOADING MEASURING DEVICE FOR TRUCKS FIELD OF THE INVENTION

This invention relates in general to heavy vehicle self-weighing systems, and more particularly to an axle load measuring system for detecting the load on individual axles of a truck and in turn the wheel load. DISCUSSION OF THE PRIOR ART

Truck self weighing systems are well known for measuring axle loads in commercial transportation vehicles. The underlying reason for an axle load measurement system is to help truck operators maximize their profits by maximizing vehicle load per trip, minimizing or eliminating axle overload fines, reducing vehicle damage caused by axle overloads, and warning the truck operators of dangerous load distributions which may adversely affect vehicle maneuverability.

For the most part, the trucking industry is forced to comply with strict axle load limitations by a system of escalating fines and penalties. The intent of the legal system is to minimize damage to the roadways and prevent dangerous vehicle overloading, a primary cost of vehicle damage and accidents. Government defined vehicle weight limits and tolerances stipulate that axle unit load, and not only gross vehicle load, is required to be regulated. Thus, prior art approaches have endeavoured to develop systems for self measurement of vehicle load on a per axle basis.

For example, U.S. Patent 3,646,512 (Borgstede) discloses a vehicle trailer bed overload indicating system comprising overload sensors in the form of intertelescoping contact carriers installed between load supporting areas, such as truck beds and the vehicle axle means therebelow that support the leaf spring means which in turn yieldably support the load supporting area or load bed. A load on the load supporting area compresses the leaf spring means and intertelescopes the contact carriers to thereby close circuit the carriers and render indicia of overload.

U.S. Patent 3,718,792 discloses an overload indicator having separate parts attached to one axle and chassis, respectively, in such a way that the chassis part lowers into contact with the axle part to actuate an overload signal upon loading of the vehicle to a predetermined cargo weight.

A sensor for detecting vehicle load or overload is disclosed in U.S. Patent 3,858,171 comprising a plurality of contact carriers mounted in relation to the load supporting area of the vehicle and supporting frame therebelow such that as the load supporting area and supporting frame are urged downwardly, responsive to a load placed thereon. The contact carriers are adapted to respond to such descent of load supporting area and frame so as to permit contact closure, thereby indicating a state of predetermined load or overload.

U.S. Patent 3,891,964 discloses a truck or trailer axle overload indicator comprising an indicator assembly that suspends a flexible indicator over the midpoint of the centre of the differential or axle housing, and functions to individually indicate an overload condition by contacting it. A transfer horizontal indicator mounting bracket is clamped to the longitudinal frame members of the vehicle and supports the indicator assembly high above the axle to be monitored to avoid damaging the indicator under severe depression of the vehicle springs. The prior art systems described in the afore- noted U.S. Patents relate to rudimentary load detection systems which rely on the compression of suspension components to give a simple indication of vehicle overloading. However, no means are disclosed for actually detecting the load on various ones of the wheels. Furthermore, such prior art systems suffer from large (5% to 10%) measurement inaccuracies which are inconsistent with existing Government regulated load maximum. For instance, overall measurement accuracy requirements of 2% would allow an error of +400 kg in the measurement of a tandem trailer axle rated at 2,000 kg. The Ontario Highway Traffic Policy specifies only +250 kg variation from the maximum.

The major problems associated with load sensors for trucks have been further discussed in various publications such as a paper entitled "A NEW LOAD SENSOR FOR TRUCK SELF-WEIGHING SYSTEMS" by Bamett et al of TRW Probe Electronics Co. Ltd., reproduced in SAE Paper No. 830103. The Bamett et al paper discusses the need to measure axle loads, payload and gross load of commercial vehicles by means of placing strain sensors on the load bearing components of trucks. Direct measurement of load using chassis mounted load cells is discussed therein. However, such systems are very expensive: Furthermore, the disclosed systems employ expensive and fragile load transducers which usually succumb to the rigorous environment encountered by the undercarriages of trucks. Pneumatic or air bag suspensions are gaining popularity due to various enhanced performance features such as a combination of high capacity, low weight, constant ride height, and excellent road vibration isolation. Additionally, many operators employ a liftable third axle ahead of the tandem rear axle to increase load capacity. This liftable axle is in almost all cases, pneumatically lowered. Mechanical springs lift the axle when the air bags are deflated. In this manner, unnecessary wear and tire scrub can be reduced or eliminated. Prior art methods of load measurement by means of measuring spring compression are not feasible with air bag suspensions due to the automatic ride height regulators employed on the system. The bag air pressure, however, is directly related to load and operating temperature. Thus, prior art manufacturers of pneumatic suspensions have included pressure measurement units for measuring the bag pressure and thereby deducing the axle load. One such system is manufactured by the Neway Division of Near Siegler Inc. and is known as the

"Scale-O-Matic"™ pressure measurement unit. Although measurements within 150 kg of full scale axle load may be obtained with this system, as discussed, the measurement system is not easily adapted for use with spring type suspensions.

Mechanical leaf springs are, by design, non¬ linear devices. Moreover, the load-deflection characteristics of mechanical leaf springs are known to change unpredictably with environmental conditions. In particular, temperature, moisture, salt, dust and rust all affect the inter-leaf friction resulting in a hysteresis effect in connection with the load-deflection characteristics. This problem is compounded by the manner in which trucks are usually loaded. Typically, a forklift carries the load onto the truck in small units (e.g. 1,000 kg) until the truck is filled or an order is complete. The load therefore increases in steps. However, the forklift also compresses the suspension with each trip, with the result that the final suspension deflection results depend not only on the final load but on how it was loaded.

Thus, prior art load sensors which merely detect leaf spring deflection, result in inaccurate load measurements. Survey data is also available from an NTIS

Publication PB83-163881 entitled "OFF-HIGHWAY HAULAGE TRUCK OVERLOAD DETECTION" by Bartol. This publication also discloses a model specification prepared for a truck overload detection system. As with the system of Bamett et al, the Bartol apparatus is complex and costly, and is characterized by relatively low accuracy and reliability under harsh operating conditions. In particular, the aforementioned publication lists some 37 systems capable of weighing trucks. Of these, 14 are on-board truck self-weighing systems.

The most commercially successful of the on- board truck self-weighing systems are as follows:

(i) The ASEA Load Indicator System from

Sweden; (ii) The International Weighing Systems CMT 200 load sensor; and (iϋ) The Nordisk ATON load sensor, also from Sweden. The ASEA and International Weighing Systems devices are of complex design and are expensive. Furthermore, the International Weighing Systems CMT 200 device is a specialized unit directed at point-of-sale billing for bulk commodities. As such, periodic calibration is required to ensure conformance with applicable weights and standards.

The Nordisk ATON system is of lower cost, and the load sensors are pre-calibrated making the system immediately usable upon installation. A vibrating wire load cell is used to sense the load as a shift in the natural frequency of a pre-tensioned steel stripe.

All of the three aforementioned commercially successful systems use load cells of various types to measure the vehicle load. Basically, prior art load cells measure force by measuring the deflection of a known spring when loads are applied to it. The spring is usually in the form of an extremely stiff beam or surface which operates well within its elastic limits. The deflection of the spring can be measured in many ways; such as by means of strain gauges and Linear Variable Differential Transformers (LVDTs) . One such prior art load measuring system is described in U.S. Patent 4,215,754 (Hagedorn) .

A major shortcoming of load cells is that they must support the entire weight being measured, must be correctly sized so as to offer adequate measurement accuracy over an expected range, yet must also be rugged enough to withstand repeated shock loads up to many times their rated load without sustaining physical damage or calibration drift.

More particularly, it has been found that an off-axis load can destroy, or at least adversely affect calibration of such prior art load cells. In general, the load must be restrained in all but the axis of measurement. This requirement makes the use of standard load cells very difficult. In addition, the configurations of most load cells do not lend themselves to easy installation.

Other examples of truck load measuring systems are disclosed in United States Patents 3,078,937 (Mehki et al) ; 4,836,036 (Jetter) and 4,747,456 (Kitagawa et al) . According to each of these additional prior art systems, compressive forces are applied across the entire length of the load cell due to bending moments under load. Therefore, in the system such as shown in Kitagawa et al, a large number of strain gauges are required to detect deflection of the load cell. Furthermore, each of these prior art truck load weighing systems must be used in conjunction with a customized U-bolt. SUMMARY OF THE INVENTION

According to the present invention, an axle load measuring device is provided comprising a plurality of load sensors mounted between the suspension and corresponding axle of respective wheels of a vehicle, such as a truck. The load sensors detect pressure load of the suspension on the corresponding axle and in response generate respective output signals indicative of the load. A truck operator is provided with an on-board controller in the vehicle cab by which the operator may selectively display the load on respective ones of the wheels. In this way, the operator is provided with an accurate indication of load distribution as well as an indication of whether the axle load on any one of the axles exceeds a legal limit.

The system of the present invention is characterized by much greater accuracy than the systems disclosed in the aforementioned prior art patents and publications, and is much less costly and easier to install than such prior art systems.

According to the preferred embodiment, the load cell of the present invention comprises a plurality of stepped recesses on the under surface thereof, and a plurality of strain gauges configured in the form of a Wheatstone bridge are mounted on the centre most ones of the stepped recesses on either side of a centrally disposed hole in which a bolt passes to hold the leaf spring structure in place. The strain gauges detect deflection of the load cell due to compression and tension forces applied thereto by the pressure load, and in response generate signals which are received by an on¬ board microprocessor incorporated into the display system. The stepped recesses allow for mounting of the strain gauges close to the point of maximum bending of the load cell and also concentrate the bending of the load cell at the location of the strain gauges. In addition, the stepped recessed provide localized compression points about which bending occurs, in order that a simple strain gauge bridge may be used. BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will be described in detail below with reference to the following drawings, in which:

Figure 1 is a block diagram of an axle load measuring system in accordance with the present invention;

Figure 2 is a schematic diagram of a sensor interface unit in accordance with the preferred embodiment; Figure 3 shows a load sensor mounted between a leaf spring suspension and axle in accordance with the preferred embodiment;

Figure 4 is a perspective view of the load sensor in Figure 3; and

Figure 5 is a schematic diagram illustrating a Wheatstone bridge configuration of the strain gauge shown in Figure 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning to Figure 1, the axle load measurement system is shown generally comprising a front right load sensor 1 and front left load sensor 3 connected to a sensor interface unit, a rear right load sensor 7 and rear left load sensor 9 connected to a sensor interface unit 11, and a calibration load sensor 13. The sensor interface units 5 and 11, and the calibration sensor 13 are connected to a sensor signal conditioning and measurement circuit 15 which, in turn, is connected to a controller 17. Similarly, the load sensors 1 and 3 and 7 and 9, along with sensor interface units 5 and 11 are mounted on a tractor portion of the tractor-trailor combination. Additional load sensors and sensor interface units may be included for interfacing further wheel load sensors associated with the wheels of one or more trailers (not shown) , which may be hauled by the tractor.

In operation, the load sensors 1, 3, 9 and 7 each generate an analog voltage proportional to the load carried thereby. Each sensor interface unit 5 and 11 receives the analogue voltage signals from each of the associated right and left load sensors, and transmits the received signals to sensor signal conditioning and measurement circuit 15 for digitization and processing. The sensor interface units 5 and 11 also provide regulated sensor excitation voltages for operating the load sensors 1, 3, 9 and 7, as discussed in greater detail below. The sensor signal conditioning and measurement circuit is preferably an off-the-shelf circuit such as the model PC-AN12-8 or PC-AN16-8 analog input card manufactured by Daytronics. Alternatively, customized hardware may be utilized for signal conditioning and measurement. The circuit 15 is provided with a plurality of differential analogue inputs for receiving signals from the sensor interface units 5 and 11 and the calibration load sensor 13, and a plurality of digital input/output lines are provided, forming a data bus, connected to the controller 17.

The analog voltage signals received by sensor signal conditioning and measurement circuit 15 are conditioned for noise rejection and then converted into digital signals for use by the controller 17. Controller 17 may be in the form of a microcomputer incorporating a CPU and memory in the usual manner. A digital look-up table is stored on memory within the controller 17 for generating correct load data in response to receiving the digitized load signals from respective ones of the load sensors 1, 3, 7 and 9. The look-up table values are selected to generate the appropriate correct load data for each specific tractor and trailer of the truck, depending on the number of wheels (and hence the number of load sensors) of the tractor trailer, etc. Thus, re- calibration of the sensors 1, 3, 7 and 9 is not required in the event that the tractor-trailer combination is re¬ arranged. This look-up table for each trailer can be added in software to the control unit 17. The look-up tables are preferably stored on the hard disc in the control unit 17. These can easily be modified, and adaptations can be made as operating conditions and parameters change.

The controller 17 includes data entry means such as a key pad (not shown) as well as a visual display (e.g. LCD screen) , of a well known design. The correct load data generated via the linearization look-up table in controller 17 may be displayed in the truck cab under control of the truck operator. Thus, the operator may select a predetermined one of the tractor or trailer wheels for load measurement in response to data entry at controller 17.

Research has indicated that the accuracy required for axle load measurement can be realized by an eight-bit digital-to-analog conversion, since one bit would represent a 0.4 percent resolution. However, sensor signal and conditioning measurement 15 preferably incorporates a 12-bit or greater resolution converter. Since conversion speed of the analog-to-digital converter is very fast relative to the rate at which the load is added. Data averaging can be used to filter noise for more accurate final signals.

In operation, the selected digital load sensor signal from the sensor signal conditioning and measurement circuit 15 is used to address the look-up table within controller 17 which, in response, generates a corrected digital wheel load value in appropriate units (i.e. kilograms) . Of course, the look-up table can alternatively be scaled with imperial or S.I. units. The digital wheel load value output from the look-up table is then transmitted for display to the truck operator. The controller program contains means for accommodating specific operating conditions such as zero-load correction, temperature effects, gain, etc.

The load sensors 1, 3, 7 and 9 plus additional units if a trailer is used, may be calibrated by mounting calibration load sensor 13 to a hydraulic jack system which is used to raise the axle for suspension against the frame, and simultaneously measuring the output signals from calibration sensor 13 and the load sensor 1, 3, 7 or 9 being tested. Calibration load sensor 13 is preferably a standard (e.g. 20,000 lb.) flat plate load cell. Turning to the schematic diagram of Figure 2, circuitry for the sensor interface unit is shown in greater detail comprising a plurality of differential terminals for applying the necessary sensor excitation voltages to respective ones of the load sensors and receiving the analog DC load sensor voltages and transmitting same to the sensor signal conditioning and measurement circuit 15.

More particularly, the sensor interface unit receives a differential DC power voltage signal (V⁺ and V IN) on terminals 1 and 2. The received power signal is applied differentially to a voltage regulator 19 (e.g. Motorola model 7805) via the IN and GND inputs thereof. In response, regulator 19 generates a DC regulated sensor excitation voltage V⁺ which is applied to the right and left load sensors via terminals 9 and 13, respectively. The VIN signal is also applied to the right and left load sensors via terminals 10 and 14, respectively.

Capacitors 21 and 23 are connected to the voltage regulator 19, in the usual manner.

A first analog load signal is received from the right load sensor via terminals 11 and 12, and transmitted to the first channel (CHI) of sensor signal conditioning and measurement circuit 15 via terminals 3 and 4 (i.e. connected to terminals 11 and 12 via respective jumpers) . In a similar manner, an analog load signal is received from the left load sensor on terminals 15 and 16, which is then transmitted to the second channel (CH2) of sensor signal conditioning and measurement circuit 15 via terminals 6 and 7, respectively.

Terminals 17, 18 and 19 are connected to the sensor signal conditioning and measurement circuit 15, the right load sensor (e.g. sensor 1 or 7) and the left load sensor (e.g. sensor 3 or 9) and are also connected together to form a ground shield. The sensor interface unit also includes a pair of calibration resistors 25 and 27 which are connected to terminal 2 for receiving the VIN signal from sensor signal conditioning and measurement circuit 15, and on opposite sides thereof to terminals 5 and 8 for connection to the CH 1 and CH 2 channels of the sensor signal conditioning and measurement circuit 15. The resistors 25 and 27 are utilized by circuit 15 for calibrating the signals output from the right and left load sensors.

An important aspect of the present invention is the load sensor discussed in greater detail below with reference to Figures 3-5. Prior art load sensors have been directed to measuring loads in one of either pneumatic suspension or leaf spring suspension vehicles. It is believed that a low cost, reliable load sensor which is applicable to leaf spring suspensions has not been hitherto developed.

According to the present invention, load sensors are mounted in the space found between the axle and suspension, as illustrated in Figure 3. This location of load sensor is common for all axle types (e.g. steering, driving, trailing) as well as suspension types. Figure 3 illustrates the mounting of a load sensor 52 within an axle attachment "U" bolt 57 or similar clamping means, for sensing the load between leaf spring 53 and axle 55. The length of the load sensor is sufficient to cover the spring mounting surface on the axle 55. The "U" bolt 57 is attached to leaf spring 53 via nuts 59 and 61 through a top plate 63, in a well known manner, and a spacer block 51 is provided intermediate the load cell or load sensor 52 and axle 55. A load applied to the sensor 52 results in a combination of compression and bending forces which are sensed and transmitted to an associated sensor interface circuit 5 or 11 (Figure 2) for processing. Turning to Figure 4, the load sensor 52 comprises a first pair of strain gauges A,B which are in compression or tension when the axle is loaded, and a further pair of strain gauges C and D which are arranged orthogonally to the gauges A,B for effecting temperature compensation measurements in the event of expansion or contraction of the steel block 52 due to changing weather conditions, etc. The strain gauges are arranged in a Wheatstone bridge, as illustrated in Figure 5. Placement of the load sensor intermediate the axle 55 and leaf spring assembly 53 is an important aspect of the invention since it has been found that such placement is at the region of highest spring deflection (i.e. maximum signal output from the strain gauges) as well as minimum spring hysteresis (i.e. greater accuracy of measured results) . Such placement allows for simple retrofitting of an existing truck suspension system to accommodate the load sensor of the present invention, without requirement for custom designed U-bolts, etc. As shown in Figure 5, the strain gauges are mounted within stepped recess portions of the load sensor 52. The stepped recesses provide two functions; firstly, the recesses allow the strain gauges to be mounted close to the centre of the load sensor where maximum bending occurs, and secondly, the recesses concentrate maximum bending of the load cell 52 at the location of the strain gauges. Furthermore, the use of stepped recesses results in localized compression points during bending. Specifically, in the configuration illustrated in Figure 4, bending moments are applied across the top surface of the load sensor as indicated by the downwardly directed arrows, whereas compression occurs at only three points on the underside as shown by the upwardly directed arrows, thereby permitting the use of a single full bridge strain gauge arrangement localized at the point of maximum deflection of the load sensor. The exact location of the stepped recesses may vary and should be chosen to optimize signal output of the strain gauges.

At one end of the load sensor 52, a pair of holes 65 and 66 are provided for connecting a pair of leads (not shown) from the strain gauges to the associated sensor interface unit. The holes 65 and 66 are preferably threaded to receive flexible tube fittings for protecting the pair of leads. Also, a groove 67 is provided through the centre raised portion of load sensor 52 for feeding the lead from strain gauges B and D to the appropriate holes 65 or 66.

The raised portions centrally of load sensor 52 at opposite ends thereof are used to support the clamping bolt loads that fasten the spring 53 to the axle 55. A hole 68 is located in the middle of the centre raised section of load sensor 52 for receiving a spring assembly bolt (not shown) for holding the leaf spring assembly 53 together. A pin 69 is inserted into the hole 68 for preventing the spring (not shown) from sliding on the mating axle plate from its clamped position. The pin 69 is shown with reduced diameter which can be selected to an appropriate dimension for fitting the top flat surface of spacer block 51.

Silicon rubber covers 71 and 72 are preferably fitted, cast or glued into the stepped recess portions of the load sensor 52 for protecting the strain gauges.

The strain gauges A, B, C and D are preferably in the form of 350 Ohm variable resistance strain gauge devices, arranged in a full bridge configuration (wheatstone bridge) such that the measured DC voltage V is proportional to the deflection of the strain gauges A and B. This DC voltage is transmitted to a corresponding sensor interface circuit (5 or 11) , and therefrom to interface circuit 15 and computer 17 for further processing. The full bridge configuration (wheatstone bridge) provides temperature change compensation. The voltage regulator 19 (Figure 2) provides a constant voltage (v⁺ - VIN) across the bridge in spite of long lead lengths.

Other modifications and variations of the present invention are possible. For examples, although the load sensor 52 is shown as being in the form of a plate, the sensor may be of curved shape to accommodate different spring shapes. Also, the width of the load sensor 52 may be increased with spacers to accommodate different sizes of springs. All such embodiments or variations are believed to be within the sphere and scope of the present invention as defined in the claims appended hereto.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An axle load measuring system for use on a vehicle having a plurality of axles and suspension means therefor, comprising: a) a plurality of load sensors mounted between the suspension means and a corresponding one of said axles for detecting pressure load on said axles and in response generating respective analog output signals; b) sensor signal conditioning and measurement means for converting said analog output signals to corresponding digital signals indicative of said pressure load on said axles; and c) controller means for receiving said digital signals and in response visually displaying said pressure load on said axles.

2. The axle load measuring system of claim 1, wherein said controller means further comprises table look-up means and program for receiving said digital signals and in response generating corrected versions of said digital signals in appropriate weight units.

3. The axle load measuring system of claim 3, wherein said table look-up means and program includes data for correcting said digital signals responsive to predetermined operating conditions including one or more of temperature variances, zero shift, gain shift, mechanical non-linearities and type of trailer.

4. The axle load measuring system of claim 1, further including sensor interface means connected to successive pairs of said load sensors and to said sensor signal conditioning and measurement means for transmitting said analog output signals to said sensor signal conditioning and measurement means and for providing sensor excitation voltage to said successive pairs of said load sensors.

5. The axle load measuring system of claim 4, wherein each of said sensor interface means further includes voltage regulator means for regulating said sensor excitation voltage provided to said successive pairs of said load sensors.

6. The axle load measuring system of claim 1, wherein each of said load sensors comprises a block having stepped recess portions and a plurality of strain gauges mounted to said block in said stepped recess portions.

7. The axle load measuring system of claim 6, wherein said strain gauges are connected in a full bridge configuration.

8. A load sensor comprising: a) a generally rectangular block; b) a pair of recessed portions on opposite sides of centre of said block; and c) a plurality of strain gauges mounted to said block in said stepped recess portions.

9. The load sensor of claim 8, wherein said recessed portions are stepped.

10. The load sensor of claim 8, wherein said plurality of strain gauges are connected in a full bridge configuration.

11. The load sensor of claim 10, wherein a first pair of said strain gauges is mounted in a first one of said pair of recessed portions and a second pair of said strain gauges is mounted in the other of said pair of recessed portions.

12. The load sensor of claim 11, wherein said first pair of strain gauges are oriented perpendicular to one another.

13. The load sensor of claim 11, wherein said second pair of strain gauges are oriented perpendicular to one another.

14. The load sensor of claim 8, wherein said block is fabricated from steel.