WO1991002226A1

WO1991002226A1 - Force measurement device for trailer couplings

Info

Publication number: WO1991002226A1
Application number: PCT/SE1990/000502
Authority: WO
Inventors: Bertil Hök; Stig Kristensen
Original assignee: Siarr Sweden Ab
Priority date: 1989-08-04
Filing date: 1990-07-25
Publication date: 1991-02-21
Also published as: SE464374B; SE8902673L; SE8902673D0; AU6149890A

Abstract

Force measurement device for measuring the vertical force component (P) acting on a coupling (1) between a towing vehicle and a towed vehicle. The coupling includes two stiff holder plates (4, 6) and an intermediate sensor means (5) having at least one deflectable or pivotable sensor plate (10). The sensor plate (10) is deflected or pivoted under the action of a vertical force (P) by the holder plates (4, 6) being restrictedly, mutually pivotable, there being spacer means (8a, 8b, 9a, 9b, 9c) which are placed in staggered relationship between the sensor plate (10) and the respective holder plates (4, 6) on each side of the fastening means keeping the holder plates and the intermediate sensor means together. Transducer means are arranged on the deflectable or pivotable sensor plate (10) for sensing the deflection or pivoting and emitting an electrical signal corresponding to the vertical force (P).

Description

FORCE MEASUREMENT DEVICE FOR TRAILER COUPLINGS.

The invention relates to a force measurement device for

measuring the vertical force component acting on a drawbar coupling between a towing vehicle and a towed vehicle,

particularly where the latter has a single axle, the coupling comprising a coupling part, e.g. a towing ball, rigidly connected to a first stiff holder plate, which is fastened by fastening means to a corresponding second, similarly stiff holder plate mounted on the towing or towed vehicle, preferably on the towing vehicle.

Enabling the measurement of the vertical force component is important, since this component effects the driving characteristics of the vehicle. Accordingly, a downwardly directed force on the towing vehicle causes deterioration in the breaking and steering capacity of the vehicle, and may also cause the vehicle headlights to blind meeting vehicle drivers during night

driving. In a corresponding way, an upwardly directed force effects the breaking function of the rear wheels and the reach of the headlights.

In addition, the vertical force varies in response to the load distribution on the towed vehicle, particularly with a singleaxle vehicle, and it is therefore desirable to measure this force continuously.

The main object of the invention is to provide against this background a force measurement device enabling continuous, reliable measurement of the vertical force component, acting on the coupling, without the horizontal component effecting the measurement results. A further object is to give the force measurement device a specially simple implementation. A still further object is that the measurement result shall as far as possible be independent of extraneous environmental parameters such as temperature, moisture etc. The stated main object is achieved with a structure of the force measurement device according to claim 1. Suitable further characterizing features are disclosed in claims 2 - 13. The invention will now be described in more detail below, with reference to the accompanying drawings illustrating an

embodiment example.

Fig. 1 is a perspective, exploded view of parts of a coupling having a towing ball and mounted on an unillustrated towing vehicle;

Fig. 2 is a vertical cross-section to a larger scale through the plates illustrated in Fig. 1;

Fig. 3a is a plane (to a smaller scale) of the left hand one of the intermediate plates of Fig. 2, seen from the right in fig. 2; Fig. 3b schematically illustrates an electrical transducer circuit mounted on the plate according to Fig. 3a;

Fig. 4 is a plane view of a modified embodiment of the intermediate plate illustrated in Figs. 1 and 2, in this case being a single plate provided with another type of transducer;

and

Fig. 4b schematically illustrates an electrical transducer circuit associated with the plate in Fig. 4a.

The coupling 1 illustrated m Fig. 1 as mounted on an

unillustrated towing vehicle and is provided with a towing ball 2, intended for being removably connected to a coupling part adapted thereto and mounted on an unillustrated towed vehicle. With the aid of a holder arm 3 the ball 2 is rigidly connected, suitably by welding, to a first, rectangular, stiff holder plate 4. Together with an intermediate, flat sensor means 5 this plate is fastened to a second, similarly rectangular, stiff holder plate 6 mounted on the towing vehicle with the aid of a holder arm 7. Each of the plates 4, 5 and 6 has two through holes 4a, 4b (4b not being visible in the figures), 5a, 5b and 6a, 6b, through which unillustrated bolts extend along the respective hole axis line A,B and are tightened with the aids of nuts, when the coupling is in an assembled state, such that the plates are mutually pre-stressed against each other (cf. Fig. 2) with the intermediary of spacers in the form of strips 8a, 8b, 9a, 9b, 9c.

The hole axis lines A and B for the bolts are in a horizontal plane of symmetry for the plates 4, 5, 6, the stiff holder plates 4 and 6 thus being restrictedly pivotable in relation to each other (about a horizontal axis which is at right angles to the lines A and B) under the action of a vertical force

component (the arrow P i Fig. 1) acting on the ball 2 and causing deflection of a sensor plate 10 included in the sensor means 5. In the sensor means 5 according to Figs. 1 and 2 there is also included a directly engaging sensor plate 11, i.e. one without the intermediary of spacers. Both these sensor plates 10, 11 are made of a material which has substantially greater elasticity than the steel holder plates 4 and 6, namely a glass fibre reinforced epoxy plastics laminate, as is normally used for circuit boards. The spacer strips 8a, 8b, 9a, 9b and 9c also consist of such material and are glued to the sensor plates 10, 11.

Horizontal, tensional forces (perpendicular to the arrow P in Fig. 1) acting on the ball 2 are substantially taken up by the unillustrated bolts. With a suitable implementation of the coupling part on the towed vehicle, the horizontally acting tensional force components can be caused to act in the horizontal plane defined by the axis lines A and B. The spacer strips 8a, 8b, 9a, 9b, 9c extend horizontally

(parallel to the plane through the axis lines A and B) and are positioned such that when the ball is subjected to a downwardly directed force, the relatively elastic sensor plate 10 will be deflected in a lower portion, below the strip 9b, by the strips 8b and 9c pressing against the plate from opposite directions, so that the region adjacent to the strip 8b approaches the constantly flat sensor plate 11 (which is kept flat by the engagement against the stiff holder plate 4). At the same time, the upper portion of the sensor plate 10, above the strip 9b, will be kept substantially flat, since the strip 8a then eases away from the holder plate 6.

If, on the other hand, the ball 2 is subjected to an upwardly directed force, the upper portion of the sensor plate 10 will be deflected in a corresponding way in the region of the strip 8a in a direction towards the constantly flat sensor plate 11, while its lower portion is kept substantially flat and positioned at a constant distance from the sensor plate 11.

To obtain such a deflection for upwardly or downwardly directed forces on the ball 2, the spacer strips are placed such that on one side of the deflectable sensor plate 10 (to the right in Fig. 2) they are situated centrally along a straight line in the region for the unillustrated bolts and the holder arm 3 attachment to the holder plate 4 (the strip 9b) and also along the opposite edge portions of the sensor plate 10 parallel to said straight line (the strips 9a and 9c), the strips on the other side of the deflectable sensor plate 10 (to the left in Fig. 2), being situated in two intermedxate regions, suitably half way between said straight line and the respective edge portion (the strips 8a and 8b).

In an alternative embodiment, the intermediate space between the holder plate 4 and the sensor plate 11 is also provided with two spacer strips completely corresponding to the strips 8a, 8b depicted in Fig. 2. In this case both sensor plates 10, 11 will be deflected by a vertical force component acting on the ball. For the remainder, the function will be the same.

In the illustrated embodiment (in Figs. 1, 2, 3a, 3b) the deflecting movement of the sensor plate 10 is sensed with the aid of capacitor plates 10a, 11a and 10b, lib situated directly opposite each other in the respective deflecting region. These capacitor plates consist of metal coatings on the sensor plates 10 and 11 and are connected to an electrical transducer circuit, which will now be described with reference to Figs. 3a and 3b.

In Fig. 3a the sensor plate 10 is illustrated seen from the right in Fig. 2, although in a smaller scale. The capacitor plates 10a, 10b together with the opposing capacitor plates 11a, lib placed on the constantly flat sensor plate 11 form two capacitors C₁ and C₂, the capacitances of which are responsive to the mutual spacing between the respective plate pairs 10a, 11a and 10b, 11b. As will be seen from Fig. 3b, the capacitors C₁ and C₂ are connected in series which each other to form an electrical transducer circuit, the conductors and components of which are mounted on the sensor plate 10, also serving as a circuit board (apart from the capacitor plates 10a, 10b only the components OP, R_F and C_F are depicted in Fig. 3a). Alternatively, the components of the transducer circuit may of course be mounted on the constantly flat sensor plate 11.

The transducer circuit according to Fig. 3b essentially comprises an oscillator circuit 12, which includes the capacitors C₁ and C₂, an operational amplifier OP, a resistor R_F and a capacitor C_F, which are connected as shown in the figure, there also being a monostable flip-flop or Schmitt trigger 13 connected to the output of the operational amplifier OP, and a low-pass filter 14. With the aid of this circuit 12-14 there is obtained on the output of the oscillator 12 a signal with an intermediate frequency (e.g. in the order of 500 kHz), this frequency being responsive to the quotient between the

capacitance values for C₁ and C₂ and thus to the plate spacing of the respective capacitor and thereby to the magnitude and direction (upwards and downwards) of the vertical force

component. As an example, it may be mentioned that the

capacitance values can vary from about 100 pF in an unloaded state to about 120-130 pF for maximum deflection (with a decrease of the plate spacing from about 0.5 mm to about 0.2 mm).

By measuring the quotient (C₁/C₂) there is obtained the

advantage that the temperature and moisture variations will effect both capacitors in the same way without influencing the quotient value. The device is thus fairly insensitive to external environmental conditions.

The frequency, which is responsive to the vertical force component, is converted in the trigger and low-pass filter circuits 13, 14 to a direct voltage, which after calibration (in post-coupled detector circuits in an electronic instrument part installed in the towing vehicle) corresponds to the magnitude and direction of the vertical force component, and which can be taken from the connection terminals of the sensor plate 10 (earth, signal and voltage feed) via an unillustrated cable connected between the sensor means 5 and the electrical system of the towing vehicle (with an associated instrument for displaying the vertical force value). The sensor plates and spacer means can of course be arranged in another way than the one shown, provided that the required deflection or pivoting of the movable sensor plate is achieved (optionally two or more movable plates can also be arranged), preferably symmetrically for separating upwardly and downwardly directed forces. It is also possible to place the holder plates 4 and 6 with the intermediate sensor means 5 oriented horizontally in the tensional force plane, although the bolts will here be subjected to shear forces and must therefore be dimensioned accordingly.

An alternative transducer embodiment is illustrated in Figs. 4a and 4b, the sensor plate 5' being intended to replace both plates 10, 11 in Fig. 2 (although with spacer strips 8a, 8b, 9a, 9b, 9c placed in a corresponding manner for similar deflection of the upper end lower portions of the plate 5' under the action of the vertical force component). As in the previous embodiment, the sensor plate 5' consists of a circuit board laminate of glass fibre reinfoced epoxy plastics and provided with resistor elements R₁ and R₂, which consist of metal foil loops etched directly out of the sensor plate, and which are included in a bridge circuit (Fig. 4b) for providing an output voltage V_ut . This output voltage V_ut is responsive to the deflection of the plate 5' in the region of the respective resistor element R₁, R₂, since the latter alter their resistance in response to the strain in the vertically extending portions caused by deflection of the plate, these portions suitably being situated relatively close to each other. A quotient is also measured in this case, namely R₁/R₂ so that the magnitude and direction (upwards and downwards) of the vertical force component can be indicated reliably with the aid of the bridge circuit in Fig. 4b and a connected detector circuit in the instrument part installed in the towing vehicle.

Instead of the etched metal foil loops, conventional wire strain gauges can of course be applied to the deflecting plate 5' or to corresponding separate plates on each side of the plane defined by the axis lines A, B (Fig. 1).

The force measuring device described above can also be modified by one skilled in the art within the scope of the following claims. For. example, the sensor means can be placed between two holder plates on the connecting part associated with the towed vehicle. To obtain the desired movement of the sensor plates it is also conceivable to combine stiff and relatively elastic spacer means, so that the sensor plate is pivoted under the action of the vertical force component. Capacitive measurement is preferably utilized in this case, e.g. according to Figs. 3a and 3b.

Claims

1. Force measurement device for measuring the vertical force component (P) acting on a drawbar coupling (1) between a towing vehicle and a towed vehicle, particularly where the latter has a single axle, the coupling comprising a coupling part, e.g. a towing ball (2), rigidly connected to a first stiff holder plate (4), which is fastened by fastening means to a corresponding second, similarly stiff holder plate (6) mounted on the towing or towed vehicle, preferably on the towing vehicle,

c h a r a c t e r i z e d in that the fastening means between the two holder plates are placed along a straight line, and in that a sensor means (5) having at least one sensor plate

(10,11), preferably of elastic material, is inserted between the holder plates (4,6) with intermediate spacer means (8a, 8b, 9a, 9b, 9c) placed such that the holder plates (4,6) are restrictedly, mutually elastically pivotable about said straight line under the action of said vertical force component, at least one sensor plate (10) being deflected or pivoted when the holder plates are pivoted mutually, and electrical transducer means (10a-11a,

10b-11b) being disposed for sensing the deflection or pivoting of the sensor plate (10) for emitting an electrical signal corresponding to said vertical force component.

2. Force measurement device as claimed in claim 1,

c h a r a c t e r i z e d in that said straight line passes through the region where the coupling part (2,3) is fastened to said first holder plate (4).

3. Force measurement device as claimed in claim 2,

c h a r a c t e r i z e d in that said straight line passes through a substantially horizontal line of symmetry for both holder plates (4,6).

4. Force measurement device as claimed in any one of claims

1 - 3, c h a r a c t e r i z e d in that each of the holder plates (4,6) is located in a substantially vertical plane.

5. Force measurement device as claimed in any one of claims 1 - 4, c h a r a c t e r i z e d in that said straight line extends substantially horizontally between two opposite edge portions of each of the holder plates (4,6), so that on the respective side of said straight line the sensor plate (10) is deflected or pivoted in response to whether the vertical force component (P) is upwardly or downwardly directed.

6. Force measurement device as claimed in claim 5,

c h a r a c t e r i z e d in that said transducer means

(10a-11a, 10b-11b; R₁,R₂) are placed on each side of said straight line, and said electrical signal is responsive to the quotient between the magnitudes sensed by the transducer means.

7. Force measurement device as claimed in claim 5 or 6, c h a r a c t e r i z e d in that the spacer means are placed on each side of said sensor plate or plates (10), namely such that on one side there are first spacer means (9a, 9b, 9c) situated in the region of said straight line, and also in the region of said edge portions, and on the other side of said plate or plates there being other spacer means (8a, 8b) situated in two intermediate regions between the straight line and the respective edge portion.

8. Force measurement device as claimed in any one of the preceding claims, c h a r a c t e r i z e d in that said sensor means (5) includes, on the one hand, at least one sensor plate (11) engaging against one holder plate (4) and thus being immobile, and on the other hand, at least one sensor plate (10) which is movable by deflection or pivoting and which is kept spaced from the immobile sensor plate (11) and from the other holder plate (6) with the aid of said spacer means (8a, 8b, 9a, 9c), the sides facing each other of the movable and immobile sensor plates (10,11) being provided with capacitor plates (10a-11a, 10b-11b), the mutual spacing of which varies with said vertical force component (P) and which constitute said

transducer means.

9. Force measurement device as claimed in claim 8, c h a r a c t e r i z e d in that, said capacitor plates

(10a,10b, 11a,11b) consist of metal coatings on said sensor plates (10,11).

10. Force measurement, device as claimed in any one of claims 1 - 7, c h a r a c t e r i z e d in that at least one sensor plate (5') is flexible and is provided with at least one

resistor element (R₁,R₂) constituting said transducer means and functioning as a strain gauge for sensing the bending of the sensor plate.

11. Force measurement device as claimed in claim 10,

c h a r a c t e r i z e d in that said resistor element

(R₁,R₂) consists of a metal foil loop etched directly out of the sensor plate (5').

12. Force measurement device as claimed in any one of the preceding claims, c h a r a c t e r i z e d in that said at least one sensor plate and/or said spacer means comprise a fibre reinforced epoxy plastics laminate.

13. Force measurement device as claimed in any one of the preceding claims, c h a r a c t e r i z e d in that at least one sensor plate included in the sensor means also serves as a circuit board for electrical circuit means connected to said electrical transducer means.