Force measuring device

Abstract: A novel force measuring device of simple design is provided with complementary stop surfaces protecting the device against over­ loading. Alternatively, overloading is prevented by duplicating the force measuring device with two portions operating in opposite directions of force application. Such force measuring devices are advantageously incorporated in couplings between a motor driven vehicle and a further vehicle to be coupled thereto. Signals derived from these force measuring devices are used in a board computer for controlling the operation of the motor driven vehicle and the further vehicle coupled thereto.

Description: The invention relates to a force measuring device using at least one pressure sensor and adapted for application of forces into opposite directions. DE-A-36 25 842 discloses force measuring devices using a pressure sensor operating on a piezo-electric basis or by using strain gauges embedded in an elastomeric material. DE-A-35 34 211 relates to a vehicle and a method for operating such a vehicle. Force measuring devices are used for optimizing the driving characteristics of the vehicle by determining forces exerted to the wheels, wheel suspensions, etc.. In general, the force measuring devices are loaded in one direction only,i.e. by the force exerted to a wheel. Under extreme conditions as with cars used in rallyes forces may be exerted onto the vehicle in the opposite direction, for example, when a wheel is freely elevated above ground or when driving the car over a very uneven ground. In general, when using force measuring devices as defined above, only forces acting in one direction may be determined. Furthermore, there is a danger that the force measuring device is torn to pieces when pulling forces are applied theronto. Beside determining forces acting in vertical direction there are applications where forces in horizontal direction are to be determined. For example, it would be desirable to determine tensional forces exerted by a trailer coupled to a motor car. When going down a slope the direction of the force will be reversed and the trailer may push the motor car. WO-A-86/02323 discloses a device used in a trailer coupling for determining threshold values of pulling and pushing forces acting onto this coupling. There are provided signal generating means by contacts indicating whether the tractor vehicle or the trailer vehicle is more heavily braked. However, the pulling and pushing forces are not directly measured therein. WO-A-86/03584 discloses a force measuring device comprising a pressure sensor in contact with an elastomeric material in a cylindrical narrow gap. However, this force measuring device is loaded in one direction only, namely the weight of large containers or the load exerted onto the wheel or axle of a vehicle. DE-A-21 17 424 discloses a force measuring device including two bars for applying pressure or tension forces onto a piston mounted within a cylindrical housing. In order to prevent damage of the piston having a sensing portion with a reduced diameter limiting means are provided which limit the movement of the piston relative to the cylindrical housing in the direction of tension forces. EP-A-0 294 803, which is a state of the art according to Article 54 (3) EPC, describes a face measuring device with a pressure sensor arranged in a gap filled with elastomeric material and formed between complementary tapered surfaces of a tubular housing and a piston, respectively. It is an object of the present invention to provide a force measuring device protected against destruction by overloading. It is a further object of the present invention to provide a force measuring device for determining forces acting in two opposite directions. According to the invention there is provided a force measuring device comprising the features of claim 1. Fig. 1 is a sectional view of a first embodiment of a force measuring device according to the invention; Fig. 2 is a sectional view of a second embodiment of the force measuring device according to the invention; and Fig. 3 is a schematic sectional view of a trailer coupling using a force measuring device according to Fig. 1. Fig. 1 shows a first embodiment of a force measuring device 1 including a cylindrical housing 2 having arranged in its interior a force introduction piston 3, preferably of tube-like shape. Elastomeric material 5, preferably silicon rubber fills a circular gap formed between complementary tapered annular surfaces 21 and 22 formed at the exterior peripheral surfaces of the piston 3 and the interior peripheral surfaces of the housing 2, respectively. The force measuring device 1 as explained above may be loaded in the main force direction 6 and will output a signal corresponding to the force applied thereto. Upon force application in the direction 6 the force introduction piston 3 tends to be shifted slightly to the right (Fig. 1). The force measuring device 1 shown in Fig. 1 may be used for rather high forces or corresponding pressures up to several hundred bars in the main force direction 6. It should be noted, that a circular sealing 9 may be provided between the cylindrical housing 2 and the force introduction piston 3 for preventing any penetration of humidity or aggressive substances which may deteriorate the elastomeric material 5. It should be understood, that the movement of the piston 3 is relative to the cylindrical housing 2. If a force is applied to the piston 3 in the opposite direction 7 the piston 3 would be drawn out of the housing 2. Therefore, a stop means 8 is provided limiting such a movement of the piston 3 relative to the housing 2. Specifically, opposing circular flanges 23, 24 extending radially may be formed complementary to each other on the exterior peripheral surfaces of the piston 3 and the interior peripheral surfaces of the housing 2,respectively. Thus, any force exerted onto the piston 3 in the direction 7 will move the piston 3 in Fig. 1 to the left until engagement of the annular surfaces 23,24. Fig. 2 shows a second embodiment of a force measuring device according to the invention using as a stop means for limiting the movement of a piston 13 in a housing 12 a second force measuring device 11' in addition to a force measuring device 11. The force measuring devices 11 and 11' have a similar, preferably an identical design and are combined with each other in an opposing manner. Accordingly, considerable forces may be applied to the piston 13 both in the main force direction 6 or the opposite direction 7. Since the elastomeric material 5 is essentially incompressible there will be only a very slight relative movement between the housing 12 and the piston 13. With a force measuring device according to the embodiment of Fig. 2 forces in two opposite directions may be determined. Furthermore, using preloaded pressure sensor 14 permits a compensation of any differences in the characteristics of the pressure sensors in particular in a measuring region close to zero. Fig. 3 shows an application of a force measuring device according to Fig. 1 for a trailer coupling 42 shown as a ball head coupling and used for coupling a trailer to a motor car. A piston 43 has attached thereto, f.i. by screws a coupling part 44 provided with the coupling ball 45. An engaging part 46 of a trailer (not shown) engages the ball 45 in a well known manner. In operation any force acting on the coupling 42 will be transmitted to the pressure sensor 4 generating an output signal in dependence on the force applied. The output signal of the pressure sensor 4 is transmitted to board computer means, especially a microprocessor 47 of the automatic control system of the vehicle for evaluation therein and for controlling the motor, the gearing, the steering and/or the brakes of the vehicle and/or the brakes of the trailer. Under normal conditions pulling forces will be measured by the pressure sensor 4 and transmitted to the microprocessor 47. However, when braking the vehicle the trailer may exert a pushing force on the coupling 42 urging the force introduction piston 43 in Fig. 3 to the left, which may result in an engagement of the surfaces 23, 24. For indicating this situation a contact 51, f.i. piezo crystal contact, may be provided on the surfaces 24 or 23 connected to the microprocessor 47. Upon engagement of the surfaces 23, 24 the contact 51 of an electrical circuit (not shown) will be closed transmitting a signal to the microprocessor 47, indicating a loading of the coupling in the pushing direction 7. Now, the microprocessor 47 may issue an acoustic or visual alarm signal indicated by reference numeral 53 in Fig. 3 and/or may increase the braking of the trailer. Alternatively, a design of a force measuring device according to Fig. 2 may be used for more accurate control by determining both the pushing and the pulling forces acting on the coupling 42 and using these force for the control of the operation of the vehicle. As a further alternative the pressure sensor 4 may be preloaded in order to enable it to measure forces in opposite directions. As indicated in Fig. 3, the board computer 47 may generate signals for controlling the breaking pressure p and/or a timing signal Δt indicating the coincidence or precession of the begin of braking of the trailer in respect of the vehicle. Preferably, the microprocessor 47 has stored therein corresponding reference values which become effective in dependence on the loading of the vehicle and the momentary operational parameters of the vehicle and the characteristics of the road, as an upward or downward slope thereof. It should be noted, that when using the design according to Fig. 2 for a coupling according to Fig. 3 the operation of the vehicle in combination with the trailer may be controlled more accurately. Specifically, the brakes of the trailer may be operated more strongly as compared with those of the vehicle in order to maintain the combination under pulling condition. In particular, the control of the braking may be such that a settable fixed force is maintained on the coupling independent from the condition of the road or intensity of braking. As a modification of the embodiment according to Fig. 3 the operation of the force measuring device 1 may be reversed such that the stop means 8 (Fig. 1) would be affected under normal pulling condition whilst the pressure sensor 4 would measure any pushing forces acting on the coupling 42, when going down an inclined road or upon braking of the vehicle. Whilst with the embodiment of Fig. 3 the force measuring device according to the invention is used in connection with a trailer coupled to a vehicle, many other application will be apparent to a skilled person in connection with vehicles or any other apparatuses. For example, the force measuring device according to the invention may be integrated to other vehicle parts, as the spring legs, wheel suspensions, the steering system or the motor drive (for determining the torque acting theron). An example for an application to other apparatuses than vehicles is the measurement of the advancing force to be applied to tools of milling or tooling machines or for measuring the compression forces for pressing machines.

Claims: A force measuring device comprising: a tubular housing (2, 12) having a cylindrical recess; a piston (3, 13, 43) inserted into said recess and penetrating through said housing (2, 12); at least one pressure sensor (4, 14) provided in a circular gap filled with elastomeric material (5) and formed between complementary tapered annular surfaces (21, 22) formed at the exterior peripheral surfaces of said piston (3, 13, 43) and the interior peripheral surfaces of the housing (2, 12), respectively; whereby said elastomeric material (5) fixedly adheres to the peripheral surfaces of said piston (3, 13, 43) and said housing (2, 12), and said pressure sensor (4, 14) is arranged in contact with said elastomeric material (5) and measures forces applied to said piston (3, 13, 43) in a main force direction (6) of a longitudinal axis of said piston (3, 13, 43) and transmitted by said elastomeric material (5) to said pressure sensor (4, 14); and limiting means (8) for limiting movement of said piston (3, 13, 43) relative to said housing (2, 12) in a direction (7) opposite to the main force direction (6). The force measuring device of claim 1, wherein said limiting means (8) is formed by complementary opposing radial projections (23, 24) formed at said peripheral surfaces of said piston (3, 43) and said cylindrical recess. The force measuring device of claim 1 or 2, wherein said limiting means (8) is provided with a signal generating means (51) generating a signal indicative of an excessive force exerted onto said force measuring device (1, 11). The force measuring device of claim 1, wherein said limiting means (8) is formed by duplicating said force measuring device (11, 11') including first and second pressure sensors (14) measuring forces applied to said force measuring device (11, 11') in opposite directions (6, 7). The force measuring device according to any of the claims 1 to 4 wherein the force measuring device (1, 11) is incorporated in a trailer coupling (42) of a vehicle for measuring vertical and/or horizontal forces exerted to the trailer coupling (42). The force measuring device of claim 5 further comprising a board computer means (47) for controlling a vehicle in operation with said force measuring device (1, 11) supplying signals to said board computer means (47) indicative of said forces exerted to said trailer coupling (42), said board computer means (47) controlling the operation of said vehicle in response of said signals. The force measuring device of claim 6, wherein said board computer means (47) further controls a braking system of said trailer coupled to said motor driven vehicle by said trailer coupling (42).

Inventors: HAEFNER HANS W, HAEFNER, HANS W., Häfner, Hans W.
Applicants: PFISTER GMBH
Application year: 1988
Main classification: G01L1
IPC codes: G01L1, B60D1, B60T7, G01L5