GB2265983A

GB2265983A - Method and apparatus for measuring forces

Info

Publication number: GB2265983A
Application number: GB9207572A
Authority: GB
Inventors: Vladimir Shterenzon; Alexey Egorov
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-12-17
Filing date: 1992-04-07
Publication date: 1993-10-13
Also published as: DE4140993A1; GB9207572D0; IL96692A0; IL96692A

Abstract

The apparatus comprises two blocks 7 and 8 each having a rough planar surface which contact thus forming a contact zone. One of the blocks contains a first recess 13 close to the applied force and a second recess 10 close to the contact zone I, both recesses terminating in smooth surfaces parallel to the contact zone. A piezo-electric transducer 9 is attached to the surface of the first recess 7 and emits pulses of elastic waves into the block towards the contact zone I and the second surface. The contact zone reflects to the transducer waves only from those spots which form clearances between the two blocks 7 and 8, while the surface of the second recess 10 reflects all waves received, the latter arriving at the transducer before those from the contact zone due to their shorter way. The transducer 9 converts the two kinds of waves into electric signals which are converted into digital output by an electronic circuit which compares the waves to standard signals defining zero load and normal conditions respectively, thus correcting errors due to influence of detortion, temperature and humidity changes and others. <IMAGE>

Description

2265983 METHOD AND APPARATUS FOR MEASURING FORCES

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for measuring large forces, as for instance the force enacted on steel sheets during a sheetrolling process, measuring the pressure on the rails by a train Lravelling over the designed area, or. checking the overweight of lorries moving along a toll road. It relat-es parti.ulary to an apparatus producing elastic, sxjper...-soD.-i.c waves by means of a,:.-,ezo-transducer, irTir)-i..nging them onto the contacting surfaces of two solid bodies, receiving the reflected waves by means of the piezo- transducer which converts them into electrical output and transmits J1- to a processor for recording and printing.

is Up to now forces are being measured by strain gages or magnetoanisotropic instruments. These apparatus have some fundamental deficiencies such as low reliability under Long-lasting pulsating and alternatinq loading, which stem from the -cl.ectrical stra.in gages themselves. The latter consist (_,J7 spcialized transducers which must undergo deformation while they are tightly connected to the forceabsorbing components, and thev are frequently damaged owi ng o break-down of their connection to these components. The m ag ne to - a. ni s c 1 ro p i c i ns truments are 54 compose,! of special Ceel plates q't-iich have the habit of s ca 1 ing a nd de terior.-,lti ng, thus causing iticorrect indical,ion of the stress or lond.

More recent methods include ultrasonic waver, In impi-le upon the contacting, rough surfaces of two solid bodies and to measure the proporti.,n of waves reflected by the non-contacting portions of the surfaces. Increasing pressure increases the amount of reflected waves and p,.rin,it.s computation of the load or stress. after ccalibration. Two such methods are described in two U.S.

Patents:

U.S. 4484475 describes the steps of causing an ultrasonic wave to impinge on the contacting surfaces as follows:

comparing an acoustic wave of the ultrasonic wave reflected from the contacting surfaces with an acoustic wave transmitted through the contacting surfaces; and vreasuring the contact stress using 1 h c. va-lu,-- of Lhe is comparison as the index of evaluation.

U.S. 5016488 describes a stress wave load cell which comprises a propagation member acoustically coupled to a transducer. Electric pulses are supplied to the transducer from a pulse generator and the electric pulses are converted in-to stress wave signals which propagate through the propagation member. The transducer also detects the stress waves after propagation through the propagation member and supplies an electrical signal to a processor which gives a measure of the load applied to the load cell. Damping members which have profiled surfaces are caused to move into damping contact with the propagation member when a load is applied to the load cell. The damping members damp the propagation of the stress waves to the area of damping contact, the area of daniping contact increasing with the load applied.

These methods and apparatus are apt to produce incorrect results for the following reasons: The reflected waves do ^ly determine the contact stress but also the stress not onL deformation of the propagation member itself. The results are also influenced by changes in temperature and by the deformation of the surface to which the transducer is attached.

It is the object of the present invention to avoid these drawbacks and to produce Correct data on the load applied to the apparatus, independent of the changes in temperature and independent of the deformation of the body to which the transducer is firmly connected.

SUMMARY OF THAE INVENTION

The force-measuring apparatus according to the present is invention to be positioned between a loa.d and a load support comprises: two solid blocks each having two parallel, planp.r surfaces perpendicular to the direction of forc-i caused by the load, of which their outer surfaces are in contact with the lo ad and the support respectively, while Lheir inner surfaces form a contact zone, wherein at least one of these surfaces is rough; one of the two blocks contains a first smooth, planar surface at a di.st>nce L] A from the contact zone and a second smooth, -- planar surface, a reflector, parallel to the contact zone at a distance H 0 from the first planar surface, H A being 1 -Larger than Hot while the area of the second surface is smaller than that of the firRt surface; a piezo-electric transducer firmly attached to the first planar surface ser-ing to emit elastic waves into this block at intervals and to detect, during the intervals, the amplitude of the elastic waves reflected from those portions of the contact zone which are not in direct contact with the surface of the other, secnnd block and of those waves reflected by the second, reflector surface and to convert beth wave amplitudes into electric signals; a generator serving to excite the piezo-electric transducer by short electrical pulses within intervals of between 0.1 and 20MHz; a p,-ocessor unit serving to analyze the electrical signals emitted by the transducer and to compare them with a referende signal defining zero force, and to issue a signal defining the actual force enacted by said first Indy ca said second body.

The piezo-electrical transducer is firmly attached to the is first planar surface by a liquid such as oil or water or bly special glues The transducer emits short bursts of ultra-sonic elastic waves into the block, as excited by the generator, which are reflected int.- 4- - _) L-lie transducer by both the reflector surface and the contact surface. The waves from the reflector surface arrive before those from the contact surface at substantially unchanged amplitude, while the amplitude of the waves reflected from the contact surface decreases with increasing load, i.e. with increased contact area. Calibration is made at zeroload and, optionally, by means of different standard loads placed onto the device.

The outer surface of the second block is preferably recessed in conical shape which serves to scatter and diverge the waves passing through the contact zone and to prevent them from impinging on, and returning through the con-'----ict zone to the transducer.

SHORT DESCRIPTION OF THE DRAWINGS

Figtire l is a section through the load measuring device, Figure 2 is a detail of the device of Figure 1 indicating dimensions relevant to the evaluation of the measurements, and Figure 3 is a diagram of the electronic circuit through the piezo- electric transducer and 1he. processing unit.

DETAILED DESCRIPTION OF THE DRAWINGS

The mechanical portion of the load measuring apparatus is comprises a first solid block 7 and a second solid block 8, both preferably metallic, and provided with parallel inner and outer surfaces. The blocks contact along their inner surfaces thus forming a contact zone I; these surfaces are shown to be rough, but it is obvious that one of the. surfaces may be smooth and the other rough with similar results. A recess 1.3 is provided in the outer surface of block 7 which contains a piezo-electric transducer 9 firmly attached to its bottom surface III, by means of a liquid or a glue. A second iecess 10 extends from the inner surface of block 7 in upward direction which terminates in a smooth reflector stirf;ce II. The blocl,. is covered by a solid top 12 protecting the transducer against the surrounding media. The outer surface of block 8 is recessed in conical shape (11) serving to diverge the waves arriving from the contact zone and to prevent them from being reflected back to and through the contact zone.

The permeabilit- of the contact surface to the waves received from the transducer changes with the magnitude of the force N acting on the de-ice, and it is obvious that the amplitude of the waves reflected from the contact zone decreases with increased pressure between the contact surfaces causing the reflecting micro-clearances between the surfaces to be more and more reduced. Therefore, the operation of the load measuring device is as follows:

The piezo-electric transducer 9 is excited by short electrical pulses V B emitted by generator 1, which is preferably lie between 0.1 and 20 MHz, causing the transducer to emit elastic vibrations P into the body of block 7 in the direction of the contact zone I and the reflector surface II. In the contact zone,he waves are diffracted in forward direction (P 3) into block 8 through the actual contact spots and in rever,e direction P 2 onto the tranducer at the micro-clearances. The vibrations received by the reflector surface are reflected to the transducer at full magnitude (P 1). The reflected waves reach the transducer after each pulse emission, wc-i ves P arriving before waves P 2 due Lo the shorter distance H 0 of the reflector from the transducer than distance H A between contact zone and transducer.

The generation of the ultrasoric waves by the piezo electric transducer and the conversion of the reflected 38 waves received by the transducer into digital or analog output signals will be described in the following s-,7ith reference to Figure 3 of the drawings; The synchronous generator employs a quartz crystal and governs timing of the pulses traveling around the circuit, in order to ensure a predictable sequence of events. The sync generator has four output lines going to the pulse generator, to the peak detector, to sample/hold 1 and to sample/hold 2.

The pulse generator crea.tes strong pulses V B serving to excite the piezo-transducer -which transmit ultra-sonic waves P towards the contact zone and the reflector respectively. The reflected waves P 1 and P 2 generate the corresponding signals V 1 and V 21 comprised in the transducer output V U Signals V BY V1, V 21 comprised in V U is are conveyed to the gain-controlled amplifier which also receives an error-correction signal from the comparator.

The errors referred to arc those, other than the force measuring signals, that may occur in the piezo-transducer as a result of changes in temperature, atmospheric pressure, humidity and ageing. The output of the amplifier is transmitted to the peak detector which minimizes the pulse V B since this does not carry any useful information.

The peak detector likewise receives from the syn chrc.ious generator a reset signal V Rr and transmit its output V D to the two sample/hold units. These units select time-spaced samples from the output V and convert them into signals D V 1 and V 2, while time spacing is controlled by signals V Lill and V H2 received from the sync generator.

Signal V called "Constant base signal,, is sent to the com parator where it is compared to a stable constant voltage V K As a result, signal V 2 becomes an error-free signal dependent solely on the changes in the force N. V 1 corresponds to signal V resulting from the wave P 1 reflected by the reflector surface, while V 2 corresponds to V 2 resulting from thewave P 2 reflected from the contact 7.one.

The A/D converter converts the signal V 2 received from the smaple/hold 2 from analog to digital and transmits it to the micro-controller for processing into display and the I/D port. The display reads the force N in appropriate units as determined by the setting of the micro controller, while the I/D port unit consistsof a is bidi--ectional data line.

It will be understood that the embodiment as illustrated and described herein before should be regarded as only one example of the apparatus of the invention and that changes and modifications may be carried out to both the mechanical as well to the electronic portion, provided the principle of the contact zone and the reflector surface are maintained.

Claims

C L A I M S: -9-

1. An apparatus for measuring a force enacted by a first body on a second body utilizes the fact that elastic waves travelling in a solid body towards a rough surface in contact with the surface of a second body travel into said second body through those surface portions in actual contact with said second body, while they are reflected by non contacting surface portions, and that with increasing load the contacting surface portions increase and more waves travel into said second body, said apparatus comprising two solid blocks each having two parallel planar surfaces perpendicular to the direction of said force, placed between said first and said second body, a first block in contact with said first body and a second block in contact with said second body, while said two blocks form a contact zone alonq their planar surfaces, of which at least one surface is rough, and wherein said first block is provided with a first smooth surface at a distance H A from said contact zone and with a second smooth planar surface, a reflector surface, at a distance H 0 < H A from said first surface, a piezo-electric transducer attached to said first smooth surface in said first block serving to emit pulses of elastic waves into said first block towards said contact zone and towards said reflector, and to receive, after each pulse, elastic waves reflected from those portions of said contact zone not in contact with said second block surface and from s:i.d refloctor surface and to ronvert- these waves into electric signals, a pul- generator serving to excite said piezo-electric transiucer by;hort electrical pulses of between 0.1. to 20 MHz, an electronic analyzing circuit adapted to receive the signals emitted by said piezo-electric transducer and p rog r'. 1 ITI rll e d to 1n,ilyz( said Lo compare, Ihem with a reference signal defining zero-force and to issue a signal defining the actual force exacted by said first body on said second body.

2. The apparatus as defined in Claim 1 wherein said first smooth surface containing said transducer is located in a recess extending from said surface proximate said first body.

3. The apparatus as defined in Claim 1 wherein said reflector surface is provided in a recess in said first block perpendicular to the direction of force, proximate said contact zone.

4. The apparatus as defined in Claim 1, wherein the surface of said second block adjoining said second body is provided with a conical recess serving to diverge the elastic waves received from said contact zone to the sides, in order to prevent their reflection into said contact zone.

5. The apparatus as defined in C1dim 2, whercin said piezo-electric transducer is attached to said first smooth surface of said first block by an elastic glue.

6. The apparatus as defined in Claim 1 -wherein said electronic analyzing circuit comprises comparator means serving to correct the errors occurring due to changes in temperature, humidity, distortion of the block material, by comparing the amplitude of the elastic waves reflected from said reflector with a standard value measured at standard temperature and at zero load.

7. An apparatus for measuring a force substantially as hereinbefore described with reference to the accompanying drawings.