IL96692A

IL96692A - Method and apparatus for measuring forces

Info

Publication number: IL96692A
Application number: IL9669290A
Authority: IL
Original assignee: Shternsohn Vladimir; Egorov Nesher; Egorov Alexey
Priority date: 1990-12-17
Filing date: 1990-12-17
Publication date: 1994-02-27
Also published as: GB2265983A; DE4140993A1; GB9207572D0; IL96692A0

Description

METHOD AND A PARATUS 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 sheet-rolling process, measuring the pressure on the rails by a train travelling over the designed area, or checking the overweight of lorries moving along a toll road. It relates particulary to an apparatus producing elastic, super-sonic waves by means of a piezo-transducer , impinging them onto the contacting surfaces of two solid bodies, receiving the rsflected waves by means of the piezo-transducer which converts them into electrical output and transmits it to a processor for recording and printing.

Up to now forces are being measured by strain gages or mag neto-anisotropic instruments. These apparatus have some fundamental deficiencies such as low reliability under long-lasting pulsating and alternating loading, which stem from the electrical strain gages themselves. The latter consist of spcialized transducers which must undergo deformation while they are tightly connected to the force-absorbing components, and they are frequently damaged owing to break-down of their connection to these components. The magneto-anisotropic instruments are composed of special steel plates which have the habit of scaling and deteriorating, thus causing incorrect indication of the stress or load.

More recent methods include ultrasonic waves to impinge upon the contacting, rough surfaces of two solid bodies and to measure the proportion of waves reflected by the non-contacting portion? of the surfaces. Increasing pressure increases the amount of reflected waves and permits computation of the load or stress, after calibration. Tv/o 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 measuring the contact stress using the value of the comparison as the index of evaluation.

U.S. 501648 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 into 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 damping contact increasi g with the load applied.

These methods and apparatus are apt to produce incorrect results for the following reasons: The reflected waves do not only determine the contact stress but also the stress 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 i^ firmly connected.

SUMMARY OF THE INVENTION The force-measuring apparatus according to the present invention to be positioned betwe-en a load and a load-support comprises: two solid blocks each having two parallel, planar surfaces perpendicular to the direction of force caused by the load, of which their outer surfaces are in contact with the load and the support respectively, while their 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 distance H, from the contact zone and a second smooth, planar surface, a reflector, parallel to the contact zone at a distance from the first planar surface, H being larger than HQ, while the area of the second surface is smaller than that of the first surface; a piezo-electric transducer firmly attached to the first planar surface serving 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, second block and of those waves reflected by the second, reflector -surface and to convert both wave amplitudes into electric signals; a generator serving to excite the piezo-electric transducer by short electrical pulses within intervals of betrween 0.1 and 20MHz; a processor unit sei'ving to analyze the electrical signals emitted by the transducer and to compare them with a reference signal defining zero force, and to issue a signal defining the actual force enacted by said first body on said second body.

The piezo-electri cal transducer is firmly attached to the first planar surface by a liquid such as oil or water or by special glues.

The transducer emits short bursts of ultra-sonic elastic waves into the block, as excited by the generator, which are reflected into the 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 zero-load and, optionally, by means of different standard loads placed onto the device. - 5 - 096692/2 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 contact zone to the transducer.

SH0i¾T DESC IPTION OF THE DRAMIM6S Figure 1 is a section through the load measuring device,, Figure 2 is a detail of the device of Figure indicating dimensions relevant to the evaluation of the measurementSs Figure 3 is a diagram of the electronic circuit through the piezo-electric transducer and the processing unit, and Figure 4 illustrates the sequence of the excitation signal and the signals received from the reflector surface and the contact surface respectively.

DETAILED DESCRIPTION OF THE DRAUINGS The mechanical portion of the load measuring apparatus comprises a first solid block 7 and a second solid block 8S 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 13 is provided in the outer surface of block 7 which contains a piezo-electric transducer 9 firmly attached to its bottom surface HI, by means of a liquid or a glue. A second recess 10 extends from the inner surface of block 7 in upward direction - 6 * 096692/2 which terminates in a smooth reflector surface ΙΙ» The block is covered by a solid top 112 protecting the transducer against the surrounding media0 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 permeability of the contact surface to the waves received from the transducer changes with the magnitude of the force M acting on the device0 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 Vg emitted by generator 1D which 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 n„ In the contact zone the waves are diffracted in forward direction (P^) into block 8 through the actual contact spots and in reverse direction Pg onto the tranducer 9 at the micro-clearances » The vibrations received by the reflector surface are likewise reflected to the transducer at full magnitude (P^ The reflected waves reach the transducer after each pulse emission, waves Ρ arriving before waves Pg due to the shorter distance HQ of the reflector from the transducer than distance Hfl between contact zone and transducer. - 7 - 096692/2 The generation of the ultra-sonic waves by the piezoelectric transducer 9 and the conversion of the reflected waves received by the transducer into digital or analog output signals will be described in the following with reference to Figure 3 of the drawings The synchronous generator 4 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 4 has four output lines going to the pulse generator, to the peak detector, to sample/hold 2 and to sample/hold 3.

The pulse generator 1 creates strong pulses Vg serving to excite the piezo-transducer which transmit ultra-sonic waves P towards the contact zone and the reflector respectively. The reflected waves P¾ and P2 generate the corresponding signals and V2, comprised in the transducer output Vy. Signals Vg, V^, V2» comprised in Vy are conveyed to the gain-controlled amplifier 5 which also receives an error-correction signal from the comparator 6. The errors referred to are 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 14 which minimizes the pulse Vg since this does not carry any useful information,. The peak detector likewise receives from the synchronous generator 4 a reset signal VR, and transmit its output VD to the two sample/hold units. These units select time-spaced samples from the output VQ and convert them into signals ^* and Vg** while time spacing is controlled by signals Vm and VH received from the sync generator. 096692/2 Signal ν^» called "constant base signal" is sent to the comparator 6 where it is compared to a stable constant voltage VK„ As a result, signal V2* becomes an error-free signal dependent solely on the changes in the force NO corresponds to signal V

Claims

C L A I M S: -

1. An apparatus for measuring a force enacted by a first body on a second body 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 along 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 from said contact zone and with a second smooth planar reflector surface, at ' a distance HQ < HA 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 said reflecto surface and to convert these waves into electric signals, a pulse generator serving to excite said piezo-electric transducer by short 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 programmed tc analyze said signals, to compare then 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 Claim 2, wherein 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 Clai ml , 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. The apparatus for measuring a force enacted by a first body on a second body as claimed in Claims 1 through 6, and substantially as hereinbefore described and illustrated in the accompanying drawings. For the Applicants, Patent Attorney