GB2385938A

GB2385938A - Load cell

Info

Publication number: GB2385938A
Application number: GB0304618A
Authority: GB
Inventors: A Maddison
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-03-01
Filing date: 2003-02-28
Publication date: 2003-09-03
Also published as: GB0204871D0; GB0304618D0

Abstract

A hydraulic load cell comprises a first portion 100, a second portion 200 and a sensor 50. The first portion comprises a cylinder 12 having a central chamber 16 and is connected via PTFE coated spacers 36 to a load bearing base member 32. The second portion comprises a load bearing disc 40 connected to a central piston 24 which slides within the central chamber 16. The central chamber is filled with hydraulic fluid which, upon loads applied to the base member 32 and disc 40, transfers fluid pressure to a pressure sensor removably mounted in the cylinder. The arrangement enables both tensile and compressive forces to be measured.

Description

Load Cell The present invention relates to a load cell.

Load cells are force conversion devices, which translate an applied force (such as weight) into convenient means for measuring or monitoring the force. They come in several different designs geared towards a number of different applications. For example, load cells can be purely mechanical such as lever based weighing scales. Alternatively they can also be pneumatic or hydraulic force-balancing devices. More commonly, however, load cells are electro-mechanical devices, which translate an applied force into an output voltage. This output voltage can be measured and calibrated against known forces and hence any applied force, within the range of the load cell, can be measured and changes in it monitored over time.

Currently, the most widely used types of load cells are electro-mechanical devices which include an electronic sensor comprising a set of bonded foil strain gauges in a balanced Wheatstone bridge arrangement and associated compensation circuitry. Such strain gauge sensor circuitry is typically sensitive because any slight imbalance in the bridge arrangement will affect a significant change in the output voltage. Hence, the strain gauge circuitry is protectively sealed within the load cell. However, the sensitive nature of the circuitry also means that its performance can degrade significantly over time until the load cell no longer functions acceptably. In these circumstances the entire load cell has to be replaced.

The output voltage of a strain gauge Wheatstone bridge circuit is proportional to the force applied to the load cell. This voltage is also dependent upon the input voltage and is typically small, in the range of only a few millivolts output per volt input. Output voltages therefore require amplification prior to measurement and interpretation. Furthermore, relatively large output voltages are desirable, prior to amplification, to maximize the signal to noise ratio of the strain gauge circuit, and hence correspondingly large input voltages are often necessary.

Additionally, in order for the load cell to function properly, a minimum activation voltage is required across the input terminals.

Additionally, each load cell has a predefined set of operational parameters (for example load capacity, sensitivity and accuracy), which are determined by the characteristics and limitations of the electronic sensor. Hence, a wide range of different load cells would be required to cover a diverse set of measurements.

Hence, there is a requirement for a load cell in which the above problems are overcome or at least mitigated.

According to the present invention there is provided a load cell for measuring forces comprising: a first portion and a second portion; and force transfer means through which a force applied to either one of the first and second portions is transferable to the other of the first and second portions; and a removable sensor for sensing an effect of a force acting between the first portion and the second portion; Advantageously the first portion includes a chamber; and the second portion includes a piston slidably received within the chamber.

Advantageously the first portion includes a first end and a second end; and the second portion includes a force-application element located intermediate the first and second ends of the first portion, the element being adapted such that a force may be applied to the first portion from either the first or the second end and a corresponding opposing force may be applied to the force-application element.

Advantageously the first portion comprises a cylinder with a cylinder wall and a cylinder top arranged to form part of the chamber, and a base member rigidly connected to the cylinder in spaced relationship to an end of the cylinder wall distal from the cylinder top; the second portion comprises a force application element associated with the piston and through which forces can be transmitted to or from the piston between the cylinder and the base member.

Advantageously a plurality of longitudinal bores extend through the cylinder wall and the base member is spaced from the cylinder wall by a plurality of tubular spacers, each spacer being aligned with a corresponding one of the bores.

Advantageously the cylinder is rigidly connected to the base member by means of fasteners, each fastener extending through a respective bore in the cylinder wall and a tubular spacer.

Advantageously the force-application element is slidably received on the tubular spacers.

Advantageously the force-application element is a disc having a plurality of through bores, each spacer being slidably received in a respective through bore in the disc.

Advantageously the disc is a separate component from the piston.

Advantageously the force transfer means is a fluid, the chamber being at least partially filled with the fluid, such that when opposing forces are applied to the first and second portions, a corresponding pressure is produced in the fluid.

Advantageously the sensor is a pressure sensor.

Advantageously the load cell makes up part of a structure comprising one or more similar load cells.

The invention provides an improved load cell, which may be used for the measurement of static and dynamic loads, in tension or compression, under a variety of conditions. The invention allows the easy and inexpensive replacement of the electronic sensor either in the event of serious performance degradation or to permit changes in operational parameters such as load capacity or sensitivity. Furthermore, the invention allows the power supply and output monitoring circuit requirements to be simplified.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure I shows a longitudinal cross-section of a load cell according to the present invention, Figure 2 is a plan view of the load cell illustrated in figure 1,

Figure 3 is a perspective view of the load cell illustrated in figure 1, Figure 4 shows diagrammatically, the application of the load cell in a generic tensile test apparatus, Figure 5 shows diagrammatically, the application of the load cell in a generic compressive test apparatus and Figure 6 shows diagrammatically, the application of the load cell in a known stress test apparatus.

In figures 1 to 3 a load cell is designated generally 10. The load cell comprises a first portion 100, a second portion 200 and a sensor 50. The first portion 100 includes a cylinder 12 with a cylinder wall 14 and a cylinder top 18. The lower surface of the cylinder top 18 and the inner face of the cylinder wall 14, form part of a central chamber 16, as seen in figure 1. A plurality of cylindrical vertical bore holes 20, are provided in an annular ring around the central chamber 16, as seen on figure 2. The vertical bore holes 20 extend from the upper surface of the cylinder top 18 longitudinally down through the cylinder wall 14 to its lower surface.

The cylinder top 18, is centrally provided with a threaded sensor aperture 22 into which a conventional commercially available pressure sensor 50 is fitted. Typically, the pressure sensor 50 may be an inexpensive industrial hydraulic pressure sensor, such as a transducer or transmitter. Such sensors typically have an integral signal amplifier and are capable of being powered by a standard dc battery cell, thus negating the need for an external amplifier or a mains operated power supply.

As shown in figure 1, the lower end of the first portion 100 further includes a plurality of tubular spacers 36 and a base member 32. The base member 32 is rigidly coupled to the cylinder 12, by bolts 34, which extend longitudinally down, firstly through the vertical bore holes 20, and then through the tubular spacers 36. The tubular spacers 36 provide longitudinal clearance between the base member 32 and the cylinder wall 14. They also act as a smooth, low friction, bearing onto which a disc 40 may be slidably received, as described

later.

The base member 32 includes a spigot 48, which is provided with a specimen aperture 38 for mounting a test specimen to the base member either indirectly, via an adaptor, or directly. In order to allow the test specimen, or adaptor, to be securely fixed, the aperture 38 may be threaded. Alternatively, or additionally, the spigot may be provided with transverse holes through which fixing pins may be inserted (neither illustrated) to secure the test specimen.

The second portion 200 includes a piston 24 comprising a piston crown 44 at its upper end and a piston skirt 46. The piston skirt 46 extends down from the piston crown 44, as seen in figure 1. The piston 24 is slidably received within the central chamber 16 thus forming a cavity 26, of variable volume, at the upper end of the central chamber 16. The piston 24 is provided with a seal 28, substantially adjacent to the piston crown 44, to hydraulically isolate the cavity 26. The piston 24 is further provided with bearing elements 30, adjacent to the piston skirt 46, to facilitate smooth vertical movement of the piston 24. Typically the seal 28 may be in the form of a ring made from PTFE or another polymer-based material, whilst the bearing elements 30 may be in the form of bronze filled PTFE strips.

The second portion 200 further includes a force-application element, which comprises a substantial disc 40 slidably received on the tubular spacers 36 via clearance holes provided for the purpose. The disc 40 is provided with a flanged portion 42 around its circumference to allow convenient interfacing with a test apparatus. Although, in the embodiment described, the disc 40 is separate to the piston 24, it will be appreciated that the disc 40 and piston 24 may form a single integral component.

The cavity 26 is filled with hydraulic fluid (such as oil or the like) prior to fitting the pressure sensor 50 such that when the sensor 50 is in place the space between the piston crown 44 and the sensor diaphragm (not shown) is fully occupied by the fluid. The volume of the cavity is set such that, when the pressure sensor 50 is fitted and the piston skirt 46 contacts the disc 40, clearance is provided between the disc 40 and both the base of the cylinder 12 and the upper surface of the base member 32.

The arrangement is such that, when a force is applied to the first portion 100, in the direction indicated by arrow B, and an opposing force is applied to the second portion 200, in the direction indicated by arrow A, the piston 24 generates a corresponding pressure within the cavity 26, which pressure is proportional to the applied force. The pressure of the fluid is sensed by the pressure sensor 50, which produces an output voltage, proportional to the pressure in the cavity 26. The, output voltage of the sensor, and therefore the pressure of the fluid, can be measured or monitored using standard equipment such as a voltmeter, oscilloscope or datalogger. Subsequently the magnitude of the force may be calculated from the known cross-sectional area of the piston 24 in combination with the measured pressure.

Alternatively, the output from the pressure sensor 50 may be calibrated, using a tensometer, to allow the measurement of force directly.

Figure 4 shows how the load cell 10 can be configured for use in monitoring a tensile load.

In figure 4, a tensile test apparatus is designated generally 58 and comprises a load cell 10 as described above, a specimen 60, a substantial support 62, a tension tube 64 and a crosshead 68.

The tension tube 64 is of a slightly larger diameter, than the flanged portion 42 of the disc 40, along most of its length to allow the insertion of the load cell 10. At one end, the tension tube 64 is provided with a lip 66 for engagement with the flange 42.

The load cell 10 is inserted into the tension tube 64 such that the lower surface of the flanged portion 42 of the disc 40 interfaces with the lip 66 of the tension tube 64 and the base member 32 is external to the tension tube 64. The end of the tension tube 64 opposite from the inset portion 66 is fixed to crosshead 68.

One end of the specimen 60 is connected to the spigot 38 either indirectly, via an adaptor designed for the purpose, or directly. The other, opposite end of the specimen 60 is connected to the substantial support 62.

In operation, a force is applied to the crosshead 68, in the direction indicated by arrow C.

This drives the disc 40 and hence the piston 24, in the same direction as the applied force, to increase the pressure of the fluid in the cavity 26. The applied force is transferred, via the fluid in the cavity 26, to the cylinder 12 and hence to the base member 32 and specimen 60.

The end of the specimen opposite to the base member 32 is fixed to the substantial support 62 and therefore a tensile force, of substantially equal magnitude to the applied force, is realised in the specimen 60.

The resulting pressure in the cavity 26 may be measured via the pressure sensor 50 and hence the magnitude of the tensile force calculated, or alternatively measured after suitable calibration with respect to the output voltage.

Figure 5 shows how the load cell 10 can be configured for use in monitoring a compressive load.

In figure 5, a compressive test apparatus is designated generally 70 and like parts to those in figure 4 are given like reference numerals. The compressive test apparatus 70 comprises a load cell 10, a specimen 60, a substantial support 62, a compression tube 72, a tubular adaptor 74 and a crosshead 68.

One end of the compression tube 72 interfaces with the cylinder top 18 whilst the opposite end is coupled to the crosshead 68. The tubular adaptor 74 includes an open end 76, which interfaces with the flanged portion 42 of the disc 40 such that the base member 32 is free to move relative to the disc 40. The tubular adaptor 74 further includes a closed end 78, which engages the upper end of the specimen 60, as viewed in figure 5. The lower end of the specimen, as viewed in figure 5, is mounted on the substantial support 62.

In operation, a force is applied to the crosshead 68, in the direction indicated by arrow D, and therefore a substantially equal magnitude compressive force is realised in the specimen 60 via the tubular adaptor 74. Hence, as with the tensile test, a corresponding force is produced between the first portion 100 and the second portion 200, thus generating a corresponding pressure within the cavity 26. The resulting pressure in the cavity 26 may be measured via the pressure sensor 50 and hence the magnitude of the compressive force calculated, or

alternatively measured after suitable calibration with respect to the output voltage.

Figure 6 shows how the load cell 10 can be configured for use in a typical stress testing apparatus.

In figure 6 a stress test apparatus is designated generally 80 and like parts to those in figures 4 & 5 are given like reference numerals. The load cell 10 is positioned at the upper end 82 of a stress tube 84 such that the upper end 82 interfaces with the underside of the flanged portion 42 of the disc 40. One end of a specimen 60 is connected to the spigot 38 either indirectly, via an adaptor designed for the purpose, or directly. Another, opposite end of the specimen 60 is connected to a substantial support 62. Typically, the stress test apparatus 80 may be in a similar form to that described in GB 2345146 in which a tubular housing is described, which is directly analogous to the stress tube 84.

In operation, a force is applied to the base of the stress tube 84 in the direction indicated by arrow E, and hence a substantially equal magnitude tensile force is realised in the specimen 60. Therefore, as with the tensile test, a corresponding force is produced between the first portion 100 and the second portion 200, thus generating a corresponding pressure within the cavity 26. The resulting pressure in the cavity 26 may be measured via the pressure sensor 50 and hence the magnitude of the tensile force calculated, or alternatively measured after suitable calibration with respect to the output voltage.

Claims

Claims 1. A load cell for measuring forces comprising a first portion and a second portion, and force transfer means through which a force applied to either one of the first and second portions is transferable to the other of the first and second portions, and a removable sensor for sensing an effect of a force acting between the first portion and the second portion.
2. A load cell as claimed in claim I in which the first portion includes a chamber, and the second portion includes a piston slidably received within the chamber.
3. A load cell as claimed in either of claims I or 2 in which the first portion includes a first end and a second end and the second portion includes a force-application element located intermediate the first and second ends of the first portion, the element being adapted such that a force may be applied to the first portion from either the first or the second end and a corresponding opposing force may be applied to the force-application element.
4. A load cell as claimed in claim 2 in which the first portion comprises a cylinder with a cylinder wall and a cylinder top arranged to form part of the chamber, and a base member rigidly connected to the cylinder in spaced relationship to an end of the cylinder wall distal from the cylinder top, the second portion comprises a force application element associated with the piston and through which forces can be transmitted to or from the piston between the cylinder and the base member.
5. A load cell as claimed in claim 4, in which a plurality of longitudinal bores extend through the cylinder wall and the base member is spaced from the cylinder wall by a plurality of tubular spacers, each spacer being aligned with a corresponding one of the bores.

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6. A load cell as claimed in claim 5, in which the cylinder is rigidly connected to the base member by means of fasteners, each fastener extending through a respective bore in the cylinder wall and a tubular spacer.
7. A load cell as claimed in claim 5 or claim 6, in which the force-application element is slidably received on the tubular spacers.
8. A load cell as claimed in claim 7, in which the force-application element is a disc having a plurality of through bores, each spacer being slidably received in a respective through bore in the disc.
9. A load cell as claimed in claim 8, in which the disc is a separate component from the piston.
10. A load cell as claimed in any of claims 2 to 9, in which the force transfer means is a fluid, the chamber being at least partially filled with the fluid, such that when opposing forces are applied to the first and second portions, a corresponding pressure is produced in the fluid.
11. A load cell as claimed in any of claims 1 to 10, in which the sensor is a pressure sensor.
12. A load cell as claimed in any of claims I to 11, in which the load cell makes up part of a structure comprising one or more similar load cells.
13. A load cell substantially as hereinbefore described, with reference to and as illustrated in Figures I to 3 of the accompanying drawings.