Title of Invention

"PLATE TYPE FORCE SENSOR FOR SENSING IN-PLANE FORCES"

Abstract

A plate type force sensor for sensing in-plane forces comprising a thin plate (1) with a slit ( 12) cut all around the said plate with linear ball bearing (3) fitted in the slit (12) for support of the overhanging portion of the plate, wherein plurality of rings (2) are cut on the overhanging sides of the plate (11), said rings are provided with strain gauges (4) connected in wheat stone bridge configuations, said force sensor in the shape of a thin rectangular plate (1) measures the applied force/component of force in the plane parallel to that of the sensor plate and the said sensor plate is insensitive to the applied force / component of force that is perpendicular to the plate sensor ( out-of-plane force).

Full Text

This invention relates to a plate type force sensor for sensing in plate forces. A rectangular plate shaped force sensor is developed to measure forces acting in a plane parallel to that of the sensor. It is a strain gauge based sensor. The sensor plate is fastened to a rigid base and then forces are applied to the sensor either directly or through a structure mounted on the sensor. The in-plane component of the force that is the component of the force in the plane parallel to that of the sensor is sensed. The magnitude as well as direction of this force is measured. The sensor is insensitive to the out-of-plane component of the force. Force sensors are transducers, which translate an input of force into equivalent electrical signal for its measurement. Force transducers of different types, based on piezoelectric crystals, electrical strain gauge, etc. were evolved over a period of time. Amongst them, those based on strain gauges became more popular. However, the applications of these sensors were quite limited. Presently, different force sensors are commercially available for wide applications. The conventional force sensors for common applications are called load ceils. They have either bending or direct type of spring elements to sense the force. The load cells are clamped at one end and force is applied from the other. The designs of different makes of standard load ceils are more or less similar. There are no sensors available commercially, of the shape of a thin rectangular plate, employing octagonal ring type spring elements to sense the in-plane forces, as these are special requirements of a sensor for some specific applications. The sensor according to the present invention is in the shape of a rectangular thin plate. The plate has countersunk holes, through which it is bolted on a base. The plate has another set of evenly spaced blind holes with internal threads. They are used, if required, for bolting another suitable structure on the sensor plate for application of force. The force, thus can be applied either on the sensor or on the structure mounted on the sensor. The sensor senses the component of the force in a plane parallel to the plane of sensor. This force is further resolved in two mutually perpendicular directions, parallel to the sides of the rectangular plate, and then measured. As the two mutually perpendicular components are measured, the direction as well as magnitude of the force is obtained. The sensor is insensitive to the out-of-plane component of the force, that is perpendicular to the plane of the sensor plate. The spring element of the sensor is in the form of an octagonal ring. There are instances of usage of such type of spring elements in some special purpose transducers like dynamometers, but not in the manner in which it is used in the present invention. The ingenuity in the arrangement of the octagonal ring in this design makes it different from other designs. It is an entirely new concept having an unique advantage over other designs. There is no patent document having a configuration of the sensor, similar to the one used in the present development. The ring configuration is such that two locations on the ring undergo strain independtly due to the two mutually perpendicular components of the in- plane force. Strain gauges are installed at these locations. The measurements of these two components of force enable determination of magnitude as well as direction on the force. Accordingly to the present invention, there is provided a plate type force sensor for sensing in-plane forces comprising a thin rectangular plate with a slit cut all around the said plate with linear ball bearing fitted in the slit for support of the overhanging portion of the plate, wherein plurality of rings are cut on the overhanging sides of the plate, said rings are provided with strain gauges connected in wheat store bridge configurations, said force sensor in the shape of a thin rectangular plate (1) measures the applied force/ component of force in the plane parallel to that of the sensor plate and the said sensor plate is insensitive to the applied force/ component of force that is perpendicular to the plate sensor (out-of-plane force). The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings: Fig.l represents the strain gauge installation on the octagonal ring; Fig.2 represents the circuit for measurement of Force Fr; Fig.3 represents the circuit for measurement of Force Ft; Fig.4 represents the plate type force sensor for sensing in-plane force. Fig, 5 is the exploded view of force sensor assembly. Fig.6 represents constructional features of plate. Fig.7 represents details of the slit in the plate. Fig.8 represents arrangement of bearing installed in the slit of plate. Fig. 9 shows photographic view of the sensor. Fig. 10. shows photographic view of the inside of the sensor. Fig. 11. shows photographic view of the testing of sensor. The sensor is in the shape of a rectangular thin plate (1). The plate (1) has countersunk holes (7), through which it is bolted on a base. The plate has another set of evenly spaced blind holes (8) with internal threads. They are used, if required, for bolting another suitable structure on the sensor plate for application of force. The force thus can be applied either on the sensor or on the structure mounted on the sensor. The sensor senses the component of the force in a plane parallel to the plane of sensor. This force is further resolved in two mutually perpendicular directions, parallel to the sides of the rectangular plate, and then measured. As the two mutually perpendicular components (Fr, Ft) are measured, the direction as weii as magnitude of the force is obtained. The sensor is insensitive to the out-of-plane component of the force, that is perpendicular to the plane of the sensor plate. The sensor as referred in Figs 1,2 and 3 uses a spring element of the octagonal ring type to sense the force. The octagonal ring undergoes strain when subjected to force. The strain response of the ring, which is proportional to the force applied, is measured by strain gages. The scheme employed to measure force is explained below. Strain gages are installed on the inside and outside of the octagonal ring as shown in the Fig.l. The locations TI, Tl', Cl and Cl'on the octagonal ring undergo strain (tensile strain at Tl, Tl' and compressive strain at Cl, CT) when the radial force Fr is applied. They do not undergo any strain when the tangential force Ft is applied. Thus, these locations are sensitive to force Fr and insensitive to force Ft. Conversely, the locations T2t T2', C2' and C2' are sensitive to force Ft (tensile strain at T2, T2' and compressive strain at C2, C2') and insensitive to force Fr. Strain gages installed at these locations sense the strain. The strain gages are connected in Wheat stone bridge circuit. The circuit for measurement of radial force Fr is shown in the Fig.2. The circuit for measurement of tangential force Ft is shown in Figs.3. Fig.4 shows a schematic of the sensor. Fig.5 shows the exploded view of the sensor assembly, where all the parts of the assembly are shown. The sensor consists essentially of a plate (1) on which strain gages (4) are installed as per scheme given in Fig.l. Linear bearing (3) are fitted in the slit portion of the plate. They are covered with bearing covers, which are part of the sealing arrangement (10). The lead wires (15) of the strain gage circuit are taken out from connecter (5) for connection to the measuring instrument. The sensor is fitted with atop cover plate (6). The constructional features of the plate are shown in Fig.6. A thin rectangular plate (1) is taken and a slit is cut all around the plate, leaving out the central portion in the middle of plate. Thus, a slit is made in the plate except for the central region. The details of the slit (12) are shown in Fig.7. The top portion of the plate above the slit is referred as Top Plate' and the lower portion of the plate is referred as 'Bottom Plate'. Four octagonal rings (2) are cut on the four overhanging sides of the Top Plate. The ring thus, have one side integrai with the un-stit central portion of the plate. Linear ball bearings (3) are fitted in the slit so that the overhanging portion of the Top Plate containing octagonal rings is supported on the Bottom Plate with help of these bearings. The arrangement of bearings is shown in Fig.8. The Bottom Plate has countersunk holes (7), through which it is bolted on a base. The Top Plate has another set of evenly spaced blind holes with internal threads (8). They are used, if required, for bolting another suitable structure on the sensor plate for application of force. The size and position of rings is decided based upon the convenience of locating the holes for insertion of fastening bolts and specific requirements of the application. Strain gauges (4) are installed on the rings as explained in Fig.i and are connected in full bridge configuration as explained in Fig.2 and Fig.3. Two sets of lead wires are taken out from connector (5) for measurement of the two mutually perpendicular components of force. These wires are connected to a two channel strain measuring instrument. Each set of lead wire has four wires, two for giving excitation of maximum up to 10V and the other two for receiving output signal of voltage. The in-plane force, when applied on the ring, produces a strain response, which is measured by the measuring instrument. The out-of-plane component of the force is passed on the base on which the plate is clamped without causing any strain in the rings. The measuring instrument can calibrated to measure the two components of forces directly in terms of engineering units. The slit containing linear ball bearings is provided with a sealing arrangement (10). The slit is filled with a rubber compound and the bearing covers are fitted pressing it, so that uo dust or liquid from outside can enter the sensor. The connector (5) is fitted to carry the iead wires (15) with proper anchoring and sealing. The lead wires may further be carried through a flexible hose, which can be fitted to the connector. The senor plate is provided with a top cover (6) as shown in Fig.5. The Top cover is fitted only after clamping the plate sensor on the base. A force sensor based on this new concept was designed, manufactured and tested for one typical application, the details of which are given here for illustration. This application is for a Tool condition monitoring system for a lathe machine. The tool condition monitoring system alerts the operator about excessive wear of the tool as also stops the machine in case of an impending breakage or collision of the tool. The system measures the cutting force during the turning operation and based upon it, assesses the condition of the tool, as the cutting force is related to wear of the tool. The system consists of a force sensor and a PC based controller. The force sensor is installed in the path of the cutting force, in between the tool post and the cross slide of the lathe. First, the sensor plate is clamped rigidly on the cross slide and then the tool post is mounted by bolting it on the sensor plate. During cutting operation, the force coming on the tool is transmitted to the cross slide through the sensor enabling its measurement by the sensor. The sensor measures the feed and radial force i e. the two components of the in-plane force. It is insensitive to the tangential cutting force, which is the out-of-plane component of the force. The dimensions of rectangular plate, the sizes and location of holes in the plate and other specifications are arrived at based on the requirements of this particular case. Specifications: Size: 273mm x 183mm x 31.5mm Force Measured: 1) Feed axis cutting force 2) Radial axis cutting force Norn inal force: 350 Kgf Measuring device: Strain gauges in full bridge configuration Nominal sensitivity: In feed axis direction 0.393 rav/v In radial axis direction 0.281 mv/v Linearity: +/-!% Excitation: Max up to 10V Mechanical over load: 600% The sensor was manufactured and tested in the laboratory and then installed on the lathe machine. Fig.9 shows the photograph of the sensor with cover. Fig 10 shows the inside of the cover. This sensor was tested in the laboratory for static loading on a specially designed test rig as shown in the photograph of Fig.ll. The test rig has a facility of applying measured values of rectangular in-plane forces. The out-of-plane force is applied by dead weights. The sensor is tested for its sensitivity, linearity etc. as per the specifications. The sensor has applications in situations, where in-plane forces are to be measured. The rectangular plate shape facilities its installation on any rigid base. It allows the mounting of another structure on it through which forces can be applied. Two typical applications are mentioned below, I) For tool condition monitoring for lathe machine. This application is already explained above. II) Another typical application is for measurement of in-plane recoil forces for a gun. The plate sensor can be made in the shape of a base plate on which the turret of the gun can be mounted and the recoil forces can then be measured during actual operation. 1 only the in-plane forces will be measured. There is no force sensor available commercially, in the shape of a thin rectangular plate, based on the arrangement of the octagonal ring spring element as used in the present case, which measures in-plane forces but is insensitive to the out-of-plane forces. The arrangement is a novel concept. It facilitates measurement of force very accurately and reliably due to the inherent characteristics of the octagonal spring element. The thin rectangular shape gives it an added advantage so that it can be used in situations, where space is a premium. It is used as a base under a structure for measurement offerees applied on the structure. It measures the direction as well as magnitude of the force. The size of the plate can be tailor-made to suit the application. The invention described herein above is in relation to non-limiting embodiments as defined by accompanying claims. CLAIM; 1: A plate type force sensor for sensing in-plane forces comprising a thin plate (1) with a slit (12) cut all around the said plate with linear ball bearing (3) fitted in the slit (12) for support of the overhanging portion of the plate, wherein plurality of rings (2) are cut on the overhanging sides of the plate (11), said rings are provided with strain gauges (4) connected in wheat stone bridge configurations, said force sensor in the shape of a thin rectangular plate (1) measures the applied force/component of force in the plane parallel to that of the sensor plate and the said sensor plate is insensitive to the applied force/component of force that is perpendicular to the plate sensor ( out - of-plane force). 2. A plate type force sensor as claimed in claim 1 wherein four octagonal shaped rings are provided on the top portion of the plate (11). 3. A plate type force sensor as claimed in claims 1 and 2 wherein the rings (2) have one side integral with the unslit central portion (14). 4. A plate type force sensor as claimed in claim 1 wherein linear ball bearings (3) are fitted in the said slit to support the overhanging portion of the top plate (1) containing the rings (2) on the bottom portion of the plate (13). 5. A plate type force sensor as claimed in claim 1 wherein the application of force to the sensor is direct or through a structure mounted on the sensor and the said in-plane force is measured by measuring the two mutually perpendicular components of the force. 6. A plate type force sensor as claimed in claim 1 is provided with countersunk holes (7) for bolting on a base. 7. A plate type force sensor as claimed in claim 1 wherein strain gauges (4) are installed in the rings (2) and connected in full bridge configuration with two sets of lead wires taken out from the connector (5) for measurement of two mutually perpendicular components of the in-plane force, the said lead wires are taken inside a flexible hose (9) fitted to the said connector. 8. A plate type force sensor as claimed in claims 1 and 5 wherein the slit opening (12) containing the linear bearings (3) is provided with a sealing arrangement (10). 9. A plate type force sensor as claimed in claim 1 wherein the sensor plate is provided with a top cover (6). 10. A plate type force sensor for sensing in-plane forces as herein described and illustrated with the accompanying drawings.

Documents:

1552-del-1999-abstract.pdf

1552-del-1999-claims.pdf

1552-del-1999-correspondence-others.pdf

1552-del-1999-correspondence-po.pdf

1552-del-1999-description (complete).pdf

1552-del-1999-drawings.pdf

1552-del-1999-form-1.pdf

1552-del-1999-form-19.pdf

1552-del-1999-form-2.pdf

1552-del-1999-form-3.pdf

1552-del-1999-gpa.pdf