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A new testing rig for force platform calibration and accuracy tests
ELSEVIER
Gait & Posture 5 (1997) 228-232
A new testing rig for force platform calibration and accuracy tests H.S. Gill*, J.J. O’Connor OxfordOrthoppcdc Engheering Centre, University of
Oxford,
Nuffield
Orthopaedic
Centre,
Oxford, 0x3 7LD, UK
Received 10 November 1995; accepted 20 February 1996
Ab Accurate estimation of resultant joint forces and moments from gait analysis data depends heavily on the accuracy of measurement of the ground reactionforce (GRF). Typically, multicompnent force platforms are used to measure the components of the GRF and the position of thecentreof pressure. A new rig, designed to allow safeand quick static testing of the vertical component and centre of pressure outputs of such force platforms, is described. This rig has been used to test the accuracy of AMTI force platforms and was found to reduce dramatically the testing time. The results of the tests are also given. Keywords: Force platform; Testing; Ground reaction force; Gait
1. -00
One of the major uses of modem gait analysis is to es-
timate the resultant forces and moments transmitted by the joints. Such estimates are derived from the use of link-segment models utilising kinematic and kinetic
measurements[ 1,2]. The kinetic measurementsof the ground reaction force are usually made using multicomponent force platforms. Recently, attention has been drawn to the necessityof in situ calibration and regular accuracy testing of these devices. Proposals for such calibration and testing procedures have been made by Bobbert and Schamhardt [3], and the CAMARC partners [4]. Thesestatic test procedures require known test loads to be applied to the force platform at a large number of known locations over the platform. Ideally, most of the platform surface must be loaded at some point during the procedure. For accuracy, the appiied loads must cover the operational range of the measurementdevice. Thesetwo requirements result in a long and potentially dangerous operation involving the manual application and removal of dead weights. The applied loads (maximum load should at least equal average adult body weight with a required testload of approximately 900 N) + Cormponding
author.
0966-6362/97/$17.00 8 1997 Elsevier Science B.V. All rights reserved PI1 SO966-6362(96)01090-9
need to be applied to over 100points on the force platform, to achieve reasonable coverage of the platform surface. This applied load needs to be precisely placed and oriented. To maintain a guarantee of output quality, suchaccuracytestsshould be performed on a regular basis. To overcometheseproblems in routine force platform testing, a new load application rig was designed. The main design criteria were safety, accuracy and shorter testing time.
The equipment allowed the movement of a setof dead weightsparallel to the floor and application of a vertical load at any point over a 1.32 m x 0.94 m rectangle on the floor. It consistedof a rigid baseframe carrying two trolleys capable of relative movement OQperpendicular tracks (Figs. 1,2). The bottom trolley was carried on guidesattached to the baseframe. The guidesconsisted of, on one side, a precision slideway (Hepco Slide Systems Limited, Tiverton, UK), and on the other side, a roller bearing riding on a machined steel bar (Fig. 3). The top trolley was carried on similar guidesattached to the bottom trolley. A loading gantry, utilii a cuntilever to achieve a mechanicaladvan- of about 1:2 betweenthe loading
H.S. Gifi, J.J. O’Connor
f Goit & Posture
5 (1997)
228432
229
Fig. I. New force platform tasting rig. For scale, the long dimension of each force piatfonn is 508 mn:.
Steel tube stock
Machined
steel bar
0
length of slideway 1 = I .706m loom
Base Frame
Fig. 2. Plan view of new test rig.
230
H.S. Gill, J.J. O’Connor
I Gait & Posture
5 (1997)
228-232
Bottom Trolley
Square
section
Hepco Slideway . _.-_ --- --I Hepco Bearing I Assembly ,
Steel bh
I Locking screw
I
I
0
100tNll
Fig. 3. Trolley articulation detail for new test rig.
weights and the applied load, was mounted on the top trolley (Fig. 4). The cantilever was pivoted at one end and loading weights were placed at the other. The load was applied to the force platform through a steel rod, the load rod. The load rod was connected to the cantilever close to its mid-point and passedthrough a guide tube in the base of the loading gantry. The tip of the load rod ended in a S-mm diameter ball bearing. The weightswere lifted and lowered using a hydraulic bottle jack (Figs. 1,4). The whole rig was placed over the force platform to be tested and levehed using four large screws, one at each bottom corner of the base frame. When level, the plane of motion of the loading gantry was parallel to the force platform and the load rod was perpendicular to the force platform. The load rod length was adjustable by screwing the tip in or out. For operation the rod length was adjusted such that it touched the force platform when the car&lever was horizontal. Also, when levelled, the whole test rig rested only on the 1eveIhng screwsand thus was held fixed. The loading rig was calibrated using an independent load transducer. The cahbration procedure allowed a linear regressionequation to be formulated to relate the loadlnasstothefeirraa~~totheforce~~~via the load rod: L = 1.88Mg + 126.85
where M was the mass of the loading weight in kilograms, g was the atxeledon due to gravity and L to the fon;e phttform in Newtons; W&SttlCiU&d theintexcopttermaroa@ tile we@ of the cantileverandthel~rod.The~~ofthetesttig~de itpo8aibktopositionamaasof6Okgwithfmgertip forceandthusapplya maxiBWnverticalloadof -1200 N to any point on the force &&form surface.
Bottle Jack
Fig. 4. Loading gantry detail for new test rig.
H.S. Gill. J.J. O’Cmvuw
3.
Tea
The test Rrarxlure was performed on an AMTI GR66-1000 face platfom, (Advanced Mechanical Technology Inc., MA). The test rig was positioned over the force platform and levelled. An accurately marked grid, drawn on draughting foil, was taped in place over the platform. The grid provided 121 points distributed over the platform (Fig. 5). The force platform amplifier was switched on and allowed to warm up for 10 min &fore the strain gauge bridges were balanced. A 20-kg mass was placed on the loading gantry and the load applied to the platform on each of the 121 points on the grid. At each loading point, data was captured on a standard 486DX33 PC (OPUS Technology plc, Surrey, UK) using a DAS-16 (Computer Boards, Mansfield, MA) analogue to digital capture (ADC) card and using a custom program written in C. Data were captured for 2 s at a data rate of 50 Hz. The average values of the six outputs of the force platform were determined. The six outputs of the platform were used to calculate the centre of pressure, from the equations provided by the manufacturer. The procedure was repeated with masses of 30 and 40 kg. At each mass level, the testing took slightly under 1 h. 4. Results The major results of the tests are given in Table 1. Load application point was described in local force plat-
/ Guit & Posture
5 (1997)
228-232
23 1
Table 1 Results of static load tests on AM-H fhrce @dms, show mean and standard deviation (SD.) vaktss of the pwe~tage error in measured vcrticpl force (Fz), cross-talk bethotital and vertical force chanaels (Fx ami Fy cross-t&~) ead errors in calculsted loading point over the 121 toad points Test mass applied (kg)
Vertical load (N) Mean Fz error (%) S.D. Fz error (%) Mean Fx cross-talk (%) S.D. Fx cross-talk (%) Mean Fy cross-talk (%) S.D. Fy cross-talk (%) Mean x error (mm) S.D. x error (mm) Mean y error (mm) S.D. y error (mm)
20
30
40
4%. 1 2.8 0.5
680.8 1.9 0.3 02 0.1 01 01 I.6 0.8 06 0.b
865.4 1.3 0.4 0.2 0.1 0.4 0.2 2.1 0.7 0.9 0.6
0.3 0.1 0.1 0.1 2.5 1.0 0.9 0.8
I---
.-.--II
-__
.-
form co-ordinates; the x-axis was parailel to the shorter edge of the platform, the y-axis parallel to the longer edge and the z-axis was perpendicular to the plate. The origin of the axis system was located at the centre of the plate and for the purposes of this report both the x and y axes were located in the plane of the platform’s surface, so for all load points z was zero. The vertical load was therefore termed Fz. The mean and standard deviations for the error in measured Fz are given in Table 1. For all test loads, these errors are small. The errors in Fz are greatest for the smallest test force and least for the largest. The cross-talk between the horizontal forces and the applied vertical load was found to be very low for all test loads. Fig. 5 shows the results of calculation of the positions of the centre of pressure as well as those of the actual load points and axis system. 5. Discussion
+------ -----. --___
464nlrn -- .---...
Fig. 5. f&Jults of c&utation of the positions of Oentre Of PreSSUE for z&kg tst on AMTI force platform; o, actual load point; +. calculated mtre ofprcww~. The e-ted arrows (five times the actual error vector magnitude) indicate the directions of the error vectors.
The new force platform testing rig has dramatically reduced the time spent in performing routine tests for assessing the accuracy of the vertical component of the GRF and centre of pressure output from standard force platforms. It has also made the process much safer. To date, no suitable method of assessing the accuracy of the horizontal components of the GRF over the whole force platform surface has been described. Since the horizontal forces have large moment arms with respect to the lower limb joints, their contributions to the estimates of joint moment are sign&ant. The test rig and procedure described only allows the static performance of a force platform to be assessed. Procedures for routine dynamic tests are also needed. Regarding the results of testing the
232
H.S. Gill. J.J. O’Connor
I Gait & Posture
AMTI force platforms, the accuracy in vertical force measurement and reconstructing the centre of pressure is seen to be acceptable. The AMTI force platforms displayed very low levels of cross-talk between the vertical and horizontal force outputs; however since the horizontal forces have much larger moment arms than the vertical force, even small percentages of vertical force may lead to significant errors in joint moment estimates. The errors in reconstruction of the centre of pressure in the y-direction were noticeably less than those in the xdirection. For these tests, the grid used was drawn on foil and the load rod placed by eye over the grid points. In the tests of Bobbert and Schamhardt, a grid marked on an aluminium plate was used to improve accuracy; with the load point locating into drill holes. Since the error vectors, Fig. 5, are all directed in the negative xdirection, it is suspected that these errors are due to a parallax error in positioning, which would be removed by use of such an aluminium plate. Other than the bias in the x-direction, the error vectors do not show any characteristic pattern, which suggests that a correction algorithm, as used by Bobbert and Schamhardt for a Kistler platform, may not be useful in this case. The uncorrected mean errors obtained for the AMTI platform in the ydirection are less than those obtained by Bobbert and Schamhardt after their results had been corrected, and those in the x-direction are comparable.
5 (1997) 228-232
The test rig presented is a prototype and during its construction and use some limitations and ideas for improvement have come to light. The main point concerns the physical size of the rig. A smaller rig would be preferable with a shorter load rod; the main size criterion is that the load rod can be placed at any point over the platform to be tested without moving the rig. Acknowledgement The work presented in this report was financially supported by the Arthritis and Rhemnatism Council (UK), Mr H.S. Gill was supported on a research grant by the Wishbone Trust (UK). The authors wish to acknowledge the assistance of Mr TungWu Lu in performing the tests. Rd[I] Elfkman H. Forces and energy in the leg during walking. Am J. Physio1 1939; 12% 339-356. [2] Winter D A. Morrmts of force and nteclmnical power in jogging. J Bbmec~ 1983; 16: 91-97. (31 Bob&t M, schamhsrdt H. Aaxracy of determinii the point of force app&ation with piaoelectric force plates. 3 &ON& 1990; 23: 705-710. [4] CAMARC II. Standards for Instrumentation and Specitkations. Technical Report, Project A-2002 C.E.C. Programme AIM, August 1994.
Gait & Posture 5 (1997) 228-232
A new testing rig for force platform calibration and accuracy tests H.S. Gill*, J.J. O’Connor OxfordOrthoppcdc Engheering Centre, University of
Oxford,
Nuffield
Orthopaedic
Centre,
Oxford, 0x3 7LD, UK
Received 10 November 1995; accepted 20 February 1996
Ab Accurate estimation of resultant joint forces and moments from gait analysis data depends heavily on the accuracy of measurement of the ground reactionforce (GRF). Typically, multicompnent force platforms are used to measure the components of the GRF and the position of thecentreof pressure. A new rig, designed to allow safeand quick static testing of the vertical component and centre of pressure outputs of such force platforms, is described. This rig has been used to test the accuracy of AMTI force platforms and was found to reduce dramatically the testing time. The results of the tests are also given. Keywords: Force platform; Testing; Ground reaction force; Gait
1. -00
One of the major uses of modem gait analysis is to es-
timate the resultant forces and moments transmitted by the joints. Such estimates are derived from the use of link-segment models utilising kinematic and kinetic
measurements[ 1,2]. The kinetic measurementsof the ground reaction force are usually made using multicomponent force platforms. Recently, attention has been drawn to the necessityof in situ calibration and regular accuracy testing of these devices. Proposals for such calibration and testing procedures have been made by Bobbert and Schamhardt [3], and the CAMARC partners [4]. Thesestatic test procedures require known test loads to be applied to the force platform at a large number of known locations over the platform. Ideally, most of the platform surface must be loaded at some point during the procedure. For accuracy, the appiied loads must cover the operational range of the measurementdevice. Thesetwo requirements result in a long and potentially dangerous operation involving the manual application and removal of dead weights. The applied loads (maximum load should at least equal average adult body weight with a required testload of approximately 900 N) + Cormponding
author.
0966-6362/97/$17.00 8 1997 Elsevier Science B.V. All rights reserved PI1 SO966-6362(96)01090-9
need to be applied to over 100points on the force platform, to achieve reasonable coverage of the platform surface. This applied load needs to be precisely placed and oriented. To maintain a guarantee of output quality, suchaccuracytestsshould be performed on a regular basis. To overcometheseproblems in routine force platform testing, a new load application rig was designed. The main design criteria were safety, accuracy and shorter testing time.
The equipment allowed the movement of a setof dead weightsparallel to the floor and application of a vertical load at any point over a 1.32 m x 0.94 m rectangle on the floor. It consistedof a rigid baseframe carrying two trolleys capable of relative movement OQperpendicular tracks (Figs. 1,2). The bottom trolley was carried on guidesattached to the baseframe. The guidesconsisted of, on one side, a precision slideway (Hepco Slide Systems Limited, Tiverton, UK), and on the other side, a roller bearing riding on a machined steel bar (Fig. 3). The top trolley was carried on similar guidesattached to the bottom trolley. A loading gantry, utilii a cuntilever to achieve a mechanicaladvan- of about 1:2 betweenthe loading
H.S. Gifi, J.J. O’Connor
f Goit & Posture
5 (1997)
228432
229
Fig. I. New force platform tasting rig. For scale, the long dimension of each force piatfonn is 508 mn:.
Steel tube stock
Machined
steel bar
0
length of slideway 1 = I .706m loom
Base Frame
Fig. 2. Plan view of new test rig.
230
H.S. Gill, J.J. O’Connor
I Gait & Posture
5 (1997)
228-232
Bottom Trolley
Square
section
Hepco Slideway . _.-_ --- --I Hepco Bearing I Assembly ,
Steel bh
I Locking screw
I
I
0
100tNll
Fig. 3. Trolley articulation detail for new test rig.
weights and the applied load, was mounted on the top trolley (Fig. 4). The cantilever was pivoted at one end and loading weights were placed at the other. The load was applied to the force platform through a steel rod, the load rod. The load rod was connected to the cantilever close to its mid-point and passedthrough a guide tube in the base of the loading gantry. The tip of the load rod ended in a S-mm diameter ball bearing. The weightswere lifted and lowered using a hydraulic bottle jack (Figs. 1,4). The whole rig was placed over the force platform to be tested and levehed using four large screws, one at each bottom corner of the base frame. When level, the plane of motion of the loading gantry was parallel to the force platform and the load rod was perpendicular to the force platform. The load rod length was adjustable by screwing the tip in or out. For operation the rod length was adjusted such that it touched the force platform when the car&lever was horizontal. Also, when levelled, the whole test rig rested only on the 1eveIhng screwsand thus was held fixed. The loading rig was calibrated using an independent load transducer. The cahbration procedure allowed a linear regressionequation to be formulated to relate the loadlnasstothefeirraa~~totheforce~~~via the load rod: L = 1.88Mg + 126.85
where M was the mass of the loading weight in kilograms, g was the atxeledon due to gravity and L to the fon;e phttform in Newtons; W&SttlCiU&d theintexcopttermaroa@ tile we@ of the cantileverandthel~rod.The~~ofthetesttig~de itpo8aibktopositionamaasof6Okgwithfmgertip forceandthusapplya maxiBWnverticalloadof -1200 N to any point on the force &&form surface.
Bottle Jack
Fig. 4. Loading gantry detail for new test rig.
H.S. Gill. J.J. O’Cmvuw
3.
Tea
The test Rrarxlure was performed on an AMTI GR66-1000 face platfom, (Advanced Mechanical Technology Inc., MA). The test rig was positioned over the force platform and levelled. An accurately marked grid, drawn on draughting foil, was taped in place over the platform. The grid provided 121 points distributed over the platform (Fig. 5). The force platform amplifier was switched on and allowed to warm up for 10 min &fore the strain gauge bridges were balanced. A 20-kg mass was placed on the loading gantry and the load applied to the platform on each of the 121 points on the grid. At each loading point, data was captured on a standard 486DX33 PC (OPUS Technology plc, Surrey, UK) using a DAS-16 (Computer Boards, Mansfield, MA) analogue to digital capture (ADC) card and using a custom program written in C. Data were captured for 2 s at a data rate of 50 Hz. The average values of the six outputs of the force platform were determined. The six outputs of the platform were used to calculate the centre of pressure, from the equations provided by the manufacturer. The procedure was repeated with masses of 30 and 40 kg. At each mass level, the testing took slightly under 1 h. 4. Results The major results of the tests are given in Table 1. Load application point was described in local force plat-
/ Guit & Posture
5 (1997)
228-232
23 1
Table 1 Results of static load tests on AM-H fhrce @dms, show mean and standard deviation (SD.) vaktss of the pwe~tage error in measured vcrticpl force (Fz), cross-talk bethotital and vertical force chanaels (Fx ami Fy cross-t&~) ead errors in calculsted loading point over the 121 toad points Test mass applied (kg)
Vertical load (N) Mean Fz error (%) S.D. Fz error (%) Mean Fx cross-talk (%) S.D. Fx cross-talk (%) Mean Fy cross-talk (%) S.D. Fy cross-talk (%) Mean x error (mm) S.D. x error (mm) Mean y error (mm) S.D. y error (mm)
20
30
40
4%. 1 2.8 0.5
680.8 1.9 0.3 02 0.1 01 01 I.6 0.8 06 0.b
865.4 1.3 0.4 0.2 0.1 0.4 0.2 2.1 0.7 0.9 0.6
0.3 0.1 0.1 0.1 2.5 1.0 0.9 0.8
I---
.-.--II
-__
.-
form co-ordinates; the x-axis was parailel to the shorter edge of the platform, the y-axis parallel to the longer edge and the z-axis was perpendicular to the plate. The origin of the axis system was located at the centre of the plate and for the purposes of this report both the x and y axes were located in the plane of the platform’s surface, so for all load points z was zero. The vertical load was therefore termed Fz. The mean and standard deviations for the error in measured Fz are given in Table 1. For all test loads, these errors are small. The errors in Fz are greatest for the smallest test force and least for the largest. The cross-talk between the horizontal forces and the applied vertical load was found to be very low for all test loads. Fig. 5 shows the results of calculation of the positions of the centre of pressure as well as those of the actual load points and axis system. 5. Discussion
+------ -----. --___
464nlrn -- .---...
Fig. 5. f&Jults of c&utation of the positions of Oentre Of PreSSUE for z&kg tst on AMTI force platform; o, actual load point; +. calculated mtre ofprcww~. The e-ted arrows (five times the actual error vector magnitude) indicate the directions of the error vectors.
The new force platform testing rig has dramatically reduced the time spent in performing routine tests for assessing the accuracy of the vertical component of the GRF and centre of pressure output from standard force platforms. It has also made the process much safer. To date, no suitable method of assessing the accuracy of the horizontal components of the GRF over the whole force platform surface has been described. Since the horizontal forces have large moment arms with respect to the lower limb joints, their contributions to the estimates of joint moment are sign&ant. The test rig and procedure described only allows the static performance of a force platform to be assessed. Procedures for routine dynamic tests are also needed. Regarding the results of testing the
232
H.S. Gill. J.J. O’Connor
I Gait & Posture
AMTI force platforms, the accuracy in vertical force measurement and reconstructing the centre of pressure is seen to be acceptable. The AMTI force platforms displayed very low levels of cross-talk between the vertical and horizontal force outputs; however since the horizontal forces have much larger moment arms than the vertical force, even small percentages of vertical force may lead to significant errors in joint moment estimates. The errors in reconstruction of the centre of pressure in the y-direction were noticeably less than those in the xdirection. For these tests, the grid used was drawn on foil and the load rod placed by eye over the grid points. In the tests of Bobbert and Schamhardt, a grid marked on an aluminium plate was used to improve accuracy; with the load point locating into drill holes. Since the error vectors, Fig. 5, are all directed in the negative xdirection, it is suspected that these errors are due to a parallax error in positioning, which would be removed by use of such an aluminium plate. Other than the bias in the x-direction, the error vectors do not show any characteristic pattern, which suggests that a correction algorithm, as used by Bobbert and Schamhardt for a Kistler platform, may not be useful in this case. The uncorrected mean errors obtained for the AMTI platform in the ydirection are less than those obtained by Bobbert and Schamhardt after their results had been corrected, and those in the x-direction are comparable.
5 (1997) 228-232
The test rig presented is a prototype and during its construction and use some limitations and ideas for improvement have come to light. The main point concerns the physical size of the rig. A smaller rig would be preferable with a shorter load rod; the main size criterion is that the load rod can be placed at any point over the platform to be tested without moving the rig. Acknowledgement The work presented in this report was financially supported by the Arthritis and Rhemnatism Council (UK), Mr H.S. Gill was supported on a research grant by the Wishbone Trust (UK). The authors wish to acknowledge the assistance of Mr TungWu Lu in performing the tests. Rd[I] Elfkman H. Forces and energy in the leg during walking. Am J. Physio1 1939; 12% 339-356. [2] Winter D A. Morrmts of force and nteclmnical power in jogging. J Bbmec~ 1983; 16: 91-97. (31 Bob&t M, schamhsrdt H. Aaxracy of determinii the point of force app&ation with piaoelectric force plates. 3 &ON& 1990; 23: 705-710. [4] CAMARC II. Standards for Instrumentation and Specitkations. Technical Report, Project A-2002 C.E.C. Programme AIM, August 1994.