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Praxim: Techanical
Analysis Report Technical review of gravity compensation
Davy Chui, Nicholas Adams, David Mountford, Ibrahim Gadala, Erica Wodzak
4/6/2010考研英语小作文模板
Gravity compensation has been revisited to asss the need for rotational gravity compensation mechanism for a four link uni-compartmental knee bone cutting device
Gravity Compensation
This ction analys gravity compensation techniques to be implemented on the device. Background
The goal of this technical analysis is to design a method to minimize the effect of unbalanced forces on the ur experience of bone cutting while using the four linkage uni-compartmental knee bone cutting device. The device is required to limit the ur input needed during operation to 2kg. This force is des
cribed as the operational weight of the device. Gravity compensation can be ud to reduce the operation weight and consists of two components;
(1) The ability to impede the vertical motion of the tool with respect to the device bone mount
(2) The ability to impede device rotation about the joint located at the device bone mount Early analysis suggested that 80% of urs are satisfied if the force required to stabilize the device is
approximately 1 kg. As a result, gravity compensation measures will be implemented to reduce the operation weight of the device to 10N. Figure 1 shows the ur satisfaction curve for the operational weight of the device.
Figure 1: Plot of the predicted ur satisfaction at different operational weights
Rotational Gravity Compensation
Figure 2 shows an overview of the rotational gravity compensation problem. The joint located at the bone mount in Figure 2 is free to rotate about the y-axis. If the tool a distance D x away from the bone mount joint the weight of the tool and surrounding links create a moment about the bone mount joint that the ur must resist. A gravity compensation mechanism ud to minimize this moment should apply a torque to the radial link that resists both clockwi and anticlockwi rotation without having significant impact on the feel of the device at the hard surface.
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Figure 2: Simplified linkage diagram for rotational gravity compensation
Technical Analysis
In order to determine the torque needed to implement effective gravity compensation, the torque produced at different angles has been determined for the four linkage mechanism. A list of input vari
ables ud in the model is included in in Appendix B. The link lengths chon fulfill all envelope requirements, and the link mass have been generated from the Solidworks model using 303 stainless steel. In the model the x component of the centre of gravity of each link with respect to the bone mount joint is determined and ud to compute the moment about the bone mount joint at multiple tool positions and device orientations.
Three angles were varied in the analysis, (1) θR – the angle of the radial links with respect to the bone mount, (2) θ12 – the angle t hat varies the linear length of the radial links, and (3) θ54 - the angle of the
Tool
Weight
x
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Gravity
amid
tool link with respect to the horizontal plane. Plots for each are shown below and the red box highlights the predicted operating range for each link.
Results
Figure 3 shows that the torque is largely symmetrical about the vertical plane - where θR = 0.The maximum torque generated around the bone mount joint with the operating range is 1.3 Nm, which corresponds to a ur force of 9N. This value is bad on the curve at 180 degrees, where the radial links are at full length and is not within the predicted operating range.
Gravity compensation, such as a resistive joint shown in Figure 4, could be implemented to reduce the torque at the extremes of the range. This reduction would shift the ideal – zero torque – operating zone away from the vertical position shown in Figure 3 to an intermediate position shown by the red circles in Figure 4. The exact zero torque position will vary with the length of the radial links. This scheme would also increa the torque when moving in the opposite direction to over 2Nm in some positions.
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hope的用法Figure 3: Plot of torque with respect to θR at increments of θ12
evolveFigure 4: Plot of torque with and without resistive joint gravity compensation
Figure 4 shows the torque generated about the bone mount and how it changes with the linear length of the radial joints. The maximum torque of 1.55Nm corresponds to the device position when the radial links are completely horizontal, and as in Figure 3 this position is not part of the predicted operating range. A resistive joint could be ud to shift the curve downwards and reduce the total torque generated by half.
fridaysFigure 5: Plot of torque with respect to θ12 at increments of
θR
Figure 5 shows the torque generated about the bone mount due to ur motion along the y-axis. The maximum torque range of 0.2N is considerably smaller than that generated by the other two links. A resistive joint would not be beneficial for this joint.
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Figure 6: Plot of torque with respect to θ54 at increments of θR
