Instrumentation Handle

Brian O’ConnorJanuary 2015

background

Test Engineering was approached to build a handle which could be retrofitted to Cybex strength equipment with minimal impact to the user experience.

The handle would transduce force and moment information which could be used to calculate the reaction forces in the user.

Two working concepts were generated:- single sensor concept- dual sensor concept

The assumption is that the data coming out of the single sensor would not be sufficient to properly calculate the user reaction forces. This is due to the potential for the user to impart a pure coupling moment which would effect the “sum of the moments equals zero” equation’s ability to predict the Center Of Pressure location with respect to the origin.

sensor (qty 2)

3 piece handle

bridge

pin adapters (qty 2)

mounting holes(pick any group of four (#10 x 32) to mount Instrumentation Handle to strength equipment)

Assembled View

clearance adjuster

Exploded View

handle has features which receive the pin adapters during assembly

x

y

z

Assembly Procedure

the “clearance adjuster” part can slide laterally then be tighted into place. This allows the pin to pin distance to be tweaked until the desired fit (no slop, no preload) is achieved.

clearance adjuster

Section View of Handle & Pin

x

z

unassembled• no contact

assembled & adjusted• “edge of contact” established

the radiused surface of the handle will maintain a line of contact regardless of small Pin movement or misalignment which may results from fabrication tolerances or deflections anticipated during use.

Section View of Handle & Pin

x

y

assembled• edges of contact are theoretically made at four edges• in reality the ideal assembled position will be achieved with the smallest gap between

parts at these edges so the part is “floating” and experiences zero pre-load

The “edges of contact” will support forces in X, Y, Z directions. Any pure coupling moments generated by the user should show up as equal and opposite shear forces in the left and right sensors. Once these are accounted for, the remaining shear force should be used to calculate the Center Of Pressure.

3 piece handlepin adapters (qty 2)

Material Selection and finish

• the “edge of contact” approach creates a higher contact stress than a typical surface to surface contact which presents a risk of denting. Therefore Tool Steel was chosen, for its high hardness combined with good machinability, for the handle and the pin adapters.

• A tiny amount of sliding is to be expected at this interface. A Surface Roughness Average of 1.6 microinches was selected to help prevent binding and surface marring

• bridge parts and handle flanges were machined from 6061-T6 aluminum to reduce weight

• Production rubber sleeve was used for the handle to help approximate a realistic user experience

this dual sensor design features clearance between the handle and the “pin adapters” when the handle is not being loaded. The handle is theoretically “floating” in its idealized position shown. (In reality the weight of the handle would bring it into contact with the pins, but this will be ignored).

X, Y directions: When a user grasps the handle and moves it from this floating position a small amount, contact is made at either the right or left end first thus establishing an “edge of contact” as shown in the previous slides. This edge alone is not sufficient to prevent further rotation so the free end then rocks into contact at the opposite side of the handle thus setting up the second “edge of contact” 5.293” away. At this position the handle is fully supported and in static equilibrium regardless of user input.

Whether the user input includes pure shear loading or a combination of shear and coupling, the reaction forces at each sensor will theoretically be shear only.

Z-direction, Handle Twist: axial rotation is possible and the square pin of the adapter can react out this input. This input will produce “z-axis” moments on the sensors. This coupling will be shared between the sensors. The extent to which this is shared depends on the machining tolerances of the assembly – whichever side develops the two points of contact first will begin to twist, and therefore develop moments until the clearance is gone at the other side – at which point any additional load will be shared.

Z-direction force will first bottom out the handle, then impart compression. Any tension produced by sliding friction at the other side, which is also measured, can be combined at the software level.

The intent is to measure the magnitude of any pure coupling being applied so that this can be accounted for in the moments and forces equations.

Explanation of reactions

Appendix:

(see following pages for design documentation)

Appendix: Design Factor

AD2.5D-500 (force sensor, qty 2 per handle)• axial capacity 500 lbs • shear capacity 250 lbs

Assumed max working static load = 100 lb (based on human capability)desired Design Factor = 5

Appendix: FEA Vert

min factor of safety on yield: 4.9

loading condition: 100 lb vertical load

max displacement: .017”

Appendix: FEA Horiz

min factor of safety on yield: 6.0

loading condition: 100 lb horizontal load

max displacement: .020”

Appendix: FEA Axial

min factor of safety on yield: 18

loading condition: 50 lb axial load

max displacement: .009”