This document describes the design of an instrumented handle that can be attached to strength equipment to measure user reaction forces. It has a dual sensor concept using two force sensors located in pin adapters on either side of a handle. This setup is intended to measure shear and moment forces from the user's grip and calculate the center of pressure. The handle features clearance between parts to allow it to float until gripped, then two contact edges are made to support loading. Material selection, assembly, and FEA analysis information is provided to validate the design.

1. Instrumentation Handle
Brian O’Connor
January 2015

2. 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.

3. 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

4. Exploded View
handle has features
which receive the pin
adapters during
assembly
x
y
z

5. 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

6. 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.

7. 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 handle
pin adapters (qty 2)

8. 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

9. 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

10. Appendix:
(see following pages for design documentation)

11. 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

12. Appendix: FEA Vert
min factor of safety on yield: 4.9
loading condition: 100 lb vertical load
max displacement: .017”

13. Appendix: FEA Horiz
min factor of safety on yield: 6.0
loading condition: 100 lb horizontal load
max displacement: .020”

14. Appendix: FEA Axial
min factor of safety on yield: 18
loading condition: 50 lb axial load
max displacement: .009”