Past Research

Quantification of Internal Wrist Forces During Joystick Controller Operation

For productivity enhancement purposes, many forest product companies have mechanized their harvesting operations through the use of joystick controlled , mobile machines.

Very little research had been conducted to identify and quantify the human factor components associated with machine use. An understanding of the variability in measures such as fingertip forces, wrist angles and joystick displacement would provide joystick designers with information which could be used to minimize the injury risk to the operator as well as damage to the machine and the surrounding environment.

Quantification of the wrist angles, fingertip and internal wrist forces and joystick kinematics indicated that the forces were very low and the risk of Carpal Tunnel Syndrome (CTS) was slight. However, force was likely exerted by other portions of the fingers and hand thereby under predicting the internal wrist forces. In fact, wrist angles observed were higher for the 'left' and 'right' motions than for the 'forward' or 'back' motions, thus posing a higher risk for CTS development. Since none of the measured or calculated variables changed over time, learning and fatigue did not affect the results. A 'handedness' effect was observed through out the experimental results, indicating that subjects were more adept at using the dominant, right hand side joystick in unskilled operators at least.

The major CTS risk associated with this type of joystick is the repetition factor.

Joystick Dynamics and Effects of Stiffness and Speed on Upper Limb Kinematics

Anecdotal evidence indicates that repetitive motion injuries of the hand, wrist, neck and shoulders are occurring in operators of joystick controlled off-road industrial machines. It is unclear why this is so when the static torque requirements are actually quite low. The actuation force requirements for joysticks used for hydraulic actuation purposes located in mobile construction and forestry machines had been assessed from a static perspective. Contributions from acceleration and velocity had been ignored. In actuality, velocity is highest when the operator moves the joystick quickly and acceleration is highest when the operator hits a hard endpoint at the end of the range of motion. No previous work had been done which examined the effect of joystick stiffness and/or speed on upper limb and/or joystick kinematics. Alterations in upper limb kinematics may actually cause the operator to make unintended joystick motions which in turn may cause the operator to be at a higher risk for RSI occurrence.

It was our purpose to develop an understanding of the dynamics of hydraulic-actuation joysticks and to quantify the effects of joystick stiffness and speed on upper limb kinematics as a first step towards the development of a joystick design protocol. The aims of this project were:

develop a mathematical model to predict the dynamic forces incurred by an operator using hydraulic actuation joysticks which rotate about two axis in which the rotation origin is a universal joint
quantify the model coefficients for a commonly used North American heavy off-road equipment hydraulic actuation joystick
use the mathematical model to predict the dynamic torques incurred by the operators using a laboratory mockup of a joystick
determine the effects of three joystick controller stiffnesses and two motion speeds on the upper limb kinematics of operators

Our conclusions showed that the dynamic torques and forces are substantial. In particular, side to side motions are the most dangerous. To correct this situation, it is recommended that joystick manufacturers should use less stiff balance and return springs for these directions than those used for forward and back motions. If the design does not minimize end-stop acceleration then spring stiffness should not be too low. If it is, then acceleration at the Joystick hard endpoint will be very high, causing the operator to incur a large palm and finger impact.