Over the past thirty years, North American manufacturers of off road equipment for the construction and mining industries have gradually made a conversion from the exclusive use of lever, button and steering wheel controls to joysticks. Surprisingly, despite their widespread use, joysticks have not been studied extensively. The purpose of this work was to develop an understanding of the dynamics of hydraulic-actuation joysticks and to quantify the effects of joystick stiffness and motion speed on upper limb kinematics as a first step towards the development of a joystick design protocol. A mathematical model was developed which allowed the prediction of the dynamic torques/forces incurred by an operator using hydraulic actuating joysticks which rotate about two axes where the rotation origin is a universal joint. The model coefficients were quantified for a commonly used North American heavy off road equipment hydraulic-actuation joystick. The mathematical model and dynamic coefficients were then used to predict the dynamic torques incurred by operators using a laboratory mock-up of a joystick. Finally, the effects of three joystick controller stiffnesses and two motion speeds on the upper limb kinematics were assessed using nine unskilled joystick operators.
Joystick stiffness and speed affected the joystick and upper limb kinematics although probably not enough to cause an unintended machine function to occur. Joystick stiffness did not have as large an effect as motion speed on the operators' upper limbs or joystick kinematics.
The most important finding of this work is that the dynamic torques incurred during hydraulic actuation joystick use are substantial. While the peak torque values were not that different between the fast and slow experimental conditions, the very high negative accelerations observed at the maximum excursion of the joystick during fast experimental conditions indicates that the dynamics do matter. One of the first things that future work must address is the development of a dynamic model for the hard endpoint.
On the basis of acceleration at the hard endpoint and peak torque, as well as the bootstrap analysis results, joystick motions which have the highest potential for causing injury to an operator are the side to side ones. This indicates that less stiff return and balance springs should be considered for these directions than for forward and back motions. However, if the design does not minimize acceleration, it is important that the spring stiffnesses not be too low since deceleration at the joystick hard endpoints will be very high causing the operator to incur large palm and finger impacts.