Abstract
Integrated single-axis force sensors allow an accurate and cost-efficient force measurement in 6 degrees of freedom for hexapod structures and kinematics. Depending on the sensor placement, the measurement is affected by internal forces that need to be compensated for by a measurement model. Since the parameters of the model can change during machine usage, a fast and easy calibration procedure is requested. This work studies parameter identification procedures for force measurement models on the example of a rigid hexapod-based end-effector. First, measurement and identification models are presented and parameter sensitivities are analysed. Next, two excitation strategies are applied and discussed: identification from quasi-static poses and identification from accelerated continuous trajectories. Both poses and trajectories are optimized by different criteria and evaluated in comparison. Finally, the procedures are validated by experimental studies with reference payloads. In conclusion, both strategies allow accurate parameter identification within a fast procedure in an operational machine state.
Highlights
Six 1 degrees of freedom (DoF) force sensors are integrated into a rigid bar framework and the measured forces are transformed to Cartesian forces and moments at the tool centre point (TCP) by a control-integrated model
Besides process force measurement and control, structure-integrated force sensors can be used for parameter identification: As mass, centre of gravity and inertia of the end-effector can change during machine usage, for example through a workpiece or tool change, a fast and easy procedure for identifying these parameters is required, which should last less than 5 min and can run in a production break
This is because a dynamic trajectory excites the parameters in a different way, while the platform joint forces that cause the force errors through platform deformation are foremost inducted by static hexapod strut forces—dynamic joint forces are much smaller and can be neglected, as separate simulations show
Summary
Many manufacturing applications, such as process diagnosis and quality assurance, or adaptive process control for milling, grinding or thermo-smoothing, require in-process force measurement, where, in particular, the measurement of spatial forces and moments in 6 degrees of freedom (DoF) is requested [1]. Integrated sensors can be supplied by the machine manufacturer and make force measurement and control an easyto-use machine-integrated feature in the future To reach this point, further studies and evidence regarding the parametrisation are required, as hardly any suitable literature on parameter identification for force measurement models is available. This work presents a one-step calibration procedure that allows the parametrisation of the measurement models first presented in [1], which are used in the new approach of structure-integrated force measurement in parallel kinematic machine tools. FT sensors can benefit from a measurement model and a parameter identification procedure when used during movement
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