CMM Gantry Torque Compensation via Force Feedforward
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Solution Overview
Problem
Coordinate measuring machines of the gantry type face challenges in compensating for resultant torques that occur during the movement of the sensor, as the center of gravity shifts, leading to unaccounted mass moments of inertia and inadequate torque compensation by existing controllers.
Innovation Solution
Incorporating a force feedforward control mechanism in the controller that adjusts the first and second drives based on the position of the second measurement slide and setpoint acceleration to compensate for torques relative to a rotation axis perpendicular to the coordinate directions, ensuring that torques are either partly or completely avoided.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a standard position regulator is used to control the gantry position, then the control system is simple, but the torques generated by the drives cause the gantry to rotate about the vertical axis due to unaccounted mass moments of inertia from center of gravity shifts
Solution Approach 1:
The controller calculates and applies compensation torques in advance based on the detected position of the second measurement slide and the setpoint acceleration. This preliminary action counteracts the mass moments of inertia before they cause gantry rotation, preventing positioning errors rather than correcting them after occurrence.
Solution Approach 2:
The system continuously detects the actual position of the second measurement slide and uses this feedback to adjust the compensation torques dynamically. This closed-loop feedback ensures that the compensation adapts to real-time changes in the system configuration and maintains positioning accuracy throughout the measurement process.
2Reliability
If the center of gravity of the mechanism is taken into account in the regulator, then torque compensation improves, but the device complexity increases significantly
Solution Approach 1:
Instead of implementing a fully complex dynamic model of the entire mechanism, the system applies compensation only to the specific local effect that causes problems - the mass moments of inertia related to the position of the second measurement slide. This selective approach addresses the critical issue without requiring complete system modeling.
Solution Approach 2:
The system changes the control parameters dynamically based on the position of the second measurement slide. By adjusting the compensation torques as a function of this position parameter and the setpoint acceleration, the controller adapts to varying mass distribution without requiring a permanently complex regulator structure.
3Measurement precision
If force feedforward control is implemented to compensate for torques, then positioning accuracy improves, but the computational requirements and control complexity increase
Solution Approach 1:
The force feedforward control calculates the required compensation torques in advance based on the setpoint acceleration and current position, applying them before the motion occurs. This preliminary calculation approach improves positioning accuracy by preventing errors rather than correcting them, while keeping the computational load manageable through focused calculations.
Solution Approach 2:
The controller acts as an intermediary that translates the setpoint acceleration and position information into compensation torques. This intermediary function simplifies the overall control architecture by handling the complex torque compensation calculations in a dedicated control layer, separating this function from the basic position regulation.
Data Source
AI summary
A coordinate measuring machine including a workpiece support for mounting a workpiece to be measured; a mechanism for moving a sensor in a first coordinate direction and a second coordinate direction perpendicular thereto. The mechanism includes: a first measurement slide guided in the first coordinate direction along two parallel guides arranged on opposite sides of the workpiece support. The first measurement slide spans the support. The first slide is driven via a first drive, which drives the first slide along a first guide of the guides, and is driven via a second drive, which drives the first slide along the second of the guides; a second measurement slide guided movably in the second coordinate direction along the first measurement slide. The second slide is assigned a position measuring system, via which the position of the second measurement slide relative to the first measurement slide can be determined; a controller, which actuates the first and second drive. The controller includes a force feedforward control, which, depending on the detected position (xactual) of the second measurement slide and depending on a setpoint acceleration (asetpoint) to be set of the first measurement slide, brings about an actuation of the first drive and/or of the second drive in such a way that the torques which as a result of the acceleration of the first measurement slide by the first drive and by the second drive relative to a rotation axis (Rz) which is perpendicular to the first and second coordinate directions at least partly or completely compensate for one another.