As the Light Weight Robot (LWR) is a highly integrated system of mechanical and electronic components collaboration between electronic and mechanical design from the start is essential.
At project start the basic design of integrating the controllers and power converters in the joints was determined as well as communication and supply concepts and power distribution. In the design process of the joints and links permanent interaction between mechanical and electrical design was necessary to achieve a high integration level. First board size and shape are defined according to various requirements, such as
- required area for the PCB layout,
- connector and grounding positions,
- PCB shape for fitting into the robot structure
- heat sinks for power components,
- component positions for minimum stacking height
- cable and mounting directions
- sensor and peripherals attachment
This close interaction can only be performed using interchange modules between the different design applications as CAD and PCB layout software in several iteration loops.
Collaboration between engineering disciplines
Finite Element Analysis
Designing a Light Weight Robot demands explicit knowledge of the stiffness of the structure inherited. As a light construction is always elastic, the thinning of the structure may only be performed to a grade of stiffness that is least required for good controllability. The Finite Element Analysis was used to examine all the critical components of the structure as well as to layout the torque sensors for maximum linearity and minimum influence of transverse forces. The tools that are used for this analysis are Pro/MECHANICA and ANSYS 7.0.
Light-weight robot concept
A robot with a load-to-weight ration similar to that of the human arm (1:1) is the design goal. The usage of Harmonic Drive gears, of motors with high power density and of light materials, as well as a consequent light-weight oriented mechanical design are key issues in reaching this goal.
The effects of the inherent flexibility which appears mainly in the joints (effects such as vibrations or absolute positioning errors of the tip), are compensated using additional sensors in model based control structures.
In addition to usual motor position sensing, the link side position and the joint torque are measured in each joint.
Especially the joint torque sensor plays a key in this context, being used for:
- Active vibration damping. The torque sensor measures the vibrations caused by the elasticity on the link side and allows active vibration suppression with a full state feedback controller.
- Actively adjustable compliance. The compliance of the arm can be adjusted by measuring the external torques acting on the robot and by using these measurements within an impedance controller. In this way, the robot can compensate for uncertainties in the environment perception and reduce the contact or impact forces. In directions with high uncertainty, the robot should be rather soft, while in the directions in which high precision is required, the stiffness should be high.
- Collision detection. Based on the torque sensor information and on an accurate robot model, collisions of the arm with the environment can be detected not only at the tip, but also along the entire robot structure. The robot can then react by switching to the low impedance mode.