The DLR Light Weight Robot III (LWR III) represents the third generation of light-weight robots developed at the DLR Institute of Robotics and Mechatronics. Like its predecessors it has an outstanding load-to-weight ratio. The robot weighs just 14 kg and is able to handle loads up to 14 kg. Thereby it has turned the dream of a robot with a load to weight ratio of at least 1:1 into reality. 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 were key issues in reaching this goal.
Similar to the human arm, the robot has seven degrees of freedom which results in advanced flexibility in comparison to standard industrial robots. The individual joints are mechanically connected via carbon-fiber structures and communicate via a fiber optical bus system. The innovative hand-axis design enables the configuration as pitch-pitch as well as pitch-roll unit. The electronics, including the power converters, the power cables and the communication bus system, is integrated into the robot arm. Neither a bulky external rack, known from standard systems, nor external cabling is needed.
Each joint is equipped with a motor position sensor, a link side position sensor and a joint torque sensor. Thus the robot can be operated position, velocity and torque controlled. Especially the joint torque sensor plays a key role in this context, being used for:
LWR Robot System
The LWR III is a light-weight, flexible, revolute joint robot, which is due to its sensoric equipment fit for robot-human interaction. The robots size, power and manipulation capabilities are fairly similar to that of a human arm. The robot can be connected to any gripper or tool by a standard robot interface flange, which can also be operated over internal supply lines.
The LWR III has 7 revolute joints for good manipulation capabilities in a changing workspace with unpredictable obstacles. As the arm is dedicated to operate the DLR artificial Hand II for research on human haptics, its kinematics is similar to that of a human arm. In this context the first roll-pitch-roll combination can be seen as shoulder and upper arm, followed by pitch-roll for the elbow and forearm and concluded by a pitch-pitch combination with intersecting axes (kardanic) as a wrist. For applications using tools or grippers mainly, an alternative configuration as pitch-roll wrist is provided in the mechanical design.
LWR III Robot Joints
The robot is made up of intelligent joint units with integrated electronics, which are connected by supply lines, an emergency loop circuit and an optical SERCOS bus for data transfer. The robot structure consists of different structure links made of carbon fibre composite.
The joint units consist of
For operating the LWR a brushless DC motor is used, which was specially developed for this task by DLR. All the parameters forming these motors have been optimized for the controlled operation in a robot and for light weight. As the required torque decreases over the length of the arm, three different types of the motor design are used (85, 70 and 50 mm Type).
In the LWR III different HarmonicDrive gear units are used due to their high gear ratio and torque vs. weight. The gear ratio is 1:160 (Joint 5: 1:100), which allows torque output of 200 (165), 100 (70) and 40 (30) Nm maximum (measuring range) and up to 1.9 rad/sec angular velocity for the joint.
Each joint in the LWR is equipped with a torque sensor on its output side between gear unit and structure. The torque sensor measures with a full bridge of strain gauges. The measuring range of the different joints is (+/-) 165, 70 and 30 Nm for axes 1/2, 3-5 and 6/7.
Each joint contains two different position sensors. One incremental sensor with high resolution on the motor side, for motor commutation and joint control, and another, absolute sensor on the output side of the gear for joint angle reference.
Power Supply unit
Each joint has its own power supply unit. The galvanically isolated supply voltages are generated from a 48V-DC-Input. The supply unit powers the controller board, the power converter and all sensors. An overall of six voltages are generated.
Joint- and Motorcontroller Board
The controller board contains two DSPs. The joint controller runs with 330µs cycle time on a TMS320VC33 floating point DSP from TI. This DSP is responsible for the communication with the robot controller over a SERCOS bus, memorize and calibration of sensor data, and the calculation of current commands for the motor controller via a dual port ram. The motor controller, a DSP56F807 from Freescale/Motorola, calculates the motor position and speed with 25us cycle time and measures temperatures, motor currents and hall signals.
The power converter has been developed for three phase motors. Two phase currents and the bridge voltage are measured galvanically isolated.
Besides the motor the safety brake is controlled via the power converter board.
The robot link structure is an exoskeleton made of carbon fibre composite. The different links consist of single direction carbon fibre rowings, which are sewed onto a carrier fabric. These preforms are put into a negative, soaked with epoxy resin and finally pressed to eliminate surplus resin to reduce weight. With this technique light and stable free form links can be produced, in which the carrying fibres can be placed optimally to take the expected loads.
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. 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.
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.