LWR Robot System
The DLR Light Weight Robot (LWR) is a light-weight, flexible, revolute joint robot, which by its overall sensoric equipment is especially dedicated for work in the sensitive area of 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 3rd generation LWR is built up as a classic dextrous arm with 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 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
- a DLR brushless DC Motor @RoboDrive with integrated power converter and electromagnetic emergency brake
- gear unit from Harmonic Drive
- a torque sensor on the output side of the gear
- position sensors on motor and output side of the gear
- an electronics stack consisting of power supply, joint and motion control DSPs and power converter for complete state feedback control of the joint
- the joint bearing consisting of a thin section cross roller bearing
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).
For the LWR different types and sizes of gear units of HarmonicDrive are used due to their high gear ratio and torque vs. weight. The gear ratio is 1:160 (J5: 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.
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.
The supply board was developed by the Fraunhofer Institute of Integrated Systems and Device Technology (IISB) in Erlangen within the “Bayerisches Kompetenzwerk für Mechatronik”.
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 first layout and board design was done by the Fraunhofer IZM department Micro-Mechatronic within the “Bayerischen Kompetenzwerk für Mechatronik”.
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.
||2 motor phase currents (galvanically isolated) |
||Bridge voltage (galvanically isolated)|
Torque and Position Sensor
This sensor measures joint torque with a full bridge of strain gauges and absolute joint position with a potentiometer. The analog sensor signals are converted with a 12 bit ADC and processed with a Ubicom SX Microcontroller. The values are transmitted to the joint controller with a high-speed serial differential bus.
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. The technique for building these structures was supplied by HighTex GmbH from Dresden (Germany).