A Motion Control Solution for a Seven-Joint Arm Robot

Collaborative robots created to work closely with people are becoming common on the factory production floor. These robots are well suited to accurate, efficient and precise tasks in human–robot environments.

Utilizing force sensors, cameras or other sensing elements, these robots intuit the presence of humans and react quickly, often totally stopping to avoid injuring humans.

These robots are frequently designed to handle small parts with high dexterity, and help in constructing consumer electronics items, instead of carrying out heavy duty operations like painting, lifting materials, or welding.

The robot design is quite small and lightweight, but also must be able to lift relatively heavy human payloads of up to 100 kg. Along with the required performance to carry out intricate work, the robot can understand whether parts are correctly and accurately constructed, which was unimaginable in former generations of heavy industrial robots.

Traditional factory six joint robots commonly moved their heavy arms with a large amount of force which could cause serious injury to any nearby humans. The new generation of robots has an additional joint which adds a level of freedom.

In comparison with the traditional 6-Degrees of Freedom (6-DOF) robot, the 7-joint (7-DOF) robot enables the arm to lengthen for a particular part in several orientations which avoids human collision and allows it to continue the task at hand.

Moreover, the kinematic redundancy is highly beneficial for controlling multiple robots within a limited space, since mitigating obstructions is simply managed. An additional characteristic of these robots is the ability to sense force and limitations. The robot senses and reacts to the smallest amount of external force allowing it to prevent collision.

In certain cases, force sensors located after the motor gearbox are utilized to observe any quick increase in external force. In other cases, the robot diagnoses the necessary amount of force to heighten a load and transport it from one position to another.

When the robot observes a necessary increase in torque for movement, for example in a collision, the robot ceases. An extra feature for the avoidance and detection of collision is its ability to modify the control mode from the inflexible, full-speed mode (such as CSP - cyclic synchronous position or CSV - cyclic synchronous velocity), to a torque mode (CST- cyclic synchronous torque) when unforeseen contact with a human or an object is recognized.

The soft, compliant CST mode enables the robot to be easily moved away by a human hand. The robot can be programmed to enter the compliant mode as soon as it identifies contact, or can be disarmed to continue in inflexible mode at optimal speed.

An additional important mode is the Teach mode, where the operator can move the robot to desired locations, while the Elmo controller documents these positions for trajectories in future operational modes.

It is important to note that there is an astounding degree of sensitivity and resolution precision during the teaching process. Even more incredible is the simplicity of each feature, and the fact that most tasks can be performed with no previous experience in programming.

Elmo Solution

Elmo's incredibly small and powerful network established on the Gold family of servo drives was chosen for the task. The drives were fixed directly onto the robot joints, and were an ideal choice for keeping the small size and compactness of the robot.

Two powerful Gold SOLO GUITAR drives were utilized to control the two base joint motors that assist the entire mechanical structure of the robot. The capability to drive the motors with a continuous current of 50 amps, and a high peak current of up to 100 amps gave the necessary high speed, acceleration, and deceleration, operation rates.

An additional five pin based Gold SOLO WHISTLE drives enabled to run up to 20 amps of continuous current, and up to 40 amps of peak current, were utilized to control the other five robot joints.

Every drive in the system operates at the highest speeds, acceleration and deceleration rates. However, they can also operate at low speeds with the highest precision and accuracy because of the incredible 1:2000 dynamic current range of the drive.

A drive with an adequately small physical footprint to allow it to be mounted directly onto the robot joint was the only choice. Positioning the drives close to the servo feedback meant that cabling was kept to a minimum, with the reduction of external noise, a low EMI and RFI, and an overall sharp increase in the reliability of the system.

A further characteristic of the drive that enhanced integration within the robot joints is the innate ruggedness and ability to cope with the high mechanical acceleration and deceleration rates.

By utilizing the EASII (Elmo Application Studio tool), the drives’ tuning procedure is raised to the highest level to optimize servo performance for each separate drive in the system.

System identification and adequate controller design, along with utilizing high order filters used to fight imperfections in the mechanical structure, were only some of the tools used to make the system’s servo performance highly efficient.

Multi axis identifications utilizing simple and advanced identification techniques with special position gain scheduling to mitigate the crossover-effects between the range of axes were just part of the facilities used in this application for operating the system with the greatest possible bandwidth, quickest response times,  along with maintaining the stability and seamless operation of the robot with high margins.

Every axis in the system performs in a dual loop servo control architecture to increase the position accuracy of the system after the gear box.

Closing dual loop control with an incremental encoder along with digital hall sensors for the inner velocity control loop before the gear box, while closing the outer position control using 19-bit high resolution absolute feedback is only one of the multi cross-feedback connection options available in all Elmo Gold drives as a standard feature.

The dual loop structure enhanced the local servo operation to its most optimized performance. For easy customer connectivity, each pin based drive in the system was supplied with an integrated SOLO board. The SOLO board enables simple connectivity and integration by the customer by allowing EtherCAT, IO and feedback connectivity.

Operating the entire system is Elmo’s advanced multi axis controller, the Platinum Maestro (P-MAS) which over real time deterministic serial EtherCAT networking can optimize the whole system operation to its greatest level of motion performance.

Elmo’s success in supporting a varied range of built-in robot kinematics utilizing the P-MAS, with mechanisms such as Cartesian, SCARA, 3- link, Delta and more, enable exemplary motion performance.

The kinematic support that is built-in operates in MCS (Machine Coordinate System) or PCS (Product Coordinate System) with full synchronization to turn tables, conveyors and alternative external equipment. Moreover, the PMAS has a dedicated real-time code section for user applications.

This special code section helps robot developers to write their own specific robot kinematic equations, allowing the P-MAS to support any high-end, unrestricted robot types available now, where the user can carry out their own specific kinematics.

The P-MAS is based on the Dual Core (2 × 1.5 GHz) powerful processor, and is critical for these application types as it allows instant calculation of the robot kinematics and inverse kinematics within a network cycle time of 250 µSec.

The kinematic equations executed in the user input real-time section can determine the target positions and velocities or torque of all the axes in the system and output them on each EtherCAT cycle time. Controlling the system in one of the DS-402 cyclic synchronous modes is a built-in customary operation of each of Elmo's EtherCAT compliant servo drives.

The wide range of standardized and proprietary communication protocols between the P-MAS and Host computer, PLC, or HMI, activate efficient and easy communications with third party features such as teach pendants, touch panels, PLCs, computers and many more.

Host communication features such as Ethernet, TCP/IP, and UDP Fast Binary Protocols such as Modbus and Ethernet IP, allow for easy, quick, and effective communication with almost all of the high level hosts in the system.

The operation of the robot can be separated into two main modes; the first is the operation teaching mode, and the second is the working mode. In the teaching mode, the operator transports the robot to critical position points along the desired path trajectory.

The multi axis controller tracks the related position points to repeat the entire trajectory motion in the working mode. During this mode, the drivers operate under the cyclic synchronous torque (CST) mode of operation.

Along with the torque command, the multi axis controller outputs extra compensation current (torque) in order to overcome relating resistance, for example gravity and robot dynamics, while still maintaining a smooth dragging method.

One benefit of such a complicated robotic solution is the simplicity when implementing the teaching process for the non-programmer, where the majority of tasks can be accomplished with no programming experience or skills at all.

The second mode of operation is the working mode. This is where the multi axis controller determines the seven target position/velocities in adherence to the kinematic model of the robot (Inverse kinematic solution of DH matrix). Where necessary, compensation of the torque based on a dynamic model will be added to the total output torque.

The drivers operate under a cyclic synchronous position (CSP) or cyclic synchronous velocity (CSV) mode of operation receiving torque offset commands and target position/velocity commands.

This information has been sourced, reviewed and adapted from materials provided by Elmo Motion Control Ltd.

For more information on this source, please visit Elmo Motion Control Ltd.

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