By Jason Goerges, National Sales and Support Manager ACS Motion Control, Eden Prairie, Minn.
To meet the requirements of today’s most demanding multi-axis automation applications in the semiconductor industry, engineers are turning towards motion control platforms with the best integrated and network-based control attributes.
Many advanced machine control platforms that use both network and integrated based control are just beginning to see wide-spread implementation from the automation community, since they require significant processing power and communication bandwidth that was unachievable with the microprocessor and network technology of just a few years ago.
Much of the high-end multi-axis automation industry has known about and incorporated the benefits of the integrated multi-axis controller since the 1990s. Using a centralized high-speed processor to handle coordinated multi-axis motion control has proven an effective architecture for deterministic digital servo control, enabling the fastest update rates and tightest synchronization. Alternatively, network architectures, such as CANOpen, have successfully been implemented in motion control for solar panel scribing, semiconductor manufacturing, and general automation applications where scalability, openness to multiple vendors and devices, and cost sensitivity were the driving factors in the control system design. Network standards have been continually evolving with ever-improving bandwidths and reliability. Now, with Ethernet-based, real-time industrial networks such as EtherCAT – a deterministic real time industrial network that has sufficient bandwidth to support update rates required for high-performance coordinated motion control of many axes and I/Os, it is possible to implement a machine control solution that has the best qualities of integrated and network based control.
The following are three recent examples of applications requiring highly coordinated and accurate multi-axis motion control, each one demonstrating a unique set of challenges and constraints for the control system.
Solar Panel Scribe and Inspection Tool
Laser scribing and the inspection of flat panels or wafers often requires extremely demanding motion control including high speeds and accelerations, highly coordinated multi-axis paths for a laser, and extremely tight tolerances of motion error for maximizing photovoltaic (PV) cell density or resolving the smallest scale defects.
Large-format panels occupy more than a square meter in size; and, as the size and mass of the panel increases, so does the complexity of the machine, number of motion axes, and demands for motion performance and power. Recently, a builder of solar panel scribing and inspection machines met many challenges in designing the control system for a line of machines employing more than 15 motion axes. Some versions of the machine used additional auxiliary axes and I/O devices.
A multi-axis control module that has an integrated architecture provides excellent performance for such coordi-nated multi-axis systems. Supporting up to eight motor drives, the master controller supports complex MIMO (multi-input multi-output) control algorithms required by large gantry positioning tables, and provides the necessary 10kW of continuous electric power for each of the two linear servomotors used to rapidly move the large panels during the scribing process.
This machine is offered in a few versions that use additional support axes and I/Os; therefore, scalability was required. The master controller can be configured with an EtherCAT network controller, allowing one controller to act as a 32-axis network master. For additional motors in the network, beyond those driven by the eight drives in the master controller, the customer selected an additional MC4U configured as a seven-axis EtherCAT slave drive module. Additional 32/32 I/O points were added to the control system with low cost EtherCAT network I/O modules.
With a network-enabled integrated solution, this machine builder was able to meet demanding multi-axis performance specifications while maintaining the necessary scalability and flexibility for their ever-changing customer requirements in a cost-effective manner.
40-axis pick-and place machine
A 40-Axis high-speed, SMT pick-and-place assembly machine is evaluated on the throughput it can achieve without sacrificing the quality of the parts it processed. The extremely competitive SMT assembly capital equipment market drives the need to keep costs low. A recent SMT assembly machine operates eight high-power servo axes for controlling gantry structures spanning large work areas, 16 low-power servo axes controlling eight pick and place heads, 16 step-motor axes controlling a conveyer assembly, and 96/96 digital I/O points. The complexity of such a system dictated a robust and high-performance network solution.
The main design challenges of a control solution for such a system are:
• Significant amount of real-time information to be communicated between the motion controller and the host computer,
• Performance requirements in terms of coordination, high speed and positioning accuracy of a complex mechanical table design,
• Integrating controllers manufactured by different vendors into a single network, and
Several challenges were addressed by selecting an EtherCAT master controller that uses a modern off-the-shelf PC to handle high-level machine control tasks, such as application programs, communication with the host application, safety and error handling, and multi-axis motion profile generation. The master controller used in this application includes a real-time operating system that communicates with the host PC through shared RAM, thus providing the communication throughput required. This high-speed communication could not be achieved using any other communication protocols, such as Ethernet, USB, or even PCI. In addition, the master controller controls all motor drivers and I/Os using the standard Ethernet port of the PC via the EtherCAT protocol.
To meet further challenges, an integrated control chassis was implemented for the eight most demanding motion axes. An EtherCAT slave drive controller with 20-kHz sampling rate and advanced control algorithms provided a cost effective, low footprint solution while enabling up to eight axes of drives to be implemented from a single physical enclosure.
For the 16 conveyor axes, two integrated low-cost, eight-axis stepper drive modules were used to keep costs low.
The eight pick and place heads were controlled by a customized 16-axis solution provided by another vendor. In addition, the module complies with the standard CANOpen -over-EtherCAT (CoE) protocol that is supported by the master controller.
With no dedicated hardware for the motion controller, the control solution significantly reduces the total cost, while meeting the high-speed communication and performance requirements.
Automated Wafer Transport Robot
Automated wafer transport systems help solar cell and semiconductor manufacturers streamline their production and increase capacity of fabrication lines. Often these systems employ specially-designed robots with minimal weight and footprint, traveling on a linear translation stage to transport wafers from one location to another.
The main design challenges of a control solution for such systems are:
• Real-time kinematic calculations. The coordinate system of the moving motors is different from that of the moving wafer. The controller needs to calculate in real time the complex equations that translate the required movement of the wafer to the movement of the motors.
• Minimizing cabling of the moving robot arms while keeping weight at minimum, and
• Scalability – the ability to add axes and I/Os without affecting the basic solution.
For systems with such constraints, a fully-integrated controller topology is not ideal because of the many long, heavy cables required to connect the motors to the integrated controller and drive chassis. Long motor cables are especially susceptible to EMI noise and can significantly degrade overall control performance. Instead, a fast EtherCAT network topology (other networks, such as CANOPEN or DeviceNet, cannot meet the required network communication throughput) with a small form factor master controller housed in the main control chassis and small lightweight drives placed directly on the moving robot, provides an optimal solution. The only cable required between the master controller and the slave drives is the EtherCAT communication cable, which is not sensitive to EMI and is insignificant in terms of weight and drag to the moving system. This solution is also scalable, making it simple to add additional axes of drives or I/O modules for various models of the transport machines.
The chosen solution consists of an EtherCAT master controller and three slave EtherCAT servo drive modules (single- and dual-axis modules). The master controller supports inverse kinematic calculations and can manage up to 32 axes and many I/Os. One slave module was stationary and the other two drive modules were placed on the moving axes of the robot.
Not long ago, engineers often had to sacrifice or compromise performance and functionality because they were forced to choose between a network solution or an integrated solution. Now, with the latest control system technology, solutions can be devised where the benefits of both control methodologies can be realized. In the applications discussed here, three different versions of such solutions were presented, one employing a master controller within the host PC, another utilizing a small standalone master controller, and the other employing a master controller within an integrated drive and control chassis. The different solutions were selected in order to meet the varied challenges faced by the engineers in each application.