This article discusses servo motion systems, which are motion control systems that combine hardware and software, have innumerable applications in compact modules. Some motion controllers operate on multiple platforms and buses, with units providing analog output to a conventional amplifier, as well as units that provide current control and direct pulse width modulation (PWM) output for as many as 32 motors simultaneously. There are amplifiers that still require potentiometers to be adjusted for the digital drives’ position, velocity, and current control. All major value-adding components of motion control systems will soon have to comply with the demands for faster controllers with high-speed multi axis capabilities supplying commands in multitasking applications.
Seemingly as many ways exist of putting together servo motion systems—motion control systems that combine hardware and software- as there are applications for them. Some motion controllers operate on multiple platforms and buses, with units providing analog output to a conventional amplifier, as well as units that provide current control and direct pulse width modulation (PWM) output for as many as 32 motors simultaneously. There are amplifiers that still require potentiometers to be adjusted for the digital drives' position, velocity, and current control. A near limitless mixing and matching of these units is possible to achieve a satisfactory solution for an application
Advances in hardware continue to make possible motion control products that operate faster and more precisely. New microprocessors and digital signal processors (DSPs) provide the tools necessary to create better features, functionality, and performance for lower cost. These feature and functionality enhancements, such as increasing the number of axes on a motion controller or adding position controls to a drive, can be traced back to advances in the electronics involved. As dedicated servo and motion control performance has improved, the system-level requirements have in creased. Machines that perform servo actions now routinely operate with more advanced software and more complex functions on their host computers. Companies that provide servo or motion systems are differentiating themselves from their competition by the quality of the operating systems they deliver to their customers.
Traditional servo systems often consist of a high power, front-end computer that communicates to a high-power, DSP-based, multiaxis motion controller card interfacing with a number of drives or amplifiers, which often have their own high-end processors on board. In a number of these cases, the levels of communication between the high-end computer and the motion controller can be broken down into three categories: simple point-to-point moves with noncritical trajectories; moves requiring coordination and blending, with trajectory generation being tied to the operation of the machine; and complex moves with trajectories that are critical to the process or machine.
Increasingly, a significant case can be made for integrating motion control functionality into one compact module to enable mounting close to the motors, providing a distributed system. By centralizing the communications link to the computer from this controller/amplifier, such a solution significantly reduces the system wiring, thereby reducing cost and improving reliability. By increasing the number of power stages, the unit can drive more than one motor independently, up to the processing power of the DSP. If size reductions are significant enough, the distributed servo controller/amplifier does not have to be placed inside a control panel, thus potentially reducing system costs even further.
In order to implement such a distributed servo controller/amplifier, the unit must meet the basic system requirements: enough processing power to control all aspects of a multiple-axis move, fast enough communications between the computer and the servo controller/amplifier to do the job, a small enough size to satisfy distributed-system requirements, and a reasonable price.
Levels of Motion Complexity
The three basic levels of motion complexity require different features and functions from a distributed servo controller/amplifier. It is possible to envision two types of distributed servo controller/amplifiers for point-to-point moves with non critical trajectory applications: those for repetitive point-to-point moves with external-event-generated conditions and others for point-to-point destinations that vary based on user- or machine-generated events. A typical application for this type of system is a "pick and place" robot. The robot's purpose can vary from moving silicon wafers between trays to placing parts from an assembly line into packing boxes.
The requirements for a distributed servo controller/amplifier involve determining a trajectory based on current location and requiring either a user destination or a preprogrammed destination. Effectively, the only user requirement is to set acceleration, velocity, and jerk parameters (for S-curve), and a series of final points.
As a unit, there are minimal communication requirements between host and controller, with simple commands and feedback. Such communication requirements include a resident motion program for path planning; support for I/O functionality; support for terminal interface via RS-232; use without resident editor/ compiler; and stand-alone motion control. A complete motion control application is programmed into the distributed control module for "stand-alone mode" control of three axis applications. This mode is typically used for machines that require simple, repetitive sequences operating from an RS-232 terminal.
Communication for the stand-alone mode can be simple and is often not time-critical. Simple point-to-point RS-232 communication is often acceptable for applications requiring few motors. Multidrop RS-485 or RS-1394 is suitable for applications with many axes of motion and motor networks.
The requirel11.ents for the "blended moves with important trajectory" option involve additional communications to a host computer, as the moves involve more information, and speed and detail of feedback to the host are critical. A typical application for this requirement would be a computerized numerical control system designed to cut accurate, repeatable paths. The trajectory generation and following must be smooth, as there is a permanent record of the cut.
The distributed servo control module must be capable of full coordination of multiple axis control. Communication to the host is critical to this application. Latency times between issue of the command and motor reaction time can cause inconsistencies in the motion.
In coordinating the motion between multiple motors there are several possibilities. With a single distributed servo controller/ amplifier module using multiple amplifiers, coordination can be achieved on the same controller. For applications requiring coordination between motors connected to different controllers, it is possible to achieve this with a suitable choice of high-bandwidth network.
Agile Systems Inc. of Waterloo, Ontario, has created a distributed servo controller called MAX2000, using Fire Wire smart cards that are capable of full coordination (including gearing and following operations between any axes) of up to 36 axes of control (using 12 MAX2000 units).
The delay time between the completion of an event on a MAX2000 unit and notification to the PC user application software is about 100 microseconds.
The MAX2000 Fire Wire smart card addresses issues that are related to latency times between the time the computer issues a command and motor reaction time. Other technologies are slower and somewhat more limited than smart cards in terms of processing power, communication latency time, and coordination capabilities.
Applications that require complex moves with critical trajectories such as inverse kinematics often include the trajectory planning as part of their own intellectual property. The generation of the trajectory is done on a computer platform and the information is transferred to the motion controller. An example of this would be a stand-alone robot such as the F3 from CRS Robotics in Burlington, Ontario. Typically, significant engineering effort is put into the top-level software provided with the robot. The software precalculates the trajectory and transfers this information to the controller, with the result that a conventional motion controller will be underused.
For a distributed servo controller/ amplifier, the interface would be to send a series of set points over the network that the controller would then follow, interpolating information for higher resolution. The servo drives are configured for torque, speed, or position control. This solution requires the addition of smart cards in the PC, with DeviceNet and FireWire solutions as add-on options. This type of configuration supports synchronized motion profile streaming (Profile Streaming Mode) for users who need to specify an arbitrary trajectory.
Meanwhile, system integration continues to advance on all fronts. All major value-adding components of motion control systems will soon have to comply with the demands for faster controllers with high-speed multi axis capabilities supplying commands in multitasking applications