Understanding servo system application requirements
When selecting servo-based motion control systems, it’s all about the application.
Choosing the appropriate servo technology can make all the difference when it comes to maximizing the potential of a machine design. Each individual application has a unique set of requirements that could be satisfied in many different ways. The ability to identify key application requirements coupled with the knowledge of available servo technologies can help the designer achieve the best automation solution. The applications described in this article illustrate unique challenges and how implementing the appropriate servo technologies optimized each automation solution.
Two-axis planar shape cutting
Cutting a two-dimensional pattern out of a sheet of material is a common application with several important requirements. Whether the cutting is accomplished with laser, plasma, or water jet technology, it is crucial for the planar motion of the X- and Y-axes to be coordinated smoothly to ensure an optimized finished cut.
Consider a plasma cutting machine that cuts patterns out of sheet metal stock (see Figure 1). In this application, the X- and Y-axes are coordinated to move the cutting head through a designated path to create the desired pattern. Each axis is actuated with a standard rotary servo motor coupled to a ballscrew assembly (see Figure 2). Depending on the size of the machine, the length of travel for each axis can be relatively long. In this case, the X-axis travel length is 76 in.; the Y-axis travel length is 49 in. During operation, a machine of this size and nature can be subject to a few different types of resonant frequencies and vibrations. As a result, the tuning of each servo axis becomes critical to the finished-part cut quality.
During the commissioning of this particular machine, significant amounts of vibration on both the X- and Y-axes were being reflected up through the cutting head. The effect of this vibration can be seen in both the Y-axis scope plot and the photo of the finished part (see Figure 3).
Some of the higher end servo systems available today have built-in functionality to account for these types of mechanical disturbances that can affect finished part quality. This built-in functionality consists of high-resolution feedback devices on the servo motors coupled with advanced tuning algorithms in the servo electronics.
In this application, a servo system with type of built-in functionality was implemented. The servo motors used on the X- and Y-axes of the machine are equipped with 20-bit feedback devices, which feature 1,048,576 counts per revolution. An advanced vibration suppression algorithm in the servo amplifier uses this high-resolution feedback to effectively eliminate the machine vibrations. By analyzing the feedback on the motor and comparing it to the commanded motion, the servo amplifier is able to obtain a mechanical signature of the machine. Due to the high resolution of the feedback devices, the amplifier can detect machine disturbances in extremely small increments. The servo amplifier processes this information and injects a signal that is 180 deg out of phase with the detected resonances and vibrations, thereby eliminating the disturbances. The result is a more efficient machine with significantly smoother operation. The elimination of the machine vibrations has extended the life of the mechanical components on the machine, and the overall quality of the finished part has been improved dramatically (see Figure 4).
Glass cutting is another example of a two-axis planar shape cutting application. The glass-cutting machine’s diamond cutting head cuts patterns out of large glass sheets in the X- and Y-axes (see Figure 5). The key requirements for this application were to improve the machine throughput and accuracy as well as to increase the machine’s flexibility and ease of use. When throughput and accuracy in linear motion are of primary importance, linear servo motor technology can be a very good solution.
A linear servo motor uses the same design concept as a traditional rotary servo motor with the exception that the motor is laid out flat. Both technologies use permanent magnets and a coil assembly, which is transformed into an electromagnet when energized. The linear motor coil assembly rides above the magnet track and is separated by a defined air gap (see Figure 6). The motor is commutated by energizing the coils in the correct sequence to create linear motion.
Linear servo motor technology offers significant advantages. No mechanical transmission is required to convert from rotary to linear motion—no ballscrew assembly required. The application load can be coupled directly to the motor’s coil assembly. Complicated mechanical designs involving ballscrews, belts and pulleys, and other types of gearing can be avoided. The only limitations to the motor’s linear speed and acceleration potential are the linear bearings and the speed at which the motor can be commutated. System accuracy can be improved significantly by using a high-resolution linear feedback device, which eliminates the backlash and mechanical-compliance issues associated with traditional mechanical actuators. As a result, linear servo motor systems can be extremely accurate, achieving speeds and accelerations that are magnitudes higher than comparable rotary servo motors used in conjunction with mechanical actuators.
In the glass-cutting application, the machine builder replaced a complicated mechanical design comprised of pneumatic actuators, ballscrews, and gearheads with linear motors to actuate the cutting machine’s X- and Y-axes. As a result, the overall machine throughput increased by 33%. Cutting speeds were increased to 3 m/sec and cutting acceleration was increased to 1.0 G. The improvements equated to a yield of 200 cut pieces per hr. The finished part quality was improved and the system accuracy doubled.
Eliminating the previous mechanical design complexity helped make the machine more flexible and easier to use. The old design required a significant amount of manual setup, which made job changeovers difficult. Because of the new linear servo motor design, the machine’s setup is 99% automatic. The modified machine has more uptime, requires less maintenance, and is more profitable for its owner.
Rotary table indexer
A rotary table indexer is a classic material-handling application where a workpiece gets rotated to multiple locations in a circular path. In many instances, direct-drive servo motor technology is a practical solution for this type of application. Direct-drive servo motor design allows the load inertia to be coupled directly to the motor’s rotor (see Figure 7). This motor design eliminates the limitations associated with common mechanical components such as couplings, gearheads, ballscrews, and belt and pulley assemblies. Each mechanical component that is introduced into the system adds compliance issues and sacrifices mechanical stiffness. When mechanical stiffness is compromised, the extent to which servo system gains can be increased is limited.
When using a direct-drive motor, these mechanical limitations are removed. When this motor design is implemented in an application, the servo now has to control only a single rigid mass (motor rotor plus load inertia). When the mechanical compliance is removed, the servo system tuning gains can be increased to a point where you can take full advantage of the total bandwidth capabilities of today’s most advanced servo electronics.
Turntable operation is very similar to that of a rotary table indexer. For example, a turntable was used in an application involving a machine designed to handle three large solar panels, each measuring 49 in. by 40 in. (see Figure 8). When the 12.5-ft. diameter table is fully loaded with three solar panels, the total load inertia (the load that is rotated by the motor) is 400 kg/m2. To index a load this large with a conventional rotary servo motor would require a significant amount of gearing.
A direct-drive motor was used in this application. The inertia of the direct-drive motor’s rotor is 0.31 kg/m2. This resulted in a load-to-rotor inertia mismatch of 1,300:1. Because of the table’s rigid design and because it was directly coupled to the rotor of the direct-drive motor, the system was able to perform a reasonable move profile regardless of this incredibly large inertia mismatch. A target move time of 5 sec for a 120 deg move was achieved (with a settling window of 10 counts) in this application.
The bearings in the motor were able to support the entire fully loaded moving structure. It is important to note that the pole count, feedback resolution, and torque constant of this particular direct-drive servo motor were chosen to optimize performance for this specific application.
As the solar panel turntable application demonstrates, when applied properly, direct-drive servo motor technology can result in performance that cannot be matched with any other motor technology.
Scott Carlberg is the motion control product marketing manager at Yaskawa America Inc. He has 15 years of experience in the motion control industry.
This article appeared in the February 2013 Applied Automation supplement to Control Engineering and Plant Engineering, both part of CFE Media.