Motion control defines the capabilities and limitations of a machine. Therefore, to maximize its throughput and flexibility, and to reduce maintenance, you often must upgrade how the motion is controlled within that machine. Most reasons for converting from traditional control designs and devices to servo control are to obtain one or more of these benefits:
• Increase throughput. Servomotors produce high acceleration rates and speeds.
• Increase accuracy. Servos can offer the high accuracy necessary to process a fast-moving piece.
• Increase flexibility. Servos offer electronic versions of traditionally mechanical components. For example, electronic cam profiles can be changed almost instantly. Programmable motion profiles can adjust to varying product size and configuration. Electronic “gear” ratios can change to accommodate different machine speeds. Also with electronic gearing, the motors can be placed anywhere that is convenient to the application, as they eliminate the need for long shafts, gears, and belts.
In addition, one electrical “line shaft” can link to an almost unlimited number of axes. For machines with multiple configurations, this means that additional motion axes do not require additional mechanical linkages.
Servos also add flexibility because of the increased information available. For example, many servo controllers store a history of faults and error conditions that aid troubleshooting. Most servo systems can also display oscilloscope-style diagrams for performance analysis. • Reduce maintenance. Servos help reduce the number of mechanical parts on a machine. Electronic gears replace belts. Electronic cams are unaffected by wear. Electronic limit switches do not need occasional readjustment or replacement.
Servos do require a certain amount of study and experience. If you are new to servo control, expect to spend some time selecting and applying your first system. (A note on servo terminology: the word controller finds several uses. The system or motion controller normally runs the program that controls motion; the motor controller controls one motor. To reduce confusion, we will refer to motor controllers as drives).
Application sizing and selection
Selecting and sizing servo components may appear complex because of the number of components: motors, drives, controller, and the possibility of an industrial PC or a PLC. If your background is mechanical, this can be intimidating. Fortunately, companies — component suppliers and control system integrators — package these components together, as well as offer application assistance. Whether doing-it-yourself or buying a package, the basic process is:
First, select the motor. Start the motor selection by choosing the motor shape, Figure 1. Motors with large aspect ratios (long with a small diameter) are the most common. They can be square or round, and they provide excellent value and performance. Disc motors (short with a large diameter) fit in tight places and provide high acceleration due to their low-inertia rotors. Both of these motors are available in sealed and unsealed versions.
Frameless or integral motors, Figure 2, separate the rotor and stator for integration into the machine. These motors enable compact design, and enhance direct- drive operation by increasing accuracy and reducing vibration.
Linear motors, which replace a standard rotary motor and the associated drive mechanisms, create linear motion directly. They can simultaneously increase throughput and accuracy by several times.
Sizing the motor. Motor size is based primarily on torque: peak and continuous. (For information on calculating torques, see “The Basics of Motion Control, Part 1,” PTD, 9/95, p 43, and “Part 2,” PTD, 3/96, p 35). Sizing motors can be challenging and mistakes may not be found until late in the development cycle. As the motor size can be difficult to increase at that point, it’s wise to include margin in your calculations. If you are new to the process, you probably should rely on the application engineers at motor companies.
Select the feedback. The most common feedback devices are encoders and resolvers. Encoders are optical devices that produce a pulse train. The pulse count is proportional to the angular travel. They offer high accuracy, especially at high resolutions. Resolvers are electro-mechanical devices that sense absolute position within one revolution of the motor and are known for their ruggedness. Choose the one that best fits your application.
After you select the types of feedback sensor, you need to select its resolution. Generally a 1,000 line encoder or, equivalently, a 12-bit resolver, will provide enough resolution. Both produce about 4,000 different positions per revolution, which is equivalent to about 0.1 deg resolution. However, if your application needs higher resolution, you should select the sensor appropriately. One word of caution: differentiate between resolution and accuracy. Many servos offer selectable resolution for resolver feedback; however, the accuracy (usually between 10 and 40 arc-min) may not be affected.
Select the drive. Consider whether you want the power supply modular (separate) or integrated into a drive, Figure 3. With three or more drives of the same family in proximity, modular power supplies work well. With one axis, integrated power supplies usually fit better. With two axes, both solutions are about the same.
If you plan to enclose the drive, keep in mind that drive sizes vary considerably and may affect the overall size of the equipment. Depending on the size of the enclosure, you may also need to investigate various cooling options.
Sine commutation vs. six-step
The power wave form from the drive to the motor tends to come in two ways for brushless servo motors: six-step and sine wave. In sine-wave, the current waveform produced by the drive produces a current that approximates a sine wave. This produces smoother torque and less heating. The six-step method produces a six-segment square wave using simple electronics. Although lower in cost, six-step has rough operation at low speeds.
Tuning flexibility. Tuning, the process of selecting gains in feedback loops, is necessary for high performance and to maintain stable operation. In the past, tuning was more art than science. Now, modern servo drives provide a host of tools to aid machine designers. Auto-tuning (or self-tuning), the process where the drive excites the mechanical system and generates a set of loop gains, is almost a standard. Most drives are set with digital gains so you won’t need a soldering iron or a pot trimmer (small screwdriver). You may need the more complex methods only occasionally, but having them available provides more options.
Analog drives can be less expensive, but you may need to adjust loops by adjusting potentiometers or changing passive components. Whichever your choice, tuning is part of the learning curve and requires some study and experimentation.
Drive communication. Many drives use an analog signal to deliver the speed and torque commands. However, digital communication is gaining popularity, because it reduces communication wiring and increases the flexibility of the system. Many drives are compatible with such networks as DeviceNet, Profibus, and a new network especially for motion control called Sercos.
Voltage. Be aware that 110 Vac power may be hard to come by on the factory floor. In Europe, 460 Vac is popular; using 230 Vac drives may require a transformer in machines for use overseas. Unfortunately, 460 Vac drives can be expensive. A compromise is the universal power supply that uses power semiconductors to convert voltage levels. For systems with modular power supplies, one universal power supply can use any voltage from 230 to 480 Vac to power several 230 Vac axes.
A last point to consider, by using only a small number of drive families on a machine, you simplify the spare-parts list.
Select the controller
When selecting the controller, choose single-axis or multiple-axis. Single-axis controllers combine a motion controller, drive, and often a power supply integrated into one package. In one or two-axes systems, these controllers can reduce cost, size, wiring, and system complexity.
Multi-axis controllers are usually a better fit in more complicated systems. First, they usually reduce cost, especially as the axis count grows. Second, they reduce system complexity because one program can control all motion. These motion controllers also provide greater flexibility in synchronization since they usually let any axis link to any other axis, and they let you modify that link during program execution.
After your controller selection, you will need to choose either a “box” or “board” configuration. A box configuration is an enclosed controller capable of stand-alone operation. Board controllers plug into industrial computers. If you have an industrial computer on the machine already, a compatible board can reduce cost and enhance integration of the control and machine. If you don’t plan to use an industrial computer, the box-based controller is usually easier to add.
Evaluate the feature set
Finally, evaluate the controller features. Consider the functions discussed so far: gearing, camming, high-speed registration, and programmable limit switches. Most controllers offer these features in some form, but the specifics must be compared with the needs of your application. Do you need to change gear ratios during operation? Do you need to modify cam profiles on the fly? What registration accuracy do you require? Do you require a change of speed or target position during operation? Does the controller support enough axes for this application? Will it fit future versions of your machine?
Dealing with cost
The cost of servo components is often higher than that of the mechanical components they replace. However, some important factors mitigate this higher cost. For example, eliminating complex mechanical devices can reduce total cost and the size of the machine, which can increase the system’s value. The servo controller often replaces a PLC; in this case, the entire cost of converting to servos can be offset. The added flexibility may reduce the number of machine models, or processes required to produce a line of machines, thus reducing manufacturing costs.
Beyond motion functions, there are other questions to ask. Is the language capable of supporting your processes? Is it so complex that you will need to spend excessive time learning it? Does the product support multi-tasking? A technique that allows you to write different programs for different processes, multitasking simplifies the programming of complex machines.
All these questions can be difficult to answer, especially if you are new to electronic motion control. Most companies that offer controllers support them well. During your selection process, ask many questions. In not only helps you evaluate the product, it helps you evaluate the support. Finally, consider the future of development activity at your company. Pick vendors who can provide products and support now and over the coming years.
George Ellis is program manager, New Business Development, at Kollmorgen Corp., in Radford, Va.