There are two kinds of systems that are easy to use: Inherently simple equipment, and more highly engineered “total solution” systems. Here, we'll discuss the latter — and how to make sense of the myriad of options and features available to machine builders.
Let's say an engineer has the luxury of abstracting motors into simple devices that create motion from a voltage signal. Well, brush motors, despite their wearing parts, are certainly the easiest to setup and control. “Applying sufficient voltage to the terminals causes the shaft of the motor to rotate for simpler control schemes,” says Patrick Dell, applications engineer at MicroMo Electronics, Inc., Clearwater, Fla. Indeed, open-loop speed control requires little equipment — sometimes just a battery is enough. “Small brushed motors run in many applications requiring little more in the way of control,” says Dell. To control motor torque, however, more complex circuitry is required to regulate the current in the motor windings.
“Now, sometimes ac inverters have open-loop vector modes that works better pushing simpler constant-torque loads such as conveyors — in addition to adjustable voltage/frequency output that works well with variable torque loads, such as centrifugal pumps and fans,” says John Malinowski, Baldor Electric Co., Fort Smith, Ark. Increasingly, there is a push towards these “smart” components that incorporate multiple logic functions. And with designers today being asked to absorb more engineering responsibilities from their customers, these systems can be very helpful. “OEMs can't save money by acting as integrator. With the algorithms in many devices today, software can automatically program setup at a cost of $200 or less,” says Rich Mintz, U.S. products manager at SEW-Eurodrive, Lyman, S.C. He argues that no OEM could engineer a setup for so little. Integrated, preengineered systems can save weeks of programming and debugging complicated machine control applications. When designers purchase separate motors, encoders, reducers, and brakes, OEMs must act as integrators and are tasked with the sometimes-difficult job of making everything work together. “When, for example, gearmotor and control are a matched package, with self-tuning algorithms and application routines, then it can take as little as two minutes to set up a system,” says Mintz.
In any case, it doesn't make sense to look at any system apart from the mechanical or electrical perspective. “Just as one can never describe an elephant by looking only at an ear or a leg, looking at a motor in isolation never gives you the right solution. You have to look at the entire system and what you're trying to achieve,” explains Mintz. “When you stop seeing standalone components, then you can achieve the greatest precision and efficiency in performance,” he adds. “The motor is only a tool to make something happen; what you're really trying to control are parts to which it's connected.
If a PLC, motion controller, I/O, motion network, drive, and motor each require significant custom programming and record keeping, the control architecture is less easy to use. “On the other hand, advanced auto-tuning algorithms are invaluable to commissioning the axis as quickly as possible,” says Adam Shively, product manager at Rockwell Automation, Eden Prairie, Minn.
Some controls operate right out of the box; simply plugging in motor parameters allows these controls to autotune. “New guys don't realize that setting up a control was once difficult and painstaking, sometimes taking three or four hours,” says Jeff Lovelace of Baldor Electric Co. The software provided with controllers today greatly influences how easy a system is to setup, control, and troubleshoot. “Good setup wizards allow users to configure systems without digesting a PDF treatise,” says Dell.
After setup, if load changes, adaptive tuning continually adjusts and optimizes control loops. Some servomotors utilize feedback devices to provide automatic identification of correct motor-to-drive connectivity, reducing commissioning time and simplifying maintenance. “High-resolution encoders offer higher-bandwidth performance and smoother motion, with multi-turn options available to eliminate homing routines and associated sensors,” explains Shively.
Diagnostic tools allow quick resolution to simple wiring and even some logical issues. Real-time graphs — capable of plotting position, velocity, and other key variables — make writing effective programs much easier,” adds Dell. And any control algorithm that improves the controllability or observability of the overall motor/drive/controller system makes that system inherently easier to use. “Algorithms such as field oriented control, state space observers, and auto tuning fit this bill,” explains Rick Dye, development engineering specialist, Ormec Systems Corp., Rochester, N.Y. Auto commissioning software that can detect and adjust for offsets and phasing errors between motor and feedback cabling greatly simplifies connecting a motor to a drive. “This type of software can also be used to measure motor parameters such as pole count, feedback resolution, resistance, and inductance. This can greatly reduce the time it takes to get a motor spinning under control,” adds Dye. Diagnostics can come from sensors, too. “Some sensors also integrate diagnostics, visual LEDs, and even serial or wireless communication for local or central plant control diagnostics,” says Bo Watson, applications and field service engineer at MTS Systems Corp., Cary, N.C.
All this said, controls can be overspecified: “Their expensive bells and whistles (that can even confuse end users) leads to extras that often don't fit needs — and frustration and down time,” warns Chris H. Medinger, stock product manager at LEESON Electric Corp., Grafton, Wis. Complex routines may require a greater degree of understanding of the controller and the routine than is necessary or warranted. “And sometimes, motion control is the same whether you're moving bottles or cars. So in these situations, canned application routines developed by suppliers already familiar with these operations can be useful,” says Mintz. In fact, common applications such as turntables, winders, and indexing act similarly from application to application; simple precanned routines allow use of limited, non-realtime CPUs here. “But users shouldn't be forced to make applications fit software or hardware constraints. Otherwise, savings from a less sophisticated controller are quickly lost on creative workarounds,” warns Dell.
Down with potentiometers
Analog sensors are the original plug-and-play devices, but always require some adjustment. This used to involve tedious physical tuning or trimming via potentiometers — turned by a screwdriver — or switches,” Watson says. Other sensors with more functions have a series of small DIP switches, usually inside the sensor, that the user flips. But newer sensors can be configured remotely, so control engineers don't need access to the sensor itself: “This is particularly useful in hard-to-reach areas. “The controller stays in an accessible location, attached by wire to a sensing head located at the difficult inspection site,” says Lee Kielblock, senior applications engineer, Banner Engineering Corp., Plymouth, Minn.
Microprocessor-based units do offer more programming flexibility. “However, confusing programming and communication options can blur sensor-to-system functionality and compatibility, often leading to painstaking electrical diagnosis,” explains Watson. Sensors that incorporate analog feedback can be less complicated during the design and installation process. “Digital and fieldbus sensors require more setup time and a more intimate knowledge of the communication platform.
What sensor feature boosts ease of use most? Configuration. “Some single-function sensors have push buttons that users simply press to initialize or teach sensors — teach being an industry buzzword,” says Kielblock.
Say one motion-control engineer designs a mechanism to activate cutting on a shrink-wrapping machine. “Traditional sensors look for light reflection, which can be unreliable if there is uncontrolled motion on the line — and if the cutting machine is cutting wrong, it can stop the machine. Once the sensor is set up, encoder signals must be brought into a PLC, with sensor signals,” explains John Keating, Cognex Corp., Natick, Mass. The engineer must then do some complex ladder logic to determine the point of detection, and the proper delay in encoder counts — to ensure material is cut at the proper point, regardless of machine speed. But some smart sensors do all this, without PLC coding. How? “Quadrature encoders are wired directly into some sensors, so when they detect parts, they automatically know the encoder position,” says Keating. “The engineer only has to tell them how many encoder counts to wait before the sensors send output directly to the encoder mechanism.” Eliminating the PLC, custom ladder logic, and FIFO building simplifies setup.
Standards allow apple-to-apple component comparisons. In North America, motors are designed to NEMA specifications. “These standards alleviate misunderstanding and help engineers and end users select the proper motors,” Medinger says. But much of the world uses motors with metric dimensions, designed to International Electrotechnical Commission (IEC) specifications. “For each market, the NEMA or IEC standards allow interchanging one motor with another for comparable performance — but NEMA and IEC motors are not interchangeable,” says Malinowski.
Their different efficiency ratings can't be compared directly. “In North America, we test to IEEE 112 Method B or CSA 390-98, which account for motor losses. The closest IEC equivalent is 60034-2 and it doesn't account for these losses. Furthermore, it makes assumptions for stray load losses,” adds Malinowski.
Happily, there is a trend towards standardized mounting. “Increasingly, mounting adheres to IEC standards for metric dimensions. This configuration allows immediate interfacing to standard transmission components,” says Shively. The same standardization is happening with sensors as well. “Sensor families with mounting holes located uniformly make it easier to switch sensors, because there's no need to drill new holes or use different brackets. For this reason, some sensors are the same size and shape and have mounting holes in the same locations,” says Kielblock.
Standardization simplifies pneumatics assembly as well. “In the design phase of a pneumatic system, for example, cylinders, slides, rotary actuators, and grippers should quickly and easily connect to other components.
Say two components need adapter plates to connect. The time, effort, and money involved in acquiring (or designing) something as simple as adapter plates can cause a number of headaches. On the other hand, centering rings or pins on pneumatic components can reduce the difficulty associated with combining components. The components fasten with pairs of bolts,” says Al Turney, manager of automation industry at Bosch Rexroth Corp., Hoffman Estates, Ill.
Some fittings are equipped with plastic release rings that enable fast connection and release of compressed air tubing. Others include oval release rings simpler to use than standard circular release rings. “And even the smallest fittings are easy to handle using a raised gripping surface,” says Phil O'Neill, product manager, also of Bosch Rexroth.
In addition, some components are designed to work together within a load tolerance. “With smart configuration software, components only fit together when a calculation is sound,” says O'Neill. So, when a large cylinder doesn't fit together with a small guide unit, the configurator doesn't pair the two. “This gives the system a simple do-it-yourself quality while also preventing designers from mismatching components,” he adds.
Software configuration tools, typically available online, allow designers to input technical data about their application and obtain product recommendations. “Technical data can include workload, moment of inertia, stroke lengths, and time per stroke,” says O'Neill. From this data configurators produce a bill of materials — and sometimes even a single part number to access the entire package. “In fact, most preengineered, user-friendly parts allow online configuration software and support — before and after sale,” says O'Neill.
Once a bill of materials is established, downloadable CAD files can provide pertinent data on specified components. They reduce the amount of time it takes to get to production, especially when designing a piece of equipment around existing or proposed components. “A downloaded 2D drawing or 3D model of a motor inserted into a design saves valuable time and ensures that a motor fits without interference,” explains Medinger. There are several common file types: .DXF (Drawing exchange format) .STP (Standard for the Exchange of Product Data) and .IGS — the Initial Graphics Exchange Specification. These file types allow engineers to import downloaded data into whatever CAD software they are using, regardless of the software used to create the original drawing.
The Internet offers one additional tool for ease of use: “Bus communication allows operators to monitor performance and troubleshoot processes from anywhere on the Internet,” adds Malinowski.
Visit motionsystemdesign.com in the coming weeks for more information on ease of use.
UNTANGLING WIRING CHALLENGES
A three-phase sensorless, brushless motor requires only three wires to function. But once Hall effect devices are introduced as feedback, motors require a minimum of eight wires — and an encoder quickly raises the wire count to ten or twelve in a harness.
Much confusion arises from incorrectly connected wires between the motor and amplifier. Systems relying on feedback devices such as encoders and Hall effect devices add complexity. As the number of wires increase, so does the chance of incorrectly connecting from point A to point B. “Many support calls stem from user-created cabling that inverts the phase relationship of the motor leads or feedback devices,” explains Dell. Amplifiers for brushed motors are fairly interchangeable. However, for brushless motors they do require careful consideration if not purchased from the same motor manufacturer.
“To reduce wire count associated with encoders, some manufacturers offer linear Hall effect devices that provide absolute positioning within one revolution of a rotary brushless motor without an encoder — absolute or augmented incremental style. The absolute positioning allows stable and proper sinusoidal commutation with minimal wires,” says Dell.
“Brushless motors come with different Hall effect sensor styles, producing digital or analog output. Too, commutation sequences can vary from one motor manufacturer to the next.” Motor wiring orientation is easily changed to accommodate the amplifier, but changing the logic level of Hall effect devices may be more difficult. Purchasing from one manufacturer is one approach to ensuring all components will work together with little optimization.
If required, tuning sophisticated amplifiers for a brushless motor is not overly complex; it just requires patience. Torque amplifiers regulate current in the motor based upon a PID loop. An amplifier that scales motor velocity with applied input voltage incorporates a velocity PID loop surrounding the current PID loop. “In both cases, software setup makes the process far more enjoyable than using potentiometers,” says Dell.
MANUALS AND DISPLAYS
Because programmming in code can be labor intensive, straightforward control switches and manuals are on the rise. “Increasingly, control units have plain-English readout displays for easier programming and troubleshooting; others use symbols or letters to communicate parameters and fault codes. But the latter requires a symbols key — which isn't very user friendly,” argues Medinger. “Intuitive command languages allow users to create the program that affects the required motion,” Dell agrees.
Newer screens display multiple lines of text, and in larger font for those with vision impairment. Most are now are 128 x 64 pixels or larger. “Even color screens are on the rise, as the cell phone industry has made small screens less expensive,” explains Lovelace. With text instead of the old P parameters, there's almost no need for a manual. “User can read what is about to change, without having to recall — Was that P34 or P43? And with the cost of memory continually falling, more languages can be stored individual drives without affecting final drive price.”