Take Your Application to the Next Level With Motion Control
| By: Kristin Lewotsky, MCA Contributing Editor
Closed-loop motion gives a boost to non-traditional applications like materials handling and process control.
A decade or so ago, an innovative thinker got the bright idea to put a variable-frequency drive onto a fan. Instead of running their fan motors constantly around the clock, they were able to adjust the speed to run them only as fast as required – or not at all. The result? Lower power consumption, fewer failures, reduced downtime, less maintenance, all of which added up to a cost savings that dwarfed the initial capital outlay. Today, it's a common strategy, but back then, it brought the original innovator a huge competitive advantage. As motion control becomes increasingly user-friendly and affordable, the scenario is quietly beginning to repeat itself across a number of applications. Read on to see whether you can apply it in your own industry.
If you're not a machine builder, chances are you're focusing on your core value proposition, not on motion in and of itself. Fortunately, the latest generation of motion components is designed to simplify the entry point with a number of features:
- Connectivity: A number of motion buses and networking protocols facilitate communications among components. This simplifies wiring and updates while making installation and maintenance more efficient. Often, vendors and system integrators can log on to the machine for remote diagnostics and troubleshooting. Components not only communicate within the network but send data from shop floor to the top floor. Resilient ring architectures also prevent the failure of a single device from bringing down the whole system.
- Intelligence: The addition of onboard memory and processing power in components ranging from drives to motors has dramatically increased functionality and usability. Electronic nameplates allow the system to recognize a drive, for example, as soon as it's connected and download the correct parameters. When a component fails, it's no longer a matter of staring at a blinking red light and trying to determine the problem. Devices can not only determine and display the exact problem, they frequently guide the user through fixing it. Even better, they warn of problems in advance, for example sending a message to flag an over-current condition. Maintenance takes place on your schedule, not in response to a crisis.
- Ease-of-use: Those smart components make it easier to integrate a motion system. An application requiring only a few axes of simple motion, for example, can be served by integrated motor drives or motors with smart drives rather than the standard motion controller, drive, motor architecture. Fewer components means faster integration, decreased costs, and reduced points of failure, Self-tuning drives eliminate the laborious process of tuning PID loops, speeding integration and commissioning. Other innovations make it easy to add safety to the system.
- Higher power: There was a time servomotors were relegated to sophisticated instrumentation that only required fractional-watt motors. Today, kilowatt-class servomotors are a dime a dozen and used in heavy industry in applications like dock side cranes.
To all of this, add increasing price efficiency. In many cases, the shift to closed-loop motion from open-loop motion has been gradual, in part because electromechanical solutions are such a departure from the incumbent technology. Like the fan example, however, applications and enterprises are increasingly getting an edge by going against common practice and investigating motion control.
Healthcare and biotech
Motion control has long been a mainstay of precision medical devices like insulin pumps and biotech systems such as DNA sequencers. Now, the technology is moving into simple applications like patient positioning tables. At first glance, it might seem like a case of over engineering -- after all, getting a patient from place to place isn't a high-precision operation. Why choose motion control?
"What makes an engineer decide this is going to work for them? Usually because it solves a problem that they have or it makes a task easier," says Reid Hunt, Product Manager for Drives and Controls at Kollmorgen (Radford, Virginia). "In this case, it probably makes it easier." The traditional approach leverages open-loop motion with limit switches and in proximity sensors. Switches fail, however, and when you're dealing with patients, you don't want to take chances. "With a closed loop motion control system, you know the position at all times. If you’re halfway done with the move, your controller knows that. So if something were to happen, you could more easily figure out the status of the machine, and that would allow you to tell the operator how to better troubleshoot it."
Key drivers here are increased ease of use and reduced cost. "Computers and controllers have over the years gotten much more sophisticated," says Hunt. "The cost of the technology to have a controller that monitors three or four positions, speed, and current from all the motors on the table is not that high anymore. It comes along with the ride. Even the simplest controllers have access to a multitude of system parameters to help make judgments."
Motion control has long played a key role in systems like DNA sequencer, but these days it's adding functionality to more simple devices like centrifuges. Preparing fluid samples for genetic analysis is an essential step for accurate results. Cells need to be separated from the host liquid in a centrifuge, the liquid drawn off, new reagents added, and the process repeated in a process that is detailed, precise, time-consuming, and often tedious.
To simplify the process and eliminate human error, one manufacturer developed a closed-loop feedback system. Cassettes of test tubes are mounted on a stepper-motor-driven carousel that performs the complex ballet of moving test tubes in and out of the centrifuge and dispensing precise volumes of reagents into the tubes at the proper time. The centrifuge itself is powered by servo motor, which maintains precise and variable speed control. Controlled deceleration is particularly important in order to avoid disturbing the cells once they have separated from the liquid. The system performs the entire process automatically, repeatedly, and reliably and within 45 to 60 min. The lab obtains accurately prepared samples while freeing staff to concentrate on more important tasks.
Although robotics seems like an obvious motion control application, in this case we’re not talking about automotive assembly robots or pick-and-place units for circuit board assembly. In the area of personal services, robotics is positioned for growth, and with it, motion control. With our aging population, an increasing number of people are going to be housed in long-term care facilities. There, low-end robots have a role to play, for example retrieving objects or assisting patients in and out of beds or bathtubs.
These are simple designs with only a handful of axes. To serve them, the motion control elements need to be simple and economical. "I think the hardware is there to do this now but the barriers are the cost and the complexity of software development and integration," says Michelle Figgs, Senior Analyst - Rotating Machines & Controls, IHS Technology. "The robotics you see in a factory is very complex. You need a specialized controller and a specialized background to program it. Having things that are easier to program and implement is going to be key in taking robotics out of the factory and using it in more non-traditional ways.”
Materials handling – conveyors
Conveyors play key roles in applications ranging from production facilities to warehouses to airports. Today, motion control provides benefits in certain niches. In many cases, conveyors are considered constant torque devices -- with a fixed load, changing speed doesn't affect the torque required to run the system. In these cases, variable-frequency drives might provide savings in operational costs, but the return on investment may be longer than the rated lifetime of the component.
When the load does change significantly, it's a different situation. Conveyors transporting ore can stretch tens of kilometers from mine to distribution center. Take coal mining, for example. If it's raining at the mine and the coal gets wet, the weight of the load on the conveyor belt can change significantly, causing motors to wear and even fail. Instead, sensors can track conditions and communicate with a closed-loop motion system, adjusting conveyor speed to minimize the impact on operations while maximizing motor lifetime.
Other conveyor applications involve much shorter runs. Conveyors may interface with other machines on the factory floor. In other cases, using motion to control acceleration and deceleration can prevent product from being damaged or from changing orientation prior to passing a machine-vision inspection system. Speaking of inspection, motion is a far more obvious asset for inspection applications requiring tight positioning, such as those found in semiconductor fabrication. Stepper motors, even running open loop, can provide good performance over short distances – you command a step, you get a step. Over longer distances, servo motors might be a more effective solution.
In some cases, changing motors simply provides economic benefit. Brushless permanent magnet motors, the workhorses of motion control, deliver high torque density and compact packaging, even when run open loop.
Controlling fluid flow is essential for industries ranging from chemical and food processing to mining and oil drilling. In conventional designs, fixed speed motors run pumps that put the fluid in motion, while valves throttle down the flow. At best, it's an inefficient process--the motor runs at full speed even when it's not necessary. The real problem arises when the flow needs to be shut off. When the valve closes completely--a process known as deadheading--it generates an enormous pressure spike. Suddenly, the pump impeller blade is simply recirculating the same volume of fluid, heating the material. When deadheading takes place suddenly, it generates an enormous pressure spike, again heating the fluid.
At best, this strains the equipment, increasing wear and decreasing lifetime. A worst-case scenario arises in an application like mining that involves mud. Deadheading can cause cavitation within the pump head. If any water has contaminated the mud, the increased heat and pressure caused by deadheading can convert the water to steam, leading to catastrophic failure of the pump housing. In this type of application, downtime can be enormously expensive, far outweighing the price of replacement components. “It doesn’t make sense any more to buy a motor and run it full-bore all the time, and throttle it by a valve," says Allen Chasey, Eastern Region Sales Manager/OHV Manager at Dynapar.
Motion control provides a better alternative. In this application, an AC vector motor running closed loop controls the flow. Now, users can make adjustments by changing motor speed rather than throttling with a valve. That doesn't entirely eliminate the need for valves, but it mitigates the negative effects and removes a failure mode. This reduces downtime, cuts maintenance costs, and the cost of replacement equipment. "Granted, you have an initial cost, but the system will pay for itself over a certain period of time," says Chasey.
Pumps for water-jet cutting are likewise migrating toward using servo motors because the technology improves energy efficiency. "There are a whole slew of other pumps, like hydraulic pumps, moving to servo," says Sal Spada, research director for ARC Advisory Group. In a classical hydraulic system, a compressor/pump puts pressure on a master hydraulic cylinder that in turn transfers force through hydraulic lines to move one or more hydraulic actuators at the other end. Hydraulic cylinders generate tremendous force but they leak fluid, which can be a problem in many environments. They also tend to be inefficient and require hydraulic lines running from the pump to the actuator. Having lines that run thousands of feet, as in subsea oil exploration, can be cost prohibitive and also introduce additional points of failure. Electro-hydrostatic actuators (EHAs) provide an option.
Initially developed for aerospace and defense applications, EHAs swap out the compressor/hydraulic pump used at the master cylinder for an electrical pump situated right at the actuator. A stepper motor or servomotor drives the pump to apply force directly to the actuator. Motor, pump, and hydraulic cylinder all fit in the same package. The result is an electrically controlled device that is cleaner and more energy efficient. Any cost difference of components is often offset by elimination of hydraulic hoses, not to mention the increased precision and reliability. Although EHAs can't entirely match the force of a traditional hydraulic system, kilowatt-class servo motors have made them effective for applications like positioning the blades on wind turbines, and they can be effective for other industrial applications.
“It's probably on the cusp of taking off," says Spada. "There’s a tremendous advantage to not having all the plumbing. It becomes an electrical interface, not a fluid interface, although it gives you all the benefits of a hydraulic system. People are moving it into the industrial sector because I think it’s becoming cost-justifiable." Of course, the approach may have a negative impact on the existing trend of replacing hydraulics with conventional motion control motion. "There are limitations in terms of how much force you can generate in these types of packages, but they will have a niche play and certainly possibly curtail the adoption of traditional servo motors for those types of applications," he adds.
Adding it up
So given this, why would companies avoid using motion control? Often, it stems from two fundamental misconceptions: motion control is complicated and motion control is expensive. As we discussed above, complexity is no longer an issue in most cases. "Some end-users want as little automation as possible because if the machine goes down, they don't want to have to send an engineer out with a laptop and diagnose what’s going on," says Chasey. "But there are very basic diagnostics on a lot of today's components that say, ‘Hey, here’s where I hurt and here’s what you need to do.’ The directions might be as simple as turn a screw or replace the part. It makes things a lot easier.”
From a cost perspective, motion control solutions can be cheaper and easier to deploy than you might think. If you are operating only one or two axes, for example, you may be able to dispense with an independent motion controller and just use a smart motor with integrated drive. Fewer components means lower capital expenditure and less time spent on design and integration.
The more important point to consider is that the cost of acquisition is not necessarily the best metric to consider when designing motion into your products. Sure, you have to justify capital outlay and get approval for the expenditure, but industrial systems are designed to last years, if not decades. Over the long run, cost of operation will be the dominant metric for your organization. This is where motion control can deliver big savings.
"Electronic shafting or electronic gearing and coordinated motion and camming has finally gotten to the point that it’s reliable and cost-effective," says Hunt. "You can get a lot of control now for the money." When you build your next product or system, consider motion control and see whether it might, indeed, bring benefits to you, too.