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New Trends in Drives
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 09/20/2018
There was a time drives had a single role – to send current to the motor as dictated by the motion controller. Other functions such as condition monitoring, quality control, and planning had to be performed by other components. Today’s drives have onboard memory and processing power that enables them to offer considerably more functionality. They can detect bearing defects before the component fails, enable an operator to clear a jam without invoking lockout tag out, and even act as a master controller for a network of daisy-chained drives. The latest generation of drives includes modifications designed to improve performance, productivity, and usability. Let’s take a closer look.
Today’s drives are getting smaller. This is partly motivated by the growth in mobile applications. Widespread implementation of smart warehouses and factories has led to broad deployment of automated guided vehicles (AGVs). Telepresence robots roam hospital corridors, drones roam the skies, and recreational vehicles roam the back country. Nearly all of them are battery-operated and space constrained. Depending on the application, they may need high-torque or high-speed operation.
To serve this growing global market, a class of ultra-compact drives has emerged. Because they are operating off of battery power, they are designed to perform at high efficiency. Batteries supply significantly less voltage than wall plug power. The voltage of the drive limits the current it can produce, which in turn limits the speed and torque that the motor can generate. ATVs and recreational vehicles, for example, frequently need high-torque, which requires drives that offer high power density. “There are drives that can deliver 160 amps at 80 volts DC in a package that fits in the palm of your hand,” says John McLaughlin, president of Elmo Motion Control (Nashua, New Hampshire). “That's a huge advantage mobile wise. Why? Less weight, the ability to operate from that low voltage and deliver high power.”
In other cases, speed, not torque, is the issue. The Guided Soft Target (GST) from AB Dynamics is a foam and vinyl vehicle mockup used for testing advanced driver assistance systems (see Figure 1). It is propelled by a low-profile robotic platform. GST is designed to generate a visual and radar signal identical to that of a car. If the vehicle under test impacts the GST, the shell shears away and the test car rolls over the platform without damage.
The application was space constrained but the motors needed to run fast to propel the platform at highway speeds. The drive applies a technology known as field weakening, or phase advance to enable motor to operate at multiple times the rated speed. Under normal circumstances, when a drive energizes the stator windings, they generate a back EMF in the rotor magnets. This EMF introduces a lag in the torque generating capabilities of the current, reducing motor speed. Phase-Advanced Technology corrects this slide, enabling motors to run at significantly higher RPMs.
Global systems are not the only machines with size limitations. Real estate is always in short supply on the factory floor. Particularly in the case of robotics, drives need to be as small and lightweight as possible. Some robotic applications demand high-torque to move significant loads, putting the focus on high power density. Others need low inertia and dynamic response high speed operation. Consider a pick-and-place delta robot for electronics assembly. “If you're moving surface mount components around, you need something that's very fast and dynamic,” says John Chandler, vice president of sales, North America at Technosoft (Canton, Michigan). “You don't necessarily need something that's 2 hp.”
Every piece of equipment has natural resonance spectrum formed of frequencies excited infrastructure by motion. These residences change as a function of the load and motion characteristics like velocity, mission profile, etc. Here, too, drives can perform as smart sensors. The machine exercises the axes, captures and analyzes the spectrum, and the drive revises mission trajectory to avoid exciting resonances. The process has a variety of use cases, from preventing slosh in containers of liquid to minimizing sway in cranes (see Figure 2).
Manufacturers today put the focus on operational effectiveness (OEE), which is the product of availability, performance, and quality. Drives have an increasing number of functions that enable them to improve OEE and, as a result, productivity.
It starts with functions to boost availability. For a number of years, drives had the ability to monitor current demand from the motor. Properly analyzed, this data can provide advance warning of impending equipment failure. Problems that can be flagged in this way include jams, worn or damaged bearings, and lubricant breakdown.
Smart drives with integrated data loggers and connectivity can export data to the edge devices for further processing and alerts. This supports the transition to predictive maintenance, which enables companies to not only avoid the costs of catastrophic failure but also to make more effective use of their maintenance staff and resources.
if you’ve ever watched a NASCAR race, you will have seen the blur of activity that is the pitstop. The pit crew swarms over the car, filling the gas tank and swapping out tires. The latter task is something of an art. If there is any mismatch in the torque from one lug nut to the next, it can create an imbalance that will only manifest when the car is at speed, compromising performance and handling and potentially causing a collision. Initially, racing teams relied on skill to (hopefully) prevent problems. More recently, they have been maintaining consistent performance by comparing the torque profile from the air gun to a preestablished profile. When the two match, then the wheel is balanced.
Manufacturing has begun applying a similar technique to perform quality monitoring through the drive. Consider machine tools, for example. At the start of production, the operations team captures data on the fabrication of a good part, recording dwell times, velocities, etc. During production, the drive collects data on each subsequent part as it is machined. The drive exports the data for processing by an edge device that compares it to the standard values. If they pass, the card is considered good. If not, it needs further action.
“They have all the information they are feeding back, the current, the torque, so right off they can basically tell whether a product is within specifications or not,” says Craig Nelson, senior product marketing manager at Siemens Industry (Norcross, Georgia). “It’s a much more efficient way to monitor production. Right off the bat, you can easily tell if a product has not been produced correctly. If it has not, we can make corrections or pull that product from the production line.”
The use cases vary – the technique can be applied for process control, sample-based lot inspection, or even acceptance testing. It saves time and money that would otherwise be devoted to hundred percent inspection. It reduces the scrap and rework. And with smart drives and edge-based analytics, it is straightforward to implement.
There was a time when machine safety meant physical sensors tied to banks of relays that cut power to the equipment when certain conditions were violated. Safety today is most often implemented through motor control. It is no longer a response (cut power) but a process (only move within this envelope, don’t operate above this speed). The approach protects workers, equipment, and the process itself.
Initial implementations involved safety PLCs that controlled “dumb” drives. The original feature was Safe Torque Off, which prevented an axis from producing torque but did not pull power. This led to faster restart and reduced downtime.
More recently, safety-related drives have begun taking over safety functionality. They go beyond STO to invoke more sophisticated functions like:
- Safely Limited Speed (SLS): Sets a maximum speed.
- Safe Direction (SDI): Restricts direction of motion.
- Safely Limited Increment (SLI): Limits motion increments to support jog operations.
These types of functions can directly increase productivity by reducing downtime. SLI, for example, lets the operator use the power of the machine to clear jams. SDI and SLS enable operations involving personnel to take place rapidly while maintaining strict safety conditions. An axis moving toward the hands of an operator might be dangerous but one moving away can take place at several times faster. Being able to set thresholds for speed and direction in the drive make these types of safety schemes easy to implement.
Safety-rated drives are generally available with built in connectivity for ease of implementation. The approach reduces wiring, speeds the assembly, and removes points of failure. Nearly all of the major industrial fieldbus protocols have safety versions that are designed to detect any communications issues before they can compromise the reaction the system.
All the sources interviewed for this article agreed that the technology is moving out of the early adopter phase. “Acceptance is really broadening right now because it increases productivity,” says Nelson. “End-users can recover quickly from events that used to require a lockout tag out.” He also points to the connectivity and ease of installation as major advantages. “Before, OEMs had to order and mount safety relays for every machine they produced. They had to do all of the wiring, which can get pretty expensive, and they incurred that cost with every machine they built. With safety integrated technology over the same communications network that controls the drives, the connectivity is already there. The software needs to be written once. After that, it’s just a download on every future machine. It’s a competitive advantage for the OEM and a big differentiator.”
In addition to boosting performance and productivity, drives today are designed to make life easier for the OEM. Advances include auto tuning for fast commissioning.
Once upon a time, tuning a servo axis was as much art as science. It is a process that necessarily needs to take place during commissioning with the actual machine and loads involved. Auto-tuning involved PID loop tuning and took hours in some cases, even for experienced technicians. The drives industry responded with auto-tuning. “Our customers are commissioning new machines every day for different applications,” says Chandler. “Autotuning has become the norm. I think it is paying dividends to equipment manufacturers that need to commission machines quickly without having to call in specialists from the factory.”
The early versions of auto-tuning drives promised more than they delivered. They put you to a first approximation but a significant amount of manual effort and skill was still required. The current crop of auto-tuning drives has vastly improved (see Figure 3). “Machine builders need these tools to work and they need them to be robust,” Chandler adds. “You hit a single button and within about thirty seconds you're 95% of the way to perfection. If you're really trying to optimize it for settling or optimize it for smoothness or low noise, you can tweak it manually from there.”
Auto-tuning doesn’t just help with commissioning. More sophisticated drives can dynamically adapt to maintain performance even in the face of changing conditions, for example if a belt loosens, a bearing or ball screw nut loses its preload, etc. These types of auto-tuning drives can detect problems, modify tuning to mitigate the issue, and send an alert to maintenance so they are able to address the underlying cause when circumstances permit.
Other usability trends include unified management software for entire product lines to simplify configuration. In some cases, companies are building components that can be programmed via Web server from any connected device or which use conventional higher level languages like C++. “What we are trying to promote here is easy setup, easy maintenance, easy diagnostics, easy integration with industry 4.0,” says Joaquin Ocampo, product manager at Bosch Rexroth (Hoffman Estates, Illinois). “OEMs should just look for products that are easy to set up and get running right away.” Even wiring is getting easier with single tables that carry both power and data.
No conversation about trends in drives would be complete without a quick status check on the progress of distributed versus centralized architectures. This can be considered on two levels:
- Distributed hardware: placing the drives out on the machine next to the motor rather than locating them in a climate-controlled cabinet with the motion controller.
- Distributed intelligence: controlling axes by drives rather than using a single centralized PLC/motion controller to form path planning and transmitted to the drives for execution.
Centralized architectures have been the traditional solutions. Keeping the drives and controllers in climate control cabinets protects them from thermal swings, contamination, and shock and vibration. Centralized control lends itself to very high performance motion or synchronized motion. On the downside, centralized hardware requires more cabling, which increases cost, installation time, opportunities for mistakes, and points of failure. The cabinet takes the floor space and consumes energy for climate control to dissipate heat generated by a large number of electronic components.
Distributed hardware architectures only require a cabinet for the controller. Drives are located near the motors, which minimizes cabling and the attendant cost, time, and complexity.
Centralized control is the default approach. It’s particularly well-suited to highly complex synchronized or coordinated motion. That said, it places a tremendous processing burden on the controller, whether it is a PLC or a motion controller. This adds cost and size. Distributed control architectures provide an alternative.
A decade ago, common wisdom held distributed control was only viable for low axis counts and simple motion. Anything else required centralized control. For a number of years now, however, smart drives equipped with Ethernet connectivity have offered enough performance to handle a wide range of applications.
Adding the level of performance to the list of the above, distributed architectures should be moving into broad deployment after all this time. And yet, the ongoing refrain is?next year.
The reason behind the solution is not a function of technology, some suggest, so much as one driven by the cycles of market. “It offers a lot of advantages but it has really been slow to materialize,” says Nelson. “We really see a lot of customers just keep their traditional approach.” In some ways, it can be chalked up to the path of least resistance, he suggests. “They don't have the manpower or resources to take advantage of the technology. They say; that’s all great but I’m just too busy.”
In some cases it’s simply pragmatic. There may not be room in the current platform to fit a drive out on the machine. This may particularly bold in the case of integrated motor drives, and other seemingly promising technology that has been slow to find market share.
“Many machines do not have the foot print space at the motor shaft to be able to integrate a motor and a drive, so that's certainly part of the reason that the adoption of the integrated drives and motors is slow to take place in some industries,” says Randy Summerville, SIMOTICS motion control motors product manager for Siemens. He points to growth areas for the technology as large conveyor systems and aerospace manufacturing.
In many cases the question is not technical performance and cost benefit than simply one of timing. Machine builders typically revamp their platforms about every 10 to 15 years. That is the point at which they are more likely to embrace new components that will require changes to the design. “You have to get them at the right time,” says Ocampo. “Switching over does require a redesign of the machine. So once the OEM decides it is time to upgrade or redesign, that’s when it becomes a good conversation about the benefits.”
Before we end, one final drives trend, and it’s not a good one. Driven by the booming global economy, coupled with the skyrocketing demand from emerging technologies such as the Internet of Things, a worldwide shortage of electronic components has developed. Lead times or even mundane devices like capacitors and resistors are running at 20 and 40 weeks. This does not appear to be a bubble. Many electronics manufacturers scaled-down during the recession and change their business model for inventory. As a result, they are unable to satisfy the unexpectedly high demand.
The shortage has significantly impacted drives market. Expect limited availability, long lead times, and higher prices. It’s important to get orders and as early as possible, as lead times for drives may be nearly as hair-raising as those for the electronics.
Outside of this particular glitch, the drives market is growing strong. Machine builders and end-users can take advantage of a variety of technologies to boost performance, productivity, and usability. Particularly when shifting to the next-generation platform, OEMs and machine builders should take advantage of the opportunity to integrate the new technologies. With the added technologies, they can dramatically upgrade the value proposition of their equipment.