Don't Let This Happen to You: Avoid These Machine Design Mistakes
| By: Kristin Lewotsky, Contributing Editor
Motion control is an enabling technology. If you’re an OEM or end user, you probably need motion control technology to operate your equipment. At the same time, you really want to focus the bulk of your efforts on your core value proposition, whether that’s making chicken pot pie or machines to press and package antacid tablets to take away the heart burn you get from eating it. Motion control is only part of the delivery to market. In an attempt to simplify your life, we talked to the experts to get a list of the most common mistakes made by motion designers.
Choosing your motor
Selecting the right motor is one of the most crucial aspects of machine design, and yet it is one of the most frequent errors. First on the list of motor sizing mistakes? Oversizing.
“We estimate that about 80 percent of all European applications – and probably a similarly large number of US applications – are oversized,” says Alby King, electromechanical product manager at Lenze Americas (Uxbridge, Massachusetts). “There’s a huge communication gap in terms of what the application actually requires and what we should deliver as an optimal solution.“ A bigger motor takes up more space, consumes more power, and costs more. It also requires a larger drive, which exacerbates all three problems.
In many cases, companies go to a larger frame size in a misguided effort to protect the system. An OEM or end user might touch the housing and find it uncomfortably warm. The response is to choose a larger motor, either to swap in or to use in a future design. A larger motor will run cooler, the thinking goes. It will, but it will also consume unnecessary space, power, and money. It will also require a larger drive, which eats up space, power, and budget on its own. In reality, motors are designed to run hot. Check the specifications and find out what the component is rated for. You may be surprised.
Another tendency is to skip sizing calculations and choose a motor by simply adding a safety margin to the size used in the previous design. Say you’re building a next-generation platform designed to package soda bottles in six packs rather than four packs. Choosing a higher-torque motor to accommodate the heavier load seems like an easy solution. The problem is that previous design teams may have done the exact same thing, so that you are adding a cushion to the previous cushion, which was added to the previous cushion, and so the error is now compounded many times over. Right size the motor and you cut cost and footprint, giving you an advantage over your competitors.
Sometimes, oversizing happens by accident but sometimes it is a conscious decision and design philosophy. “Some people will oversize it just purely on how it looks,” says Derrick Stacey, solutions engineer at B&R Industrial Automation (Atlanta, Georgia). “I’ve had company owners tell me that they build their systems bigger and better than everyone else, and that people believe in their products because they are so bulky and heavy and strong looking. The issue is that using inch thick steel plate versus half-inch thick steel plate isn’t going to alter the rigidity of the machine very much. It doesn’t make sense to completely oversize because then everything is ten times more expensive.”
Oversizing isn’t limited to motors. It can happen with actuators, too, says Joel Bekkala, applications engineer at Tolomatic (Hamel, Minnesota). Actuators also can be oversized. One common error is to convert from a pneumatic/hydraulic design to an electromechanical version. Although sizing software exists, all too frequently, end-users make a one to one replacement of the servo for hydraulics without leveraging the capabilities of the program. If you don't take into account the specific benefits of electromechanical actuators, you can easily oversize the actuator, which can influence your inertia ratio and ability to control the load.
Undersizing motors can present a different sort of problem. Companies might be trying to draw down on previous inventory or minimize the total number of parts. That’s a worthy goal but undersizing can cause serious issues in terms of system control. You’re giving up control in order to reduce part numbers, and it’s a trade-off. A wheel running in a single direction might not be a problem but for a precise positioning application with direction changes, choosing the right motor for the mechanical requirements is essential.
One of the biggest errors made in motor sizing is failing to take into account the ratio of load inertia to motor (rotor) inertia. Each machine exhibits a vibration spectrum with characteristic resonance peaks appearing at specific frequencies as determined by factors like compliance. When these peaks appear in the operating frequency band of the machine, they can cause problems like ringing and overshoot at certain speeds.
Inertia ratio has a profound impact on the location of the resonance peaks. Decreasing the inertia ratio pushes the peaks higher in frequency, broadening the useful operating bandwidth of the machine (see “Understanding the Mysteries of Inertia Mismatch”). Conversely, if the inertia ratio is too high – if the motor is improperly sized – those resonances peaks can fall within the machine bandwidth and interfere with performance. Tuning the drive can help, but there’s only so much the electronics can do to compensate for mechanical issues.
Mechanical changes to the system can have a significant effect on performance. Consider an axis driving a wheel with a 20:1 inertia ratio. Now increase the diameter of the wheel. Even if the mass remains the same, the inertia will rise. “The customer makes a request and you make a few small changes,” says Stacey. “Now your inertia mismatch jumps to 50:1. All of a sudden you get sluggish control.” When it comes to motion control, torque alone is not enough. “The devil is in the details,” he says. “Yes, you can move the load but can you have tight control of it? When you have a larger and larger inertia mismatch, that’s where all of a sudden the quality of the movement starts to fall off.”
Unexpected resonances can also come into play when systems are converted from fixed-speed or induction motors to servo motors. Bob Brennan, business unit manager for WITTENSTEIN cyber motor (Germany), recalls a sheet metal cutting line that was converted from a DC motor with an encoder monitoring the material to a servo motor with the encoder mounted on a measuring wheel. “When the servo was turned on, the sheet metal began oscillating back and forth like a yo-yo,” he says. “It turned out this was due to flexing in the measuring wheel mount that had had no effect on the prior non-servo design. The mount was re-designed with more attention to the stiffness and precision. As a result, the increased production and accuracy was easily realized.”
The perils of serial design
For decades, machine design took place in an isolated environment. Mechanical engineers developed the overall design and structure, then threw it over the wall to the electrical engineers and software specialists to add the electronics and programming. When problems cropped up, as they almost inevitably did, the electrical team came back to the mechanical engineers with a list of issues that needed to be addressed. Vigorous conversation followed and eventually a few compromises. It was neither efficient nor effective.
It can be easy to introduce problems inadvertently through lack of understanding. Thermal management, for example, can be a problem. The machine might have only limited room for the motor, but enclose a motor in a space where it can’t be cooled, and you are asking for problems. “The electrical engineer says, ‘You need to give me more room.’ The mechanical engineer says, ‘That messes with our design,’” says Stacey. “They don’t understand the issue and I think it’s just a lack of cross training. They may not have to be a drive expert but they do need to at least understand fundamentals like cooling has as much to do with motor performance as the commands and the drive and the filters.”
Although the problem still exists, there is hope. Increasingly companies are working with mechatronic design techniques that emphasize a holistic approach and extensive modeling and simulation from the very beginning
Keep your options open
“It ain’t what you know that’s a problem,” Mark Twain once said. “It’s what you know for certain that just ain’t so.” Locking on to a specific mechanical characteristic or drive train early on in the process can be a problem. It becomes all too easy to resist changes for the better because of the amount of work that would need to be scrapped. A design needs to start with the needs of the application and build out with as many degrees of freedom as possible. “Surround yourself in your design with different ways to go,” said Stacey. “Don't design yourself into a corner and then stick with it because you’re so far down the line that if you were to make a change it would require a ton of rework.”
Instead of choosing components ahead of time, consider starting with three designs; for example, one with the best high-speed performance, one able to handle large environmental swings, and maybe one with greater accuracy but less speed. With multiple options, it’s possible to progress forward and develop something truly innovative. That mechatronic design software allows users to monitor designs from the physical characteristics down to the dynamic details of the environment. This makes it possible to determine factors like the torque requirements for a given motor as the system runs through its cycle. It saves time and headaches and produces a better machine overall.
The temperature modeling capabilities can be particularly important for companies serving a global customer base. A machine that works perfectly well on a shop floor in Duluth might have big problems when it’s disassembled and shipped to Brazil or Canada. Mechatronic software allows design teams to run simulations at various points in the design process to uncover and address problems early. If a machine needs more rigidity or damping to suppress vibration, it’s straightforward to do.
Being afraid of new technology
In manufacturing, time is money. Organizations tend to be risk-averse and more likely to stick with familiar technology that they trust and that their staff knows how to work with. That said, being closed minded can prevent organizations from taking advantage of solutions that can give them a big boost over the competition. “People have experience in solving certain applications and sometimes they just don’t know about some of the other options out there for solving their applications,” says Scott Carlberg, product marketing manager at Yaskawa America Inc. (Waukegan, Illinois). “We’ve gone into an application where there was just a standard rotary motor with a right angle gearhead with a relatively high reduction ratio. They were overheating the gear box and hitting the performance limit of a traditional right angle gearhead.” Carlberg’s team suggested swapping it out for a direct drive motor technology coupled directly to the load. “First, it’s nice because you’re eliminating mechanical compliance. Second, you’re eliminating the cost of that gear head and running the motor at a slower speed and supporting the load with the bearings of the motor.”
More recent motors and drives are more capable of starting and stopping at high-performance levels. A full conveyor belt, for example, needs a large amount starting torque. That used to mean wildly over sizing the motor for just those few seconds. Today, components boast as much as 400% starting torque, allowing end-users to operate with smaller devices and all of the benefits they accrue.
“We’ve had instances where we’ve offered a new technology that is absolutely capable of doing the same job as the older motor, but was a lot smaller,” says King. He points to the example of a conveyor formerly run by a 1.5 hp motor. When the customer reordered, King did not send them the usual 1 motor. Instead, he sent a sample of the new motor with around half that power to test in the application. With 800 lbs on the pallet, but the roller was there once you got it going. "It worked fabulously, but even though it functioned beautifully, they weren’t ready to take that leap of faith with the new technology, which is pretty amazing. It was a significant cost difference, too.”
Thinking outside the gearbox
When it comes to gearboxes, we usually think in terms of reduction ratios used to reduce motor speed or cut the inertia of the load “seen” by the motor. A 1:1 gearbox doesn’t introduce a reduction ratio but it can actually provide significant benefits. It can act as a mechanical block to guard the motor from reflected torque that might otherwise cause it to oscillate. The trade-off is that it can introduce backlash, but so-called zero backlash gearboxes can address that particular issue.
“You shouldn’t think of it not in terms of what a gear box does but what it is: another mechanical component that can absorb stress and not pass it on from the motor to the load and from the load to the motor,” says Stacey. “You can use it has a mechanical buffer, and in some cases it can be helpful that you’re on that border line where the design is set, but we do need something there to lower our inertia mismatch or to prevent mechanical resonance coming back into our system.”
Even reconsidering the type of gearbox can be instructional. A company might decided it needs a planetary gearbox design just from the point of view of form factor when a standard inline helical gear box could do the job for a fraction of the cost. That would not only deliver a smaller, more efficient design, but it could potentially allow the use of a smaller motor, too.
The topic of assumptions goes back to the Mark Twain quote. King, for example, points to a packaging conveyor running a belt. The design team was afraid that the belt applied too much tension to the motor shaft and spent significant time running calculations to determine whether it would cause early failure. Then, someone thought to reach out to the maker, only to find out they were wrong. “It turns out we assumed a few values on the belt tension itself,” King says. “We got in touch with the belt manufacturer and found out the true belt tension was significantly less.” Problem solved, but not without some lost time.
In the case of ball-screw actuators, it’s dangerous to make assumptions about ratings. We can say that torque applied to the lead screw converts into force applied to the nut. “I fully agree with that,” says Terry Anderson, tech sales specialist at Tolomatic. “A lot of customers tend to think of the lead screw and nut as being the bread and butter of the actuator from a performance standpoint. The problem with this ‘lead screw/nut’ mindset is that the components linking the motor to the actuator lead screw are often overlooked.” It’s easy to assume that lead screw force is all you need to know, but the reality is that there is more going on. Coupling components typically cannot handle the torque levels of the lead screw. It is essential to take that into account when sizing the actuator and ruggedizing the other components.
Another example would be taking care to check materials properties. Steel, for example, has a constant friction coefficient. Get a low-end steel to save money, however, and those materials properties can change. It won’t invalidate the design but it does need to be taken into account to avoid nasty surprises once the machine is operational. Don’t assume anything – check, confirm, document. The effort applied up front can make life significantly easier in the long run.
A final example involves jumping to conclusions on the servo motor speed-torque curve. As the name suggests, it plots torque as a function of speed. The plot consists of an outer curve that defines the peak torque envelope. The motor can be operated at these parameters for brief periods of time. The lower region identifies the safe operating zone for operating a constant speed without overheating. Of course, the curve can’t always protect engineers from themselves.
Brennan recounts the tale of a company that put a cartoning machine for chocolate inside a refrigerated room. The servo motors were set up to operate within the limits of the speed-torque curve. Then the production engineers decided to get risky. Because the machine was operating at a lower ambient temperature, they decided to ignore the speed-torque curve and try running the motor at higher currents to get greater throughput. It worked – for about an hour.
It didn’t take long to find the solution. The servo motors were able to operate under the new conditions because they were in the refrigerated environment. The cabling, however, was not, and it had been rated for the approved operating currents. After an hour of running at the higher currents, it didn’t just fail, it melted. “Lesson to be learned,” says Brennan. “Understand the lines on the motor curve and their relationship to servo motor operation.”
Anyone involved in any creative endeavor knows that no matter how many mistakes you make, you will always find new ones. However, it is our hope that this collection of tidbits and advice can help save you some time and grief on your next design project.
Thanks go to Craig Dahlquist, automation group supervisor at Lenze Americas; Aaron Dietrich, director of marketing at Tolomatic.