Simplify Motion Control with Integrated Motors
Introduction
Integrated motion systems save time, cost and streamline the motion control design process.
Engineers today are faced with the challenge of creating machines that are more robust, smaller, less expensive, and more reliable. Designers who are not experts in motion control are expected to take on challenges in the design process for which they did not necessarily study or train.
One way to address motion control design challenges without being a mechatronics expert is to use an integrated motion control system. Such a solution combines the motor and drive along with other system components — such as a motion controller/intelligence onboard and I/O — all in one unit, reducing the number of necessary parts.
By using an integrated solution, designers can spend more time developing machines and less time solving compatibility issues between various system components. With an integrated motion system, the components have already been designed and sized for use as a complete unit.
The world market for integrated motors
Simply put, an integrated motor is a motor and drive along with other system components all within a single housing. Drives and motors that are in separate enclosures but mounted onto each other are also considered integrated motors, even though they are not in the same enclosure.
Integrated motors were first introduced in the mid-1990s with AC induction motors. Today machine designers can select from numerous integrated motor products, which includes choosing the motor technology that is the best fit for an application:
- AC brushless servo
- DC stepper
- DC brushless
- AC induction
What industries make up overal use of integrated motors?
When the integrated motors market is measured by revenue, the AC induction motors account for 35.2% of total revenues, according to global market research company IHS Markit. AC brushless motors account for 32.1% of revenues in the integrated motors space, with DC brushless motors coming in at 18.9% and DC stepper motors at 13.7%.
IHS Markit estimated the worldwide market for integrated motors was $648 million in 2017, with nearly 1.7 million integrated motors shipped during the year. This was up significantly from $392 million in 2012, and $110 million in 2002.
Markets worldwide for integrated motors
Integrating electronics into motors as a solution
Machine size.
On the other hand, when looking to put an integrated motor on an existing machine design, there must be adequate space. Because the drive and other system components are mounted onto the motor, more space is needed where the motor is used. If that space is not available, it may not be cost-effective to redesign an existing machine to accept an integrated motor.
Cost.
There are definitely cost savings associated with integrated motors compared to traditional components. The cost of cabling is an expense that goes away with an integrated motor. If the drive is in a centralized cabinet and the motor is a distance away on a long packaging machine or long conveying machine, a decent amount of money is spent on the cabling. This is especially true for servo motors, where there is a power cable and cable for the feedback device back to the drive. Mounting the drive directly on the motor eliminates the cabling, contributing to some cost reduction. Distributed control via networking, such as Ethernet and CAN, also can represent a significant cost saving.
Reliability.
Reliability has been a concern in the past with integrated motors, especially with regard to heat dissipation. However, as the technology has become more advanced and accepted, users find that the motors are reliable and don’t fail at a higher rate than a regular motor and drive system. Also, by reducing the number of components in a system, and thus the number of wire connections, the overall reliability of the machine is increased.
Modular systems.
A growing trend in industrial automation is modular machine building. This essentially means developing a large machine responsible for multiple functions by combining small subsystems that are each responsible for a single task. For example, a packaging machine that’s used in the food and beverage industry may feed the packaging material in the machine, fill and form the pouch, seal and cut the pouch, and send the product out to other machinery in the system.
A modular design allows taking each one of those tasks and making an individual subsection machine that’s responsible for just that task, which operates independently from the rest of the system. This is beneficial for a couple of reasons. It allows designers to easily change just one modular section, so it’s an inexpensive way to provide customized machines. In the packaging machine example, say the designer wants to accommodate different packaging materials: the machine can now offer that without much added cost.
This modular building concept also makes shipping and delivering the product easier. Independent modules can be shipped to the factory floor for easy assembly. In addition, setting up these modular machines is made easier when the drive and motor are placed directly in the machine rather than located in some centralized cabinet. Integrated motors are one way to do that.
Distributed control.
In more industrial applications, there is a real benefit to what is called distributed control. Motor operation and synchronization are carried out through digital data signals transmitted between the motors and a master control system such as a programmable logic controller (PLC) or process automation controller (PAC). The PLC or PAC sends a command to execute a particular function, and it’s up to the controller on the motor to carry out that command.
This distributed control system provides faster response and greater accuracies than that obtained by a single central control point running all operations. Clark Hummel, engineer manager at SEM, explains: “In a centralized control system, if you want to home an axis, it has to move so it can see the sensor, roll off the sensor, and then set its position reference from there. Instead of having to do all those steps, with distributed control, you can just tell the axis to go home; because of the intelligence on board, it will go to the right position.”
Electromagnetic compatibility.
An integrated motor solution has already been designed with electromagnetic interference issues settled. And, reduced cabling minimizes electrical noise.
Leading-edge applications
As motors and drives have decreased in weight and size, areas that can benefit from integrated motors are new applications that weren’t necessarily around years ago. One example of a growth market for integrated motors are AGVs, or automated guided vehicles. These have long been used to transport materials in the pharmaceutical, automotive, and chemical industries, and are now used in hospitals and distribution centers. Where you have small machines that are moving around, integrated motors are a good solution due to their compactness and lack of cabling.
The medical market is growing for DC brushless as well as DC stepper integrated motors, in applications such as DNA analyzers. There’s demand for these products in criminal investigations, paternity tests, people who want to test their genetics for diseases, etc. Motorized prosthetics and artificial hearts are technologies that have advanced significantly in the past decade. They need small, compact motors that are easy to use, and that’s what an integrated motor can provide.
A closer look: stepper and servo motors
In the last two decades, integrated motors have become the dominant motors in the industrial market, as 65.6% of the market is integrated motors. The remainder of the market is machinemounted drives.
Stepper motors are advantageous for certain applications. They’re inexpensive compared to servo motors, and they’re a less complicated product to set up and run. It’s possible to get a high torque at the starting or low speed, but the available torque declines steeply as the speed of the motor increases. This is one of the limitations of stepper motors; if the application needs high speed, a servo might be the best option.
Other challenges with stepper motors are that they have higher vibration, they’re not as energy-efficient, and they can have slippage. If the torque exceeds the motor speed-torque rating, it loses accuracy and is no longer in the position it should have been in.
To address those three challenges, suppliers can run a stepper in a closed loop system. They put a feedback device on a stepper and actually monitor the position of the shaft and take away some of the disadvantages of that product. But because there’s an added feedback device, the cost differential between a stepper and servo motor may be smaller.
Motor technologies — defined and differentiated
There are a number of motor technologies available to machine designers, and choosing the best motor for a design is a matter of finding the best fit for the application.
Linear motors.
Linear motors take a rotor and stator and lay them flat so that there is a forcer (rotor) on a magnetic track. When the motor is excited electrically, it produces a linear force along its length.
Linear motors allow direct coupling to the load and deliver high speeds, high precision, fast response, stiffness, zero backlash, and low maintenance, as there are no contacting parts to wear. However, the biggest drawbacks to using linear motors are their comparatively high cost, higher bandwidth drives and controls, and force-per-package size.
Linear motors are good for applications that don’t need mechanical parts and require high acceleration and high accuracy. These reasons might justify the extra cost for a linear motor.
Rotary motors.
Rotary motors have a rotor and stator that are circular and produce rotational motion when they are excited electrically. It’s possible to use a rotary motor and translate the motion to linear using mechanical parts, such as ballscrews, leadscrews, belt drives, and rack and pinions. The drawbacks with these added components are that they introduce inertia, friction, compliance, and backlash to the system.
There’s a difference between applications that need linear motion using a rotary motor versus ones that
need a true linear motor. For applications that need linear motion using a rotary motor, machine designers frequently choose stepper motors and/or servo motors.
Stepper motors.
A stepper motor is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor’s position can then be commanded to move and hold at one of these steps without any feedback sensor as long as the motor is carefully sized to the application.
Servo motors.
A servo motor is a rotary actuator that allows for precise control of angular position, velocity, and acceleration. It consists of a motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servo motors.
No matter which type of motor is chosen, a complete motion system needs to include additional components, including a compatible stand-alone controller, drive, encoder, and cabling. Taking the time to source each individual component, making sure they work with each other, and then assembling and testing the entire system can take a lot of time and guesswork. There are several ways to simplify the design process, one of which is an integrated motor with a drive and other components built in.
Next-generation integrated motors
SEM’s Liberty MDrive® products are integrated stepper motors with servo-like closed loop performance. The product’s hybrid motion technology (hMT) eliminates many negatives traditionally associated with stepper motor technology.
One of the benefits stepper motors provide is their smooth motion at low speed. Stepper motors do not require tuning, allow for a greater inertia mismatch, and have very hightorque density. This torque is 100% available immediately upon start-up, which can be very advantageous when doing short quick moves, or when coupled to high-inertia loads. Because stepper motors are synchronous motors with a high pole count, they are able to run smoothly at extremely slow speeds with no cogging.
Stepper motors have some disadvantages, however. The most critical drawback is the loss of synchronization and torque if a large load exceeds the motor’s capacity to resynchronize once the load is reduced to a level within the motor’s capacity. These motors also tend to run hot because they draw constant current.
“Stalling has always been the albatross around the neck of stepper motors,” says Hummel. Many designers initially use stepper motors because they’re so much more cost-effective than using servo motors. “However,” he explains, “during use, if the motor didn’t end up in a position where you thought you were, and you had no idea you weren’t there, that causes all kinds of problems.”
Hummel notes that the Liberty MDrive has an encoder built in to not only check the position “but if you get to the point where there’s too much torque and it’s not going to be synchronous anymore, the encoder stops the field from rotating. It will sit and hold torque, meanwhile keeping track of where it should have been. So, if it does loosen up at some point, it can catch its way back up to where it is supposed to be.”
“Problems with stalling and not knowing position,” Hummel adds, “are eliminated.”
SEM’s closed loop technology, known as hMT or hybrid motion technology, delivers servo-like performance from a stepper, eliminating many negatives traditionally associated with the motor technology.
hMTechnology blends stepper and servo motor benefits
hMT is a closed loop technology that eliminates loss of synchronization, allowing safe operation of a stepper motor at its maximum torque curve. Therefore, sizing a motor with up to 50% torque margin is no longer required. This may also allow a smaller frame size or shorter stack length motor in some applications. It also enables a system to ride through known transient overloads, further eliminating the requirement for a larger motor and enabling stepper motors to be used in applications outside their traditional range.
The closed loop functionality of the Liberty MDrive also enables a stepper motor to perform under variable torque control. Designers can use it in thrust, pressing, and tension control applications—capabilities that are not usually possible with stepper technology.
Saving time, effort and money
There are many examples where the Liberty MDrive shows measurable savings and success. Applications with equipment that is sensitive to heat changes, for example, lab and imaging equipment, are good fits for the solution. A typical stepper motor might draw 2.5 A continuously, whether it’s being used or not. With a Liberty MDrive, variable current control saves energy and lowers heat by drawing only as much power as is needed to execute a move. The heat dissipation will be less if the motor only needs to draw 2.5 A for the 10% of the time it is being used, and 0.5 A for the other 90%.
Conveying applications represent another example where a Liberty MDrive shows significant power consumption cost savings. Conveyors may run 24 hours a day, seven days a week, with varying loads. The Liberty MDrive allows the motor to run at the minimum current needed to complete the motion, and then increases current only when the load demands it.
A Michigan-based manufacturer and integrator of automation equipment, assembly systems, and test equipment, relies on integrated motors for many of its applications. Their R&D Manager describes using the hybrid motion technology on a slide package for measuring how much load it takes to close a glove box door on a car.
“Using this motor gives me the ability to easily integrate an Allen-Bradley processor using EtherNet/IP to control the motor and look at the load cell feedback.” The application requires smooth actuation. Although an air cylinder was considered for this application, it was reasoned that “an air cylinder wouldn’t run smooth, or have adjustable rate and positioning capabilities.”
This line’s success may lead to installing it in others, including one that assembles sunroofs. The motors will allow the glass to be positioned correctly before being clamped onto the rails, enabling it to move back and forth. The OEM’s engineer says it is very easy to put the system together, and “after you figure out the integration of the software into the motor itself, in my case with EtherNet/IP, it’s a breeze.” He adds, “You get complete control over the motor, making it do exactly what you want.”
Many other applications with various network communication protocols are finding success using the Liberty MDrive products. A material packaging application benefits from the servo performance to insert cotton packing into plastic bottles. The machine’s shock load/resistance caused stalling in typical stepper motors but was overcome with the hybrid technology.
One company that needed to cut medical tubing now uses two Liberty MDrive products in its machine. A rotary motor drives a friction wheel to move the plastic tubing to a specified length for cutting. A linear motor raises and lowers the material platform to he friction wheel. The integrated frame size, speed/torque of the motor, and cost of the system were the main factors in choosing the integrated motor solution.
Summary
Using an integrated motor is an efficient, easy, and effective way to let designers spend more time developing machines and less time studying motion control design. With an integrated motion solution, the integrated components have already been designed and sized for use as a complete unit.
When looking at the types of integrated motors, AC servo motors are the fastest growing segment, but also the most expensive. An alternative to the premium servo motor is a hybrid product called Liberty MDrive, which is an integrated stepper motor with closed loop feedback that acts like a servo.
Liberty MDrive products address the stalling issues associated with stepper motors while keeping the benefits of smooth motion and starting torque at low speeds. These integrated motors can be used in thrust, pressing, and tension control applications.
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