The machines that make our world continue to advance. Six-axis CNC milling machines, seven-axis robots, collaborative robots, and smart presses are more powerful than ever, with micron- level accuracy. One of the key elements in these advances are servos. People in industry often bandy about the word “servo” without fully understanding all it means. That’s not really an issue unless one is trying to understand why industrial machines are so capable today, how they are going to get better—and their impact on your business.
Before exploring these trends, let’s lay the groundwork of what makes up a “servo.”
Servos deliver precision movement. They either move to a precise spot or move in a precise way. Servos also can keep a steady torque to hold a load in a set position. The four parts of an electromechanical servo are what provides that precision control. Three of these parts are:
- An electric motor, either AC or DC. The output shaft may be (often is) connected to a mechanical transmission for greater control, such as a ball screw or gear set.
- A sensor that knows the position, torque and/or speed of the motor shaft, usually a rotary encoder.
- A controller module, or servo drive unit. (This includes the amplifier needed to transform low-voltage command signals into high-voltage signals that the electrical motor can use. Technically, the amplifier is a separate unit.)
A servo drive controls its motor in a closed loop by sensing its torque, speed, or position (or any combination thereof) and adjusting the drive voltage to make what is sensed match what is desired in torque, speed, or position. There are any number of algorithms for how this works, such as the popular proportional–integral–derivative (PID) control algorithm.
What is the fourth element, you ask? Software. Both for the servo as a unit and for the mechanism in which it is embedded and directs it, such as an automation system or CNC machine tool. This element is becoming the most important of the four.
Electric Motors and Applications
Electric motors, whether they use alternating current (AC) or direct current (DC), are well understood. Many of the basic designs of the different motor classes are still unchanged after decades. All technologies reach a limit of diminishing improvements, and that is where motors are.
“Servo motor manufacturers treat motors like a black box,” said Scott Strache, product manager CNC & robotics, Mitsubishi Electric Automation Inc., Vernon Hills, Illinois. Mitsubishi Electric provides technology for use by both CNC machine builders and motion control integrators. “DC motors in general provide more torque at a lower base speed, while AC motors tend to be more high precision, run at faster rpms and faster acceleration and deceleration.” He noted that in the machine tool industry, his primary market, “everybody has gone to AC motors.”
Yet there are many applications where high power in a small package is needed, and DC motors are the solution. “We use only brushless DC motors,” said Glenn Nausley, president of Promess Inc., Brighton, Mich. Promess delivers complete turnkey monitoring and motion systems, including servo presses with up 200,000 lb-ft capacity. “We have applications where you need strength to move, but also strength to hold a position accurately. That is where brushless DC motors are so well suited,” Nausley said.
A practical tip from Nausley: “Do not undersize the motor. Motors are capable of providing torque well beyond their rated nominal, sometimes up to three times. If you rely on that peak, it generates heat it was not designed for and you risk failure. You have to pay attention to overheating.” The lesson is, match your needs to the nominal power rating of the motor, do not rely on its peak to save a few bucks.
Sensors, Improvements and Smart Manufacturing
Sensors, especially encoders, are the key element that makes a garden variety motor a servo. “The industry typically offers different resolutions of encoders,” said Strache from Mitsubishi Electric. “We offer encoders from Mitsubishi Electric that include one million, four million, and 67 million pulses per revolution,” Strache said. That translates into a maximum of 0.0000053° of resolution, easily achieving sub-micron level accuracy depending on the diameter of the unit.
“Optical encoders are good for 99 percent of applications,” said Dan Zachacki, senior product marketing engineer, servo & motion at Mitsubishi Electric Automation. “The new designs of optical encoders are what is exciting. The motors are more than capable of handling the precision demanded today, and even more.”
The next generation of Mitsubishi Electric’s servo product is its MELSERVO-J5 or MR-J5, noted Zachacki. “Our standard encoder integrated into the servo is 26 bit,” he said, which translates to more than 67 million pulses per revolution. Just as important as the encoder is the frequency at which the drive/amplifier is collecting data from the encoder—the speed frequency response rate. The faster this rate, the more responsive the servo can be. Faster movements equal faster production. “The MR-J5’s response rate is 3.5 kHz,” said Zachacki.
Another emerging trend is that servo sensor data can be used for other things. “The servo motor can be used to collect data for performance analytics of the machine,” explained Joaquin Ocampo, product manager for Bosch Rexroth Corp., Hoffman Estates, Illinois, the supplier of drive and control solutions for many industries. Since the data exists and is accessible, that makes it easier to enable smart manufacturing.
Smaller, Accurate, Stronger
Ocampo summarized a few essential trends that will drive development in servos in the future. The trend to quicker and more precise acceleration and deceleration, for one, and ever higher speeds for another, will continue. “Also, we will see higher overload capacities for very short durations,” he said. “Once a CNC drill or robot moves, for that very short duration at the beginning the servo motor will need higher overload capacity.
“Safety has also become a big concern today, not just for personnel but also recognized as improving production by keeping up-time higher,” Ocampo added. Nausley from Promess also stressed that servo drives are now smart enough to know if something is not safe, say moving down when a worker’s hands can be in the way. The servo is a sophisticated digital device that integrates safety without sole reliance on an emergency stop. “We’re seeing safety integrated into the network itself,” said Nausley.
Zachacki from Mitsubishi Electric agreed, adding that safety is coupled with higher levels of integration. “The CC-Link IE TSN network is designed to either give you a full plant-wide network for your machines, or you can integrate it into an existing one,” he said. “That includes safety. We refer to it as safety over CC-Link IE TSN. For example, a machine’s e-stop button can connect it to a TSN network. When the e-stop button is hit, it sends a command to the servo amplifier, and it cuts off the torque-producing current.”
Not only is it safe, it is also productive, because according to Zachacki, when the machine is re-cycled, there is no need to reboot the servo. It was always on, just not operating. “This is because the technology is becoming more integrated,” he said.
Reducing footprint will always be in demand. Servo motors are getting smaller as they get more powerful and designers want to take advantage of that. Most servo systems today connect to a cabinet. Bosch Rexroth now offers a cabinet-free solution with its IndraDrive MI line, an integrated motor/encoder/drive unit. “One cable runs to one unit from the power supply and multiple IndraDrive MIs can be daisy chained, which reduces both footprint and cable management,” Ocampo said. “The integrated unit is IP65 rated and that integrated drive can now be right on the machine.”
Software is the Secret Sauce
Once, servo amplifiers were tuned with screwdrivers to adjust the motion of the motors, with say three potentiometers, one for each of the elements of a PID controller. “Today, most servo drives have algorithms that autotune adjustments,” said Nausley. Promess can now position its presses within a few microns. “A few years ago, there’s no way we could have done that.”
Nausley pointed out that Promess, like others, writes its own software while purchasing the components, like servos. This is in part driven by the need to use other sensors in addition to the servo sensors, such as temperature and pressure transducers. “A good example is a press that needs to hold 1,000 lb-ft during a material cure cycle. As the material tends to shrink, we need to keep the force at 1,000 lb-ft and our monitoring system, written entirely by us, sends a signal to the servo to keep up that pressure,” Nausley said.
Bosch Rexroth has recognized this trend. “Opportunities are shifting from hardware to software. The digital arena is changing our business rapidly,” said Ocampo. “Our ctrlX DRIVE and ctrlX AUTOMATION will provide this. It is more based on software than hardware. Some of its details include multi-Ethernet Fieldbuses or its programmability with high-level, open-source languages such as C++, Java, Python or even a visual-code such as Blockly.
“Think of ctrX AUTOMATION as the smart phone of industry,” he continued. Applying this analogy, ctrlX AUTOMATION could be used to build the next generation of CNC milling machines, multi-turret/live tooling lathes, or material handling systems. Blocks of components and controllers could be used for almost anything, according to Ocampo. This is good news for those that need to build the machines of tomorrow.