Spring Assist Makes for Better Actuator
Robots are filled with motors. The actuators that make the moving parts … move need motors to provide motion.
The most desirable type of motor for robotic applications is an electric one, said Steve Collins, associate professor of mechanical engineering at Stanford, because they are versatile and highly controllable. Engineers can use a variety of advanced control techniques with them.
But electric motors have a downside: They waste energy in proportion to the square of the torque that they provide, even for tasks that could be net energy neutral. Hydraulic motors have also gained in popularity in robotics because of the higher torque output, but they also have energy drawbacks and challenges with leaking.
Details of the spring assisted actuator developed by Stanford researcher Steve Collins. Image: Steve Collins/Stanford
Springs, on the other hand, are fantastic in terms of efficiency, Collins said. They can return nearly as much energy as they capture. But they are hard to control because they produce torque in proportion to how much mass they’re deflecting.
Collins and his team have tried to bring the strengths of these two complementary robotic actuation elements together for a long time. In a March 2024 paper published in Science Robotics, he has finally cracked the code on spring-assisted actuation, making it possible to harness the raw power of a spring while still maintaining the features of an electric motor.
“About nine years ago, we invented these clutches and, at the time, it hit me like a bolt of lightning,” Collins said. “Instead of continuously variable transmissions between our output and the spring, we’ll have discretely variable transmission with lots of springs. It’s an extremely complicated system, and it took us forever, but we did it.”
After many iterations, Collins and his team developed an electroadhesive clutch. An elastomer spring is connected to two clutches, one is connected to the output, and the other is connected to the frame that holds the system together. By engaging the output clutch, the spring becomes activated, holding and storing its energy as it is stretched. When engaged, the spring has a holding force per unit mass that is about 1,000 times stronger than electromagnetic clutches. The torque can be adjusted by disengaging the output and connecting it to the frame, releasing the stored energy.
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Collins and his team used the clutches to design a robot that can perform a series of different tasks, testing how mass and speed impact the efficiency of the system. The robot is able to maintain control, using anywhere from 50 to 97 percent less power than current electric motor systems.
With more precision and control, Collins said, multiple clutches can be configured to perform all kinds of tasks.
“If you want to control torque, you can place several parallel clutches together,” he explained. “Then depending on how many you engage, you can get different amounts of joint torque. The more springs engaged, the more torque.”
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The next step for the team will be making the system practical for the industry, a process that involves redesigning the system so that it is modular and compact. They are hoping to make a module that can easily replace an electric motor, so it is just a matter of unplugging the old and plugging in the new.
Currently the system requires a bit of time to determine the best way to perform a task. To bypass that constraint, they are also looking at new control methods and how to give the system a library of options to choose from, so it can toggle between tasks with ease.
There are many possible robotic applications for this system, and Collins is particularly excited to see how the system can be used in robotic prosthetics.
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“The reason I did this research was to try to design robots that improve people’s mobility, especially people with disabilities,” he said. “I’m excited for the future.”
After almost 25 years of butting up against challenges and exploring many different paths to get to this point, Collins is incredibly proud of this work. It will be a game changer for robotics and anything that uses a motor, he said.
“I tell my students, ‘Just stick with it,’” he said. “And even if the first five things you try don’t work, eventually, something will click.”
Cassandra Kelly is a technology writer in Columbus, Ohio.
The most desirable type of motor for robotic applications is an electric one, said Steve Collins, associate professor of mechanical engineering at Stanford, because they are versatile and highly controllable. Engineers can use a variety of advanced control techniques with them.
But electric motors have a downside: They waste energy in proportion to the square of the torque that they provide, even for tasks that could be net energy neutral. Hydraulic motors have also gained in popularity in robotics because of the higher torque output, but they also have energy drawbacks and challenges with leaking.
Details of the spring assisted actuator developed by Stanford researcher Steve Collins. Image: Steve Collins/Stanford
Collins and his team have tried to bring the strengths of these two complementary robotic actuation elements together for a long time. In a March 2024 paper published in Science Robotics, he has finally cracked the code on spring-assisted actuation, making it possible to harness the raw power of a spring while still maintaining the features of an electric motor.
“About nine years ago, we invented these clutches and, at the time, it hit me like a bolt of lightning,” Collins said. “Instead of continuously variable transmissions between our output and the spring, we’ll have discretely variable transmission with lots of springs. It’s an extremely complicated system, and it took us forever, but we did it.”
After many iterations, Collins and his team developed an electroadhesive clutch. An elastomer spring is connected to two clutches, one is connected to the output, and the other is connected to the frame that holds the system together. By engaging the output clutch, the spring becomes activated, holding and storing its energy as it is stretched. When engaged, the spring has a holding force per unit mass that is about 1,000 times stronger than electromagnetic clutches. The torque can be adjusted by disengaging the output and connecting it to the frame, releasing the stored energy.
Discover the Benefits of ASME membership
Collins and his team used the clutches to design a robot that can perform a series of different tasks, testing how mass and speed impact the efficiency of the system. The robot is able to maintain control, using anywhere from 50 to 97 percent less power than current electric motor systems.
With more precision and control, Collins said, multiple clutches can be configured to perform all kinds of tasks.
“If you want to control torque, you can place several parallel clutches together,” he explained. “Then depending on how many you engage, you can get different amounts of joint torque. The more springs engaged, the more torque.”
Frontiers of Engineering: Soft Robot Moves Like a Caterpillar
The next step for the team will be making the system practical for the industry, a process that involves redesigning the system so that it is modular and compact. They are hoping to make a module that can easily replace an electric motor, so it is just a matter of unplugging the old and plugging in the new.
Currently the system requires a bit of time to determine the best way to perform a task. To bypass that constraint, they are also looking at new control methods and how to give the system a library of options to choose from, so it can toggle between tasks with ease.
There are many possible robotic applications for this system, and Collins is particularly excited to see how the system can be used in robotic prosthetics.
For You: Sensor Determines Produce Ripeness Instantly
“The reason I did this research was to try to design robots that improve people’s mobility, especially people with disabilities,” he said. “I’m excited for the future.”
After almost 25 years of butting up against challenges and exploring many different paths to get to this point, Collins is incredibly proud of this work. It will be a game changer for robotics and anything that uses a motor, he said.
“I tell my students, ‘Just stick with it,’” he said. “And even if the first five things you try don’t work, eventually, something will click.”
Cassandra Kelly is a technology writer in Columbus, Ohio.
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