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Research Papers

Comparison and Analysis of a Robotic Tendon and Jackspring™ Actuator for Wearable Robotic Systems

[+] Author and Article Information
Thomas G. Sugar

College of Technology and Innovation,
Arizona State University,
Mesa, AZ 85212
SpringActive, Inc.,
Tempe, AZ 85281

Jeffrey Ward

SpringActive, Inc.,
Tempe, AZ 85212

Manuscript received March 11, 2012; final manuscript received June 15, 2013; published online September 24, 2013. Assoc. Editor: Just L. Herder.

J. Med. Devices 7(4), 041003 (Sep 24, 2013) (11 pages) Paper No: MED-12-1035; doi: 10.1115/1.4025182 History: Received March 11, 2012; Revised June 15, 2013

Spring-based actuators are important in the design of wearable robotic systems. These actuators can store and release energy, and reduce the peak power requirements. Reducing these requirements allows the system to function with smaller and lighter-weight motors. Three actuators are compared: a lead screw actuator, a robotic tendon actuator, and a JackSpring™ actuator. The robotic tendon actuator adds a spring in series to the traditional actuator. The JackSpring actuator is a lead screw with a finite stiffness. A formal set of equations for the three actuators is added to Table 1 which summarizes the torque, angular speed, and power for each one. The traditional lead screw actuator cannot store and release energy and the power into the actuator must equal the power out of the actuator. The robotic tendon actuator stores and releases energy, and if a tuned spring is chosen, the power requirements can be greatly reduced. For example, if the desired external motion matches the natural frequency of the system, the motor does not need to rotate. The JackSpring actuator is a unique actuator because the stiffness and motion are coupled. It is shown that if the spring is tuned properly, the power requirements can be greatly reduced, as well.

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References

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Figures

Grahic Jump Location
Fig. 1

In a robotic tendon model, the motor turns moving a nut inward or outward adjusting, r. The actuator force, F, and position, x, describe the desired outputs. A spring is placed in between the nut and the force, F. The free length of the spring is given by, a.

Grahic Jump Location
Fig. 2

JackSpring Concept: as the number of active coils is decreased, the stiffness of the remaining spring is increased. The length of the spring is decreased, as well.

Grahic Jump Location
Fig. 3

The force on the actuator, F, is used to balance the inertial load, m, and the load due to gravity, u

Grahic Jump Location
Fig. 4

Case 1a: A mass is raised and lowered at a constant rate. The mass is mounted on top of the actuator applying a compressive force. The JS actuator has reduced torque requirements, but increased speed requirements resulting in higher peak power. K = 4024.3 N/m for the RT actuator, B = 130,182.3 N/m/coil for the JS actuator.

Grahic Jump Location
Fig. 5

Case 1b: A mass is raised and lowered at a constant rate. The mass is mounted underneath of the actuator applying a tensile force. The JS actuator has increased torque requirements, but decreased speed requirements resulting in lower peak power. K = 4024.3 N/m for the RT actuator, B = 15,000 N/m/coil for the JS actuator.

Grahic Jump Location
Fig. 6

Case 2a: A mass is oscillated back and forth at a constant frequency, 1 Hz. The mass is mounted on top of the actuator applying a compressive force. The total force is the sum of the inertial force and compressive load. The tuned RT actuator has reduced torque requirements and does not move as compared to the LS actuator. The tuned JS actuator has reduced torque requirements and does not move as compared to the LS actuator. K = 3947.8 N/m for the RT actuator, B = 127,708 N/m/coil for the JS actuator.

Grahic Jump Location
Fig. 7

Case 2b: A mass is oscillated back and forth at a constant frequency, 1 Hz. The mass is mounted underneath of the actuator applying a tensile force. The total force is the sum of the inertial force and tensile load. The tuned RT actuator has increased torque requirements and does not move as compared to the LS actuator. The tuned JS actuator has increased torque requirements and does not move as compared to the LS actuator. K = 3947.8 N/m for the RT actuator, B = 99,292.1 N/m/coil for the JS actuator.

Grahic Jump Location
Fig. 8

Case 3a: A mass is oscillated back and forth at a constant frequency, 1 Hz. A compressive force is added that is 90 deg out of phase with the position of the mass. The total force is the sum of the inertial force and the compressive force. The tuned RT actuator has increased speed as compared to the LS actuator. The RT actuator has greatly increased power requirements. The tuned JS actuator has increased speed and greatly increased power as compared to the LS actuator. The power curve for the JS actuator is not a simple sinusoidal pattern because it differs in compression and tension. K = 3947.8 N/m for the RT actuator, B = 127,708.8 N/m/coil for the JS actuator.

Grahic Jump Location
Fig. 9

Case 3b: A mass is oscillated back and forth at a constant frequency, 1 Hz. A tensile force is added that is ninety degrees out of phase with the position of the mass. The total force is the sum of the inertial force and the tensile force. The tuned RT actuator has increased speed as compared to the LS actuator. The RT actuator has greatly increased power. The tuned JS actuator has increased speed and greatly increased power as compared to the LS actuator. The power curve for the JS actuator is not a simple sinusoidal pattern because it differs in compression and tension. K = 3947.8 N/m for the RT actuator, B = 127,708.8 N/m/coil for the JS actuator.

Grahic Jump Location
Fig. 10

Case 4: A mass is oscillated back and forth at a constant frequency, 1 Hz. A damping force is added that is in phase with the velocity of the mass. The total force is the sum of the inertial force and the damping force. The tuned RT actuator has decreased speed as compared to the LS actuator. The LS power curve matches the output power requirements. The RT actuator has greatly decreased power requirements. The tuned JS actuator has decreased speed and somewhat decreased power as compared to the LS actuator. The power curve for the JS actuator is not a simple sinusoidal pattern because it differs in compression and tension. K = 3947.8 N/m for the RT actuator, B = 127,708.8 N/m/coil for the JS actuator.

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