1. An electrostatic clutch assembly for transmitting a torque comprising:
- a first electrode disposed on the surface of an input shaft,a second electrode aligned parallel to the surface of the input shaft,wherein the second electrode is flexible,wherein at least one of the surface of the first electrode and the surface of the second electrode is coated with a dielectric material, thereby forming a clutch;
a rigid bar connected to the second electrode;
an output connector affixed to the rigid bar; and
a power source for applying an electric field across the first electrode and the second electrode to develop an electrostatic charge, causing the first electrode and the second electrode to exist in either an attractive state or a non-attractive state,wherein a torque is transmitted through the input shaft to the output connector when in the attractive state.
An electrostatic clutch is described comprising a plurality of micron-scale thickness electrodes, adjacent electrodes being separated by a thin film of dielectric material. A power source and controller apply a voltage across two electrodes, causing an electrostatic force to develop. When engaged, a force can be transferred through the clutch. A tensioning device maintains the alignment of the clutch when the electrodes are disengaged, but permits movement in at least one direction. In some embodiments, multiple clutches are connected to an output to provide variable force control and a broad range of torque input and output values. Moreover, the clutch can be used as an energy-recycling actuator that captures mechanical energy from negative work movements, and returns energy during positive work movements.
- 1. An electrostatic clutch assembly for transmitting a torque comprising:
a first electrode disposed on the surface of an input shaft, a second electrode aligned parallel to the surface of the input shaft, wherein the second electrode is flexible, wherein at least one of the surface of the first electrode and the surface of the second electrode is coated with a dielectric material, thereby forming a clutch; a rigid bar connected to the second electrode; an output connector affixed to the rigid bar; and a power source for applying an electric field across the first electrode and the second electrode to develop an electrostatic charge, causing the first electrode and the second electrode to exist in either an attractive state or a non-attractive state, wherein a torque is transmitted through the input shaft to the output connector when in the attractive state.
- 2. The electrostatic clutch assembly of claim 11 further comprising:
a plurality of clutches arranged concentrically, wherein each clutch of the plurality of clutches is attached to the input shaft and the output connector.
This application is a divisional of U.S. Nonprovisional application Ser. No. 15/484,052 filed Apr. 10, 2017, which claims priority to Provisional Ser. No. 62/495,693, filed Sep. 21, 2016 and is a continuation of PCT Application No. PCT/US2015/055005, filed Oct. 9, 2015, which claims priority to Provisional Ser. No. 62/122,066, filed Oct. 9, 2014, and Provisional Ser. No. 62/231,818, filed Jul. 16, 2015, all of which are incorporated herein by reference.
This invention was made with government support under NSF Grant No. U.S. Pat. No. 1,355,716. The government has certain rights in this invention.
The invention relates generally to an electrostatic clutch. More specifically, the invention relates to a lightweight and high power density electrostatic clutch that can be incorporated into robotic systems, including exoskeletons and wearable devices, among other uses.
Clutches have many uses in mechanical systems, often being used to improve the functionality of springs and actuators. However, existing clutch systems suffer several drawbacks when used in mobile applications, such as robotics. For example, electromagnetic clutches feature fast activation and moderate torque density, but require continuous electrical power to stay active. Magnetorheological clutches produce large torques, but are heavy and also require continuous power to remain active. Because of the power requirements, both of these systems require large batteries or tethered electrical connections. Mechanical latches require no energy to stay active, but only engage and disengage under special conditions.
The problems associated with traditional clutches are particularly pronounced in wearable robotic systems, such as exoskeletons. Assistive robotic exoskeletons have shown positive impacts for people in a variety of applications, including physical performance augmentation and medical treatment. One challenge associated with autonomy is the metabolic cost associated with carrying the combined weight of the exoskeleton structure, energy storage, actuators, and electronics. Batteries in particular account for a significant portion of the weight of many devices, especially in devices with clutches that require constant power. In addition to the weight of batteries, significant weight penalties are experienced with commercially available actuators, such as motors and pneumatic actuators.
Walking on level ground is an example of an application where traditional actuators and motors are not well suited for robotic applications. Walking on level ground at a constant speed requires very little energy input since the potential and kinetic energies of the moving body do not change on average. However, approximately equal amounts of positive and negative work are performed by the legs during a walking cycle. Both the positive and negative work require energy, since the negative work cannot be stored and reused as an input for the positive work.
If an actuator could absorb and return mechanical energy, the total energy consumption of the system could be reduced. Ideally, energy recycling could supply all needed positive work by absorbing and reusing negative work movements. As an added benefit, using a device to absorb energy from negative work movements would reduce the metabolic cost of a human wearing a robotic device because muscles require energy to perform negative work.
Lightweight, low-power, and electrically controllable clutches would allow greater performance of many robotic systems. Improved clutches could be incorporated into actuators to substantially improve the actuator'"'"'s energy demands. Therefore a need exists for a clutch that does not exhibit any of the shortcomings of traditional clutches.
According to embodiments of the present disclosure is an electrostatic clutch that can be incorporated in many types of mechanical systems, such as robotics, wearable devices, or exoskeletons. In particular, the present invention utilizes micron-thickness electrostatic clutches that are light-weight and consume minimal power.
Electrostatic forces can be developed by applying a voltage to a set of electrodes separated by a gap. In the present invention, the gap is maintained by a layer of dielectric material deposited on the electrode. When a voltage is applied, positive and negative electrical charges develop, causing an attraction between the adjacent electrodes and preventing them from moving relative to each other. Like a capacitor, power consumption is very low once a charge is developed because additional energy is only required when switching states. A controller can manipulate the voltage, allowing electrical ‘on-off’ control of adhesion between the electrodes.
The electrodes comprise a lightweight conductive material, such as aluminum-sputtered biaxially-oriented polyethylene terephthalate. With a pair of electrodes, at least one electrode is covered in a dielectric material to maintain the gap between the conductive surfaces of the electrodes. In some embodiments, the electrodes are generally planar, having a rectangular or square shape. A frame is connected to each of the electrodes, providing a transfer point for a force acting on the clutch. For example, the frame of one electrode could be connected to a spring, while the frame of the other electrode could be connected to the body of an exoskeleton. Thus, the activation state of the clutch determines if a force is transferred from the spring to the body of the exoskeleton through the clutch, or if the electrodes will simply slide against each other without transferring the force. A tensioner maintains alignment of the electrodes, while permitting movement in one or multiple directions.
In an alternative example, three electrodes are arranged in a parallel orientation. One electrode is attached to a body of a device and a second electrode is connected to an output supplying a force. A third electrode is connected to a spring and placed between the first and second electrodes. The electrode connected to the spring can be engaged against either the body electrode or the output electrode. In this configuration, the spring can be stretched by the output force, affixed to the frame to store the energy, then later returned to the output to perform work, forming a type of energy recycling actuator. Given the micron-scale thickness of the electrodes, the actuator can be comprised of tens to hundreds of clutch/spring pairs that are individually engaged and disengaged with the output, thus allowing variable stiffness and a broad range of torque input and output values over the course of one actuator stroke.
The clutch system of the present application, depending on the particular implementation, uniquely allows for both force control and energy recycling, making it both highly controllable and highly energy efficient. In addition, this system allows variable stiffness, impedance or other state-dependent force generation at exceptionally high bandwidth and with low input of control energy. The clutch system will enable dramatic improvements in the energy efficiency and controllability of autonomous robotic systems and wearable robotic devices.
Embodiments of the present invention and its advantages are best understood by referring to the figures.
A frame 103 is attached to one end of the first electrode 101 and a separate frame 104 is attached to one end of the second electrode 102. The frames 103, 104 are positioned at opposite ends of the clutch 100, as shown in
Referring again to
As previously stated, the electrodes 101, 102 can be flexible according to some embodiments. The compliant nature allows intimate surface contact between the electrodes 101, 102 when engaged. This allows the surfaces to conform closely without relying on a high surface energy interface. In previous electrostatic devices, electrodes are embedded in soft, tacky elastomers. Releasing these types of devices requires a separate mechanism because the elastomers tend to stick to each other after being drawn together by the electrostatic forces.
When in the engaged state, a layer of dielectric material 203 maintains the gap between the conductive layers 103 on adjacent electrodes 101, 102. In one example embodiment, a thin film of dielectric material 203 is disposed on the surface of one of the electrodes 101, 102, covering the conductive layer 202. At a given voltage, the capacitance of the clutch 100 increases as the dielectric constant of the material used for the insulating layer 203 increases. As such, a high dielectric constant material is desirable to allow operation at a relatively low voltage. However, the type of dielectric constant material used can depend on the particular application. The dielectric layer 203 can be an inorganic particle impregnated polymer or a liquid-formable nanoparticle composite. In one example, a ceramic polymer composite containing barium titanate and titanium dioxide is used to create the dielectric layer 203. An example of such a material is Dupont™ LuxPrint® material, which is sold for electroluminescent applications. With a low voltage, 200-300 V for example, standard electronics hardware can be used with the clutch 100.
Testing of a liquid formable nanoparticle composite indicates the capability to produce 6 times higher pressures at 15 times lower field strengths than the inorganic polymer (Table 1).
Increasing field strength and voltage provide diminishing shear pressure after a critical value is reached. This result is attributed to the development of space charge. Space charge occurs when charge carriers are forced into the dielectric material 203 from the electrodes 101, 102 and become trapped. This creates an internal electric field that counteracts the applied field, and produces some force even after the electrodes 101, 102 are grounded, causing slow or no release. This effect is dependent on the chemical makeup of the dielectric material 203. Avoiding space charge is critical to achieving effective pressure development and fast releasing.
The detrimental effects can be reduced by maintaining low electric field strength and voltage. Consequently, decreasing the thickness and subsequently the overall voltage value can mitigate space charge effects, but the electric field strengths should also be kept low. The liquid formable nanoparticle composite is used in the example embodiment because the high dielectric of the material reduces required field strengths. Also, because the liquid formable nanoparticle composite is obtained in its uncured form, it can be incorporated onto the clutch 100 with a lower thickness.
By way of example, the process of applying the dielectric layer can include depositing a 25 micron layer of the liquid formable nanoparticle composite on one side of electrode 101 using a thin film applicator. Based on the particular dielectric material 203 used, the composite is cured to a thickness of 10 microns in a ventilated oven. A second 25 micron layer is then applied and cured to a final dielectric layer 203 of 20 microns. The film decrease in thickness occurs because a significant amount of solvent evaporates from the original mixture during curing. Other methods can be used to deposit the dielectric layer 203, such as screen printing or chemical and physical deposition.
With potentials as low as 200 V between electrodes 101, 102, a shear pressures of 15 kPa is generated across electrodes 101, 102 described in the embodiments shown in
As a result, when implemented in an actuator or other device, the low-mass, low-energy, and low-volume electrostatic clutch 100 of the present invention allows multiple clutches 100 to be used in a single device. Because of the unique geometry of these electrostatic clutches 100, many can be “stacked” into a small volume with a spacing of 1 mm or less between clutches 100. Achieving tens or hundreds of clutches in a device using traditional mechanical or electromagnetic clutches results in a slow, energy-expensive device far too large and heavy to be body-mounted.
By way of example of a system thus described, a stacked clutch implementation can comprise 5 electrode pairs, each having a thickness of 45 microns and a mass of 2 grams. The contact area of the pairs is 100 cm2 (10 cm×10 cm), resulting in a holding force of 150 N. The switching energy required to change from an engaged to disengaged state is 0.01 J. Switching can occur at a bandwidth of 160 Hz.
The clutch 100 of the present invention is designed to be generic enough to be “attach-and-play” on assistive exoskeletons, active prostheses, walking robots, and other devices. It is an aspect of the present invention that the design can be modified for use with a motor as a variable stiffness actuator, or to achieve “one-to-many” degrees of freedom by decoupling an input from an output. This can be achieved by adding single clutch 100 in series with a clutch-spring pair.
In another embodiment, multiple electrodes are arranged in parallel to create a type of energy-recycling actuator 500, which is illustrated in
In operation, selective engagement of the electrodes 501, 503, 504 can result in an energy recycling cycle. An example of an energy recycling cycle for a similar actuator 500 is illustrated in
Energy capture and return could be achieved with a single spring 301 permanently engaged with the object. However, assistance timing and periods of non-interference are important for many tasks. A device capable of periodically allowing free movement therefore offers more utility. The actuator 500, using a spring 301 and a three-way clutching mechanism provides this utility. In the example of an exoskeleton, the spring 301 could absorb energy from a human'"'"'s negative work movement. At the end of the movement, the spring 301 is engaged to the housing 502, storing the energy and allowing free movement of the object. When energy is required to assist in a positive work movement, the third electrode 504 is re-engaged with the spring 301, which shortens as it returns energy to the human.
The foregoing example describes an actuator 500 comprised of a single spring/clutch mechanism. However, multiple spring/clutch pairs can be used to create an actuator capable of providing variable stiffness. That is, if all springs 301 are engaged, actuator 500 will have a high stiffness. If only a fraction of the springs 301 are engaged, while the remainder are disengaged and free to move, the actuator 500 will have a reduced stiffness. Consequently, the stiffness of the actuator can be manipulated based on the appropriate level for different types of activities. With a higher stiffness, higher assistive torques will be provided.
By engaging increasing or decreasing numbers of springs 301 during an actuator stroke, a variety of force values can be achieved, independent of device configuration. For example,
The opposite end of the spring 301 is attached to a lower portion 603 of an exoskeleton frame 601. The exoskeleton frame 601 is a lightweight, high-strength composite frame having a hinge 602 at the ankle, connecting the lower portion 603 to an upper portion 604. Frame 103 is connected to the upper portion 604 of exoskeleton frame 601. During a walking cycle, flexing of the foot causes stretching of the spring 301 when the electrodes 101, 102 are engaged. The energy of the spring 301 can be released during other phases of the walking cycle.
In the preferred embodiment, the electrode 101 is switched between high voltage and ground at 200 Hz for 50 ms to facilitate clutch release. In this example, peak torque is about 7.3 N*m on an average step, and the device consumes about 8.7 mW of electricity.
In this example shown in
While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.