Marman clamp with a shape memory alloy actuator

A Marman clamp is disclosed. The Marman clamp includes a release mechanism with either pins or tubes formed with a shape memory alloy. The release mechanism opens and closes the clamp depending on the temperature of the pins or tubes. The temperature of the pins or tubes may be based on whether a space vehicle including the clamp is in a direct line of sight of radiation from the sun. Alternatively, a heat source may control the temperature of the pins or tubes. The Marman clamp may also open and close as a result of an independent force applied to the release mechanism.

FIELD

The disclosure is related to Marman clamps and, more particularly, to a Marman clamp having a shape memory alloy actuator.

BACKGROUND

Marman clamps are commonly used in space vehicles to connect satellite payloads to rockets, join rocket stages and antennas, and as a general clamping mechanism. A Marman clamp is a band clamp that allows two cylindrical interfaces to be clamped together. A typical Marman clamp system100is shown inFIGS. 1a-cand is also found in NASA's Marman Clamp System Design Guidelines, Guideline No. GD-ED-2214.

As shown inFIGS. 1a-c, the Marman clamp system100includes a tension strap102that creates radial forces on V-segments104, which wedge together flanges of cylindrical structures in an axial direction. A release mechanism106includes two end fittings110, each with a nut112connected to a bolt108. The Marman clamp system100remains in a closed position until the release mechanism106is opened via a force, for example, by a pyrotechnic bolt cutter. Based on this type of release mechanism106, these Marman clamps are sometimes referred to as pyrotechnic-based clamps.

With the Marman clamp system100, multiple release mechanisms are required in space applications in case one of the mechanisms fails to open. For example, up to four release mechanisms may be required depending on the size of the clamp. These additional release mechanisms add to the weight and complexity to the space vehicle that includes the Marman clamp system100.

Thus, it would be beneficial to have a Marman clamp that does not require redundant release mechanisms in space applications, eliminating both the added weight and complexity.

SUMMARY

A Marman clamp that includes a release mechanism that places a ring in either the open or closed position using a shape memory alloy actuator is disclosed. The shape memory alloy actuator may be in the form of a pin or a tube.

Also disclosed is a system, such as a space vehicle, that uses a clamp to connect a first structure to a second structure. The clamp has a shape memory alloy actuator that determines when to change positions of the clamp, from closed to open or vice versa. A temperature of the shape memory alloy actuator may determine when to change positions of the clamp. The temperature of the shape memory alloy actuator may depend on whether the space vehicle is in a direct line of sight of radiation from the sun or whether heat is applied to the actuator. Alternatively, a force applied to the shape memory alloy actuator may determine when to change positions of the clamp.

A method of actuating a clamp with a shape memory alloy actuator is also disclosed. The method includes maintaining the clamp in a first position during a first period of time and then moving the clamp to a second position when the shape memory alloy actuator receives an external trigger. The two clamp positions are opened and closed.

In one example, the external trigger is temperature and the method includes maintaining a clamp in a first position until a temperature of the shape memory alloy actuator passes a martensitic transition temperature, which is when the clamp moves to the second position. The temperature of the shape memory alloy actuator passes the martensitic transition temperature by either rising or falling. In another example, the external trigger is a force and the clamp moves to the second position when the force is applied to the shape memory alloy actuator.

The drawings are for the purpose of illustrating example embodiments, but it is understood that the inventions are not limited to the arrangements and instrumentality shown in the drawings.

DETAILED DESCRIPTION

FIG. 2is an illustration of a Marman clamp200. The Marman clamp200includes a ring202and a release mechanism204. The ring202is sometimes referred to as a band or a strap. As shown inFIG. 2, the release mechanism204is in a closed position.FIG. 3depicts the release mechanism204in the open position.

The ring202may be formed using any suitable material for space conditions. Preferably, the ring202is formed using a highly ductile material that resists stress corrosion cracking. For example, the ring202may be formed using aluminum. Other possible ring materials include steel and titanium. The ring202includes ends that connect to the release mechanism204.

The release mechanism204includes shape memory alloy (SMA) pins206. Alternatively, the release mechanism204may include SMA tubes400as shown inFIG. 4. An SMA tube400is similar to an SMA pin206except that the tube400has a hole402at its center. The SMA pins206and the SMA tubes400are referred to as actuators because they control whether the Marman clamp200is in an open or closed position. U.S. Patent Application Publication No. 2005/0198777 describes methods for making SMA pins and tubes and is incorporated in its entirety herein. While the remainder of this description refers to the SMA pins206, it is understood that the pins206may be replaced with the SMA tubes400.

Shape memory alloy is an alloy that changes shape in response to its temperature. A transition temperature defines when the alloy transitions from one shape to another. In particular, the alloy transitions from a martensitic state to an austenitic state when heated above the transition temperature, and then returns to its martensitic state when cooled below the transition temperature. For the Marman clamp200, the temperature determines whether the clamp200is in its closed position as shown inFIG. 2or its open position as shown inFIG. 3.

An example of an SMA is Nitinol. Nitinol is a nickel titanium (NiTi) alloy. There are various versions of Nitinol depending on the amount of nickel as compared to titanium in the alloy. For example, Nitinol 55 includes 55% nickel by weight. The material properties of Nitinol 55 are well known. Other known versions of Nitinol include Nitinol 57 and Nitinol 60. The amount of nickel in Nitinol impacts the alloy's transition temperature.

The SMA pins206may be formed with any type of Nitinol. The SMA pins206may also be formed using other shape memory alloys, such as an alloy formed with nickel, titanium, copper, zinc, aluminum, platinum, and palladium. For example, the SMA pins206may be formed using an alloy of copper and nickel.

The choice of alloy may be based on the operating temperature ranges, transition temperatures, and transition times. In particular, the type of alloy used for the pins206impacts when the Marman clamp200opens and closes. For example, if the alloy is Nitinol 55, the clamp200may close at a temperature of approximately 130° C. and open at approximately 30° C. depending on actuator training and alloy composition. For example, with actuator training, the Marman clamp200may open when heated as opposed to when it is cooled.

The size of the Marman clamp200may depend on the diameter of the pins206. The pin diameter may be as small as 1/10th of an inch and can be much larger depending on the temperature properties of the pin206. The pin diameter should be large enough not to be affected by surrounding materials and be able to hold heat.

FIG. 5is an illustration of a space vehicle500using a Marman clamp200to connect a payload502to the space vehicle500. In this example, the space vehicle500is a satellite and the payload502is an auxiliary satellite payload. It is understood that there are other space applications for the Marman clamp200. For example, the Marman clamp200may be used to connect satellite payloads to rockets, join rocket stages, to connect antennas to the space vehicle500, and for other clamping applications.

In space, there is no atmosphere and, thus, no heat transfer. In one example, when the space vehicle500is in direct line of sight of radiation from the sun, the Marman clamp200remains in its closed position. As a result, the payload502remains connected to the space vehicle500until the space vehicle500is located away from the sun.

In this example, when the space vehicle500is out of the direct line of sight of radiation from the sun, the surrounding temperature drops below the transition temperature of the alloy used in the pins206. At that point, the Marman clamp200opens releasing the payload502into orbit. As the release mechanism204depends only on the surrounding temperature and the transition temperature of the alloy, the Marman clamp200is not damaged during release like pyrotechnic-based clamps. Instead, the Marman clamp200closes again once the surrounding temperature increases above the transition temperature.

In another example, the Marman clamp200may include pins206that have been trained to open in the direct line of sight of radiation from the sun. The pins206may be trained using a tooling fixture during heat treatment. The space vehicle mission determines whether the Marman clamp200is designed to be in the open position when it is in or out of the direct line of radiation from the sun.

While the previous examples described the use of heat from the sun, it is also possible to use another heat source to trigger the Marman clamp200to open or close. For example, the heat source may be a heater or an electrical current. In this example, the performance of the Marman clamp200is not dependent on the direction that the space vehicle500or the clamp200is facing. Rather, the temperature and, therefore, the actuator performance are controlled as needed.

In addition to heat, another external trigger may be used to open and close the Marman clamp200. For example, an independent force may be applied to the release mechanism204, such as an electromagnetic or applied force. With either the heat source or independent force applied to the release mechanism204, the Marman clamp200may be used in non-space applications (e.g., land applications) as well as space applications.

WhileFIG. 2depicts a release mechanism204with two SMA pins206, the release mechanism204may also include a single SMA pin206. In this example, the second pin may be fabricated using an elastic or other flexible material. Alternatively, the second pin may be replaced with a spring.

In addition to not being damaged during release, the release mechanism204eliminates the need for redundant release mechanisms on the Marman clamp200. As a result, the Marman clamp200is lighter and less complex than the pyrotechnic-based clamps. Additionally, the Marman clamp200is easier to install as it functions as its own tightening device.