Clamp assembly including permanent magnets and coils for selectively magnetizing and demagnetizing the magnets

A clamp assembly comprises a first clamp including a plurality of magnet devices. Each magnet device includes a permanent magnet and a coil surrounding the permanent magnet. The clamp assembly further comprises a controller for pulsing the coils to selectively magnetize and demagnetize the permanent magnets.

BACKGROUND

Consider the example of an assembly operation in which a stack of parts are fastened together. The parts are clamped together with hundreds of pounds of force, while fasteners such as rivets or bolts are inserted into the stack and then terminated.

In this example, electromagnets are used to apply the clamping force. For instance, an array of electromagnets may be positioned on one side of the stack, while a metal plate is positioned on an opposite side of the stack. When the electromagnets are actuated, they create a magnetic field whose flux lines flow through the plate and move the plate towards a least reluctance position (towards the electromagnets). As a result, the parts are clamped together.

To obtain hundreds of pounds of clamping force, large electromagnets and high currents are used. High current is applied constantly during a clamping cycle. During a long clamping cycle, total energy usage is extremely high.

A cooling system may be needed to cool the electromagnets to avoid overheating during long clamping cycles. Air or another cooling fluid may be flowed through channels between copper windings of the electromagnets. Heat carried away by the fluid may be discharged by a heat exchanger or other secondary system.

SUMMARY

According to an embodiment herein, a clamp assembly comprises a first clamp including a plurality of magnet devices. Each magnet device includes a permanent magnet and a coil surrounding the permanent magnet. The clamp assembly further comprises a controller for pulsing the coils to selectively magnetize and demagnetize the permanent magnets.

According to another embodiment herein, a system comprises a robot end effector including a plurality of magnet devices about a process axis. Each magnet device includes a permanent magnet and a coil surrounding the permanent magnet. The system further comprises a controller for pulsing the coils to selectively magnetize and demagnetize the permanent magnets.

According to another embodiment herein, a method of clamping a stack comprises positioning permanent magnets against a first surface of the stack, placing a flux-conducting structure against a second surface of the stack, and applying external magnetic field pulses to the permanent magnets to magnetize and demagnetize the permanent magnets.

These features and functions may be achieved independently in various embodiments or may be combined in other embodiments. Further details of the embodiments can be seen with reference to the following description and drawings.

DETAILED DESCRIPTION

Reference is made toFIG. 1, which illustrates a clamp assembly110for magnetically clamping a stack. The stack may include one or more parts. Composition of the stack is not limited to any particular material.

The clamp assembly110includes a clamp120, which includes a plurality of magnet devices130. Each magnet device130includes a permanent magnet132and a coil134surrounding the permanent magnet132. The coils134are used to selectively magnetize and demagnetize their corresponding magnets132.

The clamp assembly110further includes a flux-conducting structure140that forms an air gap with the magnets132of the clamp120. During a clamping operation, the stack is located in the air gap, between the permanent magnets132and the flux-conducting structure140. When the magnets132are magnetized, the flux-conducting structure140is drawn towards the clamp120, whereby a clamping force is applied to the stack. When the magnets132are demagnetized, the clamping force is removed.

In some embodiments, the flux-conducting structure140includes a plate made of a flux-conducting material (e.g., steel). In other embodiments, the flux-conducting structure140includes a second clamp, which includes a corresponding plurality of magnetic devices. Permanent magnets in the first and second clamps are aligned to form the air gap. In still other embodiments, the flux-conducting structure140may be a flux-conducting part in the stack (e.g., the flux-conducting part that is furthest from the clamp120).

Additional reference is made toFIG. 2, which illustrates a method of using the clamp assembly110to perform magnetic clamping of a stack. At block210, the permanent magnets132are positioned over a first surface of the stack. Typically, the permanent magnets132will be demagnetized before they are placed over the first surface.

At block220, the flux-conducting structure140is placed against a second surface of the stack. At block230, the permanent magnets132are magnetized in situ. Flux flows through the flux-conducting structure140.

As illustrated inFIG. 3, when the magnets132are magnetized, magnetic flux (F) flows from one magnet132a, through the flux-conducting structure140, and to another magnet132b. (A keeper136may be magnetically coupled to the magnets132aand132bto complete the magnetic circuit.) The flux-conducting structure140is moved towards a least reluctance position, which is towards the clamp120. As a result, a clamping force is applied to a stack S, which is located in the air gap AG.

The clamp assembly110further includes a controller150for pulsing the coils134to selectively magnetize and demagnetize the permanent magnets132. The controller150supplies coil current in one direction to magnetize the magnets132, and it supplies coil current in an opposite direction to demagnetize the magnets132. When supplied with current, a coil134establishes an external field of sufficient intensity to either promote or demote magnetic domain alignment.

Pulse width is short relative to the duration of a clamping cycle. For instance, the pulse width may be on the order of milliseconds, whereas the clamping force during a cycle may be applied for ten seconds (or longer).

Amplitude and duration of the coil current are selected to create a magnetic field that changes the magnetization of the magnets132. The amplitude of the coil current might be higher than that of an electromagnet that applies the same clamping force. However, overall power consumption is lower, since the current is applied to coils134for milliseconds, whereas current would flow through a conventional electromagnet for tens of seconds (or longer). Moreover, a cooling system is not needed to cool the coils134.

A single pulse of coil current may be sent to a coil, or multiple pulses may be sent.FIG. 4Ashows a single pulse.FIG. 4Bshows train of pulses of different magnitudes and directions.

The clamping force may be maximized by maximizing the flux field. The flux field is a function of coil current amplitude and pulse width, the number of magnets, and the number of winding turns per coils. The clamping force is also a function of size of the air gap.

In some embodiments, the magnets132may include iron alloy. For example, the permanent magnets may be iron alloy magnets such as AlNiCo magnets. However, other embodiments may use permanent magnets having higher or lower magnetic flux density. For instance, other embodiments may use rare earth magnets.

The external field may saturate the magnetic material of a permanent magnet132to completely magnetize the material in either direction (to achieve maximum clamping force). However, to demagnetize or essentially nullify the magnetic field produced by a permanent magnet132, the external field may be smaller in magnitude and it may be applied in the opposite direction to what exists.

A continuum of clamping forces may be achieved by applying an external field below the saturating magnitude. For example, all permanent magnets132in the clamp assembly110are magnetized to create a uniform force distribution, but the magnetization is only partial so as not to exert a full clamping force. Amplitude and pulse width of the coil current may be controlled to achieve a specific clamping force per magnet device.

The clamp assembly110may include a sensor160for sensing the actual clamping force. As a first example, a force sensor (load cell) may measure the amount of actual clamping force generated. As a second example, a hall effect sensor may measure the magnetic flux density in the air gap, and the actual clamping force may be calculated from this measurement.

In some embodiments, the controller150may include a closed loop control for controlling the actual clamping force. For example, the closed loop control may vary the clamping force until the error between actual and desired clamping forces is within a threshold. Consider the pulse train inFIG. 4B. Two initial pulses cause full magnetization of all magnets, resulting in maximum clamping force. Subsequent pulses of reverse polarity and lower amplitudes are used to reduce the clamping force from maximum force to a lower desired force.

In other embodiments, the controller150may use an open loop control for controlling the clamping force. For example, a lookup table may be used to determine the magnitude, duration and direction of coil current to achieve a desired clamping force.

FIGS. 5 to 13illustrate different embodiments of clamp assemblies. In these embodiments, the permanent magnets are arranged symmetrically about a process axis. Such an arrangement enables uniform clamping force to be applied to a stack, while a manufacturing operation (e.g., drilling, riveting) is performed on the stack along the process axis.

Reference is made toFIGS. 5 and 6, which illustrate a clamp assembly510including upper and lower clamps520and530for clamping a stack. The stack includes an upper part (P1) and a lower part (P2).

Each clamp520and530includes four pairs of magnet devices540, with each device540including a permanent magnet542and a coil544. The magnet devices540are arranged radially about a process axis (not illustrated). Air gaps are defined by first ends of opposing magnets of542of the upper and lower clamps520and530. For each pair of magnet devices540, a keeper550magnetically couples second ends of the magnets542. The keepers550may be made of a low carbon steel or other flux-conducting material.

FIG. 6depicts a cross section of a pair of magnet devices540in the upper clamp520and a corresponding pair of magnet devices540in the lower clamp530.FIG. 6also illustrates a flux pathway (straight arrows) during clamping. Magnetic flux density within the air gap AG and corresponding clamping force between the magnet devices540is controlled by the magnitude, direction, and sequence of current pulses (circular arrows) through the coils544.

Reference is made toFIG. 7, which illustrates a clamp assembly710including a clamp720and metal plate730for clamping a single part P3. The clamp720includes two pairs of magnet devices740disposed symmetrically about a process axis (A). Each magnet device740has a square configuration, including a square-shaped permanent magnet and a square-shaped coil. The permanent magnets are connected by a single keeper750, which has arms752extending radially outward from the process axis (A). An opening754in the keeper750allows a manufacturing operation to be performed within the clamped portion of the part (P3).

FIG. 8illustrates a clamp assembly810including a metal plate830and a clamp820that are similar to the embodiment illustrated in FIG.7. In the embodiment ofFIG. 8, however, the magnet devices840of the clamp820have cylindrical configurations (the permanent magnet and coil of each magnetic device840are cylindrical). A coil having a cylindrical configuration may be easier to wind, and it may have lower tension in its windings.

FIGS. 9 and 10illustrate clamps920and1020, each having a single pair of magnet devices940and1040, and a keeper950and1050for providing a flux path between ends of the devices' permanent magnets. The clamp assembly910ofFIG. 9includes the clamp920and a steel bar930. The clamp assembly1010ofFIG. 10includes the clamp1020, and a corresponding lower clamp1030. Permanent magnets of the lower clamp1030are aligned with and form an air gap with the permanent magnets of the upper clamp1020. A higher clamping force may be achieved by using the lower clamp1030instead of the steel bar930. On the other hand, cost and complexity of the clamp910ofFIG. 9is lowered by use of the steel bar930. In addition, positioning accuracy is not as stringent, since the steel bar930doesn't have to be aligned with the permanent magnets of the clamp920.

FIG. 11illustrates a clamp assembly1110including upper and lower clamps1120and1130, where each clamp1120and1130includes a plurality of magnet devices1140arranged in a circular pattern. Each clamp1120and1130further includes a circular keeper1150for magnetically coupling the permanent magnets of the magnet devices1140. In other embodiments, a flux-conducting plate may be used instead of the clamp1130.

FIG. 12illustrates a clamp assembly1210including upper and lower clamps1220and1230, with each clamp1220and1230including a circular arrangement of magnet devices1240. Each clamp1220and1230further includes a plurality of keepers1250. Each keeper1250magnetically couples a pair of the permanent magnets.FIG. 12shows three pairs of magnetic devices1240in each clamp1220and1230.

FIG. 13illustrates a clamp assembly1310including upper and lower clamps1320and1330. The upper clamp1320includes a plurality of magnetic devices1340having permanent magnets1342that are arranged parallel to a process axis (not shown). Coils1344are wound around the permanent magnets1342. The lower clamp assembly1330includes a plurality of magnet devices1345whose permanent magnets1347have bends. One end of each bent permanent magnet1345is aligned with and forms an air gap with a permanent magnet1342of the upper clamp1320. A coil1349is wound around the other end of each bent permanent magnet1347. This arrangement reduces the profile of the lower clamp1330and enables the clamp assembly1310to be used in tighter spaces. The clamp assembly profile may be further reduced by using bent permanent magnets in the upper clamp, or by using a metal plate instead of the upper clamp1320.

Control of a clamp assembly will now be discussed. Pulse magnitude or duration or both may be modulated during magnetization to alter the peak current attained by the pulse. This, in turn, produces external fields of different strengths. Thus, by varying pulse magnitude and/or duration, a continuum of clamping forces may be produced.

Moreover, since each permanent magnet can be selectively magnetized and demagnetized, different regions of the stack can be clamped.

Reference is now made toFIG. 14A, which illustrates a clamp assembly1410including a clamp1420having six magnet devices, andFIG. 14B, which illustrates different process configurations A to E for the clamp assembly1410. The magnet devices are labeled M1to M6in a counterclockwise order. If a large clamping area and force are desired, all six magnet devices M1to M6are energized (process configuration A). If only a smaller clamping area is needed, fewer magnet devices are energized. For example, magnet devices M1and M4are magnetized while magnet devices M2, M3, M5and M6are demagnetized (process configuration B).

In process configurations A, B, C and D, the clamping force is applied on opposite sides of the process axis. The clamping force may be symmetric about the process axis if all magnets have same magnetization, or the clamping force may be non-symmetric if at least one of the magnets has a different magnetization.

In other configurations, clamping force may be applied only on one side of the process axis. For instance, in process configuration E, a clamping force may be generated by magnetizing magnets M1and M2and demagnetizing the other magnets M3to M6. Such a force might be used to clamp an edge of a stack.

Selected magnets may be magnetized simultaneously or sequentially. The magnets may be magnetized sequentially, for instance, if input energy is insufficient to magnetize all of the permanent magnets M1to M6at the same time. Consider process configuration D. To have magnets M1, M3, M4and M6achieve a specified clamping force magnet M1may be magnetized first, followed by magnet M6, then magnet M4, and them magnet M3.

Reference is now made toFIG. 15, which illustrates a system1510for supplying current to the coils of the magnet devices. The system1510includes one or more capacitors1520for storing electrical energy. For instance, at least one capacitor1520may be provided for each coil of the clamp assembly. A power supply1530(e.g., batteries) may be used to charge each capacitor1520. Each capacitor1520may be discharged through a coil by a circuit1540such as an H bridge circuit (the H-bridge circuit can control the direction of the capacitor current) to either magnetize or demagnetize its corresponding permanent magnet. A series of power resistors1550may be used to regulate capacitor charging and discharging times. The charging and discharging may be controlled by the controller150.

A clamp assembly herein is not limited to any particular application. As but one example, a clamp assembly herein may be used in a robot system that is configured to perform one or more manufacturing operations.

Reference is now made toFIG. 16, which illustrates a robot system1610. The robot system1610includes an end effector1620and a robot1630for positioning the end effector1620. The end effector1620includes a clamp1640, which includes a plurality of magnet devices arranged about a process axis. The robot system1610further includes a flux-conducting structure1625, which may be positioned by the robot1630or a separate robot1630.

The end effector1620may be configured to perform one or more manufacturing operations along the process axis. For example, the end effector1620may further include a vision system1650for accurately positioning the process axis over a target location, and a tool assembly1660for performing one or more manufacturing operations along the process axis at the target location. Operation of the robot(s)1630, clamp assembly1640, vision system1650, and tool assembly1660may be controlled by a controller1670. In some embodiments, the controller1670may be carried by the end effector1620or robot1630.