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Rotary solenoids bistable - holding force without current with permanent magnets
Bistable rotary solenoids can rotate in both directions and hold in the two end positions by means of a built-in permanent magnet without current being applied. Only a short current pulse is required to stimulate the movement. Rotating solenoids with currentless holding positions are particularly well suited for shutter applications in laser devices and optical systems, as well as switch drives for paper transport and sorting systems, e.g. ATMs and other banknote sorting mechanisms. The bistable rotary solenoids have a torque of up to 0.4Nm and a rotation angle of up to +/-55°. The integrated ball bearings ensure a long service life while eliminating axial movements.
The bistable rotary solenoid changes state with the application of a momentary pulse of electricity, and then remains in the changed state without power applied until a further pulse of reverse polarity is applied to drive it in the opposite direction. Because energy is only applied in short pulses, high power can be applied to develop high torque for fast operation without leading to heating problems. Response time of <<10ms is possible for some of these devices. Referring to the image and graph, the device is drawn in the mid-position (90° on graph), torque in the de-energized condition is represented by the black curve and arrows. Without stops, the device will try to turn towards stable equilibrium points (where two arrow heads meet) located at 0° and at 180°, and away from unstable equilibrium points (represented as a black dot) located at 90° and at 270°. In the forward energized state, the device tries to turn towards a single stable equilibrium point at 180°, in the reverse energized state it tries to turn towards a single stable equilibrium point at 0°. Bistable rotary solenoids do not normally incorporate end stops within the device, a stop should be incorporated externally in the customer application. The stop positions are represented as vertical orange lines in the graph. The mechanical end stops restrict rotation so the device cannot turn all the way to the equilibrium points (which are zero torque points), they should restrict motion to a region where developed torque is sufficient to turn the load at required speed, or to hold the load. For more efficient operation, shape of the torque curves may be modified to optimize behavior for a particular rotation angle. Subjectively, the torque behavior may more easily be understood by considering the analogy of a surface down which a ball bearing is rolled. The surfaces representing the different excitation states of the solenoid are illustrated below, in the case of a 2-pole device this would represent 180° of movement. The de-energized state is represented by the black surface, the ball-bearing will try to roll towards either end-position. As it is moved further from the end position, the force trying to restore it will initially increase, but will then reduce as it approaches the mid-position. This is an unstable equilibrium point where no force is developed, however if displaced to either side it will roll away from this point towards the end position. The Forward energized condition is represented by the red surface, the ball-bearing will try to roll to the right. The end positions are zero-force points, the force moving it rightwards will be a maximum somewhere close to the mid-position. The Reverse energized condition is represented by the blue surface, this is a mirror image of the red surface, the ball-bearing will try to roll to the left.
Installation and Use
The illustration shows a BRS5045 solenoid in its mid-position. The solenoid has a stop fitted (the green part mounted on the shaft, and red part mounted to the body of the solenoid) which limits the range of movement to 30°, 15° to either side of the mid-position (shown in this position).Without any power applied, this is an unstable position, if the shaft is turned in either direction from this mid-position, the residual torque will drive the solenoid further away from the mid position until it comes to rest against the end-stop. This is represented by the black arrows. A pulse of electrical power applied in the forward direction, will cause the solenoid to develop torque acting in the clockwise direction, and to turn in this sense until it comes to rest against the stop. This excitation condition is represented by the red arrow. If power is then removed the detent torque will cause the solenoid to remain in this position. A pulse of electrical power applied in the reverse direction, will cause the solenoid to develop torque acting in the counterclockwise direction, and to turn in this sense until it comes to rest against the stop. This excitation condition is represented by the blue arrow. If power is then removed the detent torque will cause the solenoid to remain in this position.
- For bistable operation it is important that the solenoid is mounted so that the mid-position (parts are normally drawn in this position) is located mid-way between the end stops.
- End stops are normally required to be fitted by the customer. These devices are not normally supplied with internal stops, although these may be offered as an option for some models
- Without end-stops to limit rotation of the solenoid, it will naturally try to turn into a magnetic detent position, these positions are zero-torque positions, the solenoid will develop little or no torque if energized in these positions.
- If both end stops are positioned to the same side of the mid-position, a ‘fail-safe’ design can be realized. As shown in the graph, in the case of power failure, the detent torque will drive the device clockwise, it can be energized with forward excitation to drive more quickly to this position. The device must be energized in the reverse direction to drive to the CCW position and must be kept energized to hold in this position.
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