Electricity: motive power systems – Nonmagnetic motor
Reexamination Certificate
2001-03-22
2004-06-22
Tamai, Karl (Department: 2834)
Electricity: motive power systems
Nonmagnetic motor
C318S119000, C310S309000
Reexamination Certificate
active
06753664
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of servo-mechanical actuators and, in particular, to the high precision and high speed positioning of objects using actuators.
BACKGROUND OF THE INVENTION
Electromechanical and servo-mechanical systems make extensive use of a very wide range of actuators. Magnetic actuators in particular find wide industrial use as these devices lend themselves to control via the use of appropriately controlled electric current through suitably designed electromagnets. Applications range widely. While much effort has gone into balancing systems for valves, levitation devices, and “frictionless” bearings, it is the high speed and high accuracy arena, such as the fields of data storage on various magnetic and optical media, that has provided an impetus for the development of the technology. In these applications magnetically actuated reading and writing heads have to be positioned at very high speeds to high accuracies over data tracks and the vertical positioning of the heads has to be controlled to maintain either a magnetic writing distance or an optical focal distance.
With the more recent advent of microlithographic technology and, more particularly, the development of microelectromechanical (MEMS) devices, it has also become possible to viably employ electrostatically driven actuation devices. The fact that sufficiently high actuating electrostatic field strengths may be attained using practical voltage levels at the characteristically small inter-electrode distances, lies at the root of this development. More recently magnetically actuated microelectromechanical devices have also been described.
One specialized application field for actuators that has seen much growth in recent times is the digital-on-press developments within the field of lithographic printing. In this field, image data is written directly to a blank lithographic plate using a laser head equipped with modulated laser arrays.
One of the particular challenges in this arena is the need for very precise control of the distance between the focusing lens and the printing plate surface. To establish this control, the focusing lens is affixed to the moving member of an actuator. This moving member is referred to herein as a plunger. Data is written at very high speeds in these systems and the focusing lens has to maintain a precise “flying height” in this process. Because of the large lateral distances traversed in this application compared with typical data storage devices, the actuators have to maintain this accurate separation while the plunger of the actuator traverses over a considerable stroke length to allow the focusing lens to “follow” the variation of the printing plate or medium.
In the most simple incarnation of a linear actuator, a force, either magnetic or electrostatic, is applied to the plunger. The plunger is made subject to a restoring force. In this most simple case, this force is provided by a spring with a simple spring constant. This results in the restoring force being essentially linear in the sense that the force provided by the spring is proportional to the linear displacement of the plunger. It is possible to eliminate the restoring force if a bi-directional actuator, such as a voice-coil, is used.
The actuating force, on the other hand, being magnetic or electrostatic in nature, is inherently non-linear. In particular, it is known to those skilled in the art of magnetic actuators in particular, that, in principle, the net force on the plunger in a magnetic actuator is related to the square of the magnetizing current and inversely related to the square of the magnetic gap (also called the air gap) between the magnetic member of the plunger and the fixed driving electromagnet. In practice this relationship is even more complex because, amongst other reasons, saturable magnetic elements are employed to manipulate the behavior of the actuator. The relationship between gap and force also depends upon the geometry of the actuator.
Because of these highly non-linear relationships, a significant problem arises in linearizing the net output force of the moving member of a magnetic actuator in response to an applied force command; i.e., to obtain a plunger force proportional to the input signal in order to establish adequate control over the actuator.
In providing linear force control on the plunger of magnetic actuators, previous control techniques have included flux feedback, force feedback arid current/gap feedback methods. In the case of electrostatic actuators there has been comparatively little described in respect of means to control such actuators beyond simple two-state devices. This dearth of practical analog electrostatic actuator devices is related partly to the nature of the applications that employ them, but also in particular to the difficulty in controlling them in view of the non-linear actuation forces.
In the flux feedback approach used in magnetic devices in particular, the magnetic flux experienced by the plunger is monitored continuously by a sensor, typically a Hall-effect device, and this information is fed back to the control system. Via a wide variety of electronic and computing means, an appropriate compensating current is then applied to the electromagnet driving the device.
One of the drawbacks of this approach for actuators working at high speeds over small stroke lengths is the fact that it is extremely awkward to have a sensor occupying any significant fraction of the magnetic gap (air gap). Recessing the sensor, either into the plunger or into the pole piece of the electromagnet, can cause the sensor to measure a field-strength not entirely representative of the field experienced by the plunger. The relationship between measured field and force on the plunger is therefore perturbed.
The force feedback approach, which is in concept a variation on the flux-feedback technique, incorporates a force sensor in a closed-loop configuration to linearize the net forces on the plunger. The plunger is physically tied to the payload through the force sensor. Any force exerted on the plunger is transmitted through the force sensor to the payload. Force sensors vary, but are typically quartz oscillator crystals, which vary an oscillator frequency in response to a tensile or compressive force. This frequency shift is then used to control the force on the plunger, by adjusting the magnetic flux density produced by the control electromagnet.
Among the drawbacks to this approach is the fact that force sensors capable of a high bandwidth and resolution required for low-level force control are costly, fragile, and require sophisticated support electronics. Further, because of their fragility, these sensors often require elaborate holding fixtures to protect them from damage.
As another approach to force linearization, current/gap feedback has been used. This technique is more common and utilizes the relationship between magnetizing current and air gap for a linear medium in an open force loop configuration. In this method, the force is controlled by employing the fact that the plunger force is nominally proportional to the square of the magnetizing current and inversely proportional to the square of the magnetic gap.
In this approach, any sensor capable of providing a signal proportional to gap position can be used. Previous applications have incorporated eddy current, capacitive, and inductive sensors. By employing both current and gap position sensors, the requirement for and disadvantages of a force sensor are eliminated. To remove the current non-linearity, various bias current techniques have been utilized. However, because of the open force loop configuration and square law relationships, both the position and current sensing signals, as well as squaring compensation circuitry, must be very accurate and linear over all operational conditions. Because of the nominally squared relationship, percentage force errors can be more than twice the percentage position and gap errors that cause them.
It is true in all the aforementioned
Neufeld Richard David
Trautman Christopher Earl
Creo Products Inc.
Oyen Wiggs Green & Mutala
Tamai Karl
LandOfFree
Method for linearization of an actuator via force gradient... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for linearization of an actuator via force gradient..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for linearization of an actuator via force gradient... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3329860