Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Reexamination Certificate
1999-01-27
2001-06-19
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
Reexamination Certificate
active
06249418
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the control of a large class of moving element systems described as electromagnetic actuators such as solenoids, contactors, or even trains.
2. Description of the Prior Art
Electromagnetic actuators including the solenoid are also called variable reluctance actuators. In these devices we find a movable element made of either a ferromagnetic material, a magnet, or both, that has a force exerted on it by a magnetic field which has been generated by an electrical current flowing in a coil of wire. There may also be a permanent magnet in the non-moving component, and the coil then either adds to or reduces the force produced by the magnet. The coil is also typically wound on a ferromagnetic material to increase the efficiency and force.
Common examples of such items are tubular solenoids, the individual wires in many printheads, cradle relays, some types of Maglev trains, and novelty items such as globes that magnetically float without direct mechanical contact above or below their support. Loudspeakers are included in the list of systems that can be controlled with this approach. Although the motion of a loudspeaker coil is not normally controlled with a closed loop system, it is possible to do.
Although many such systems are presently controlled by feedback systems, smooth control of the motion and position of the moving element in these systems has always been a somewhat complicated combination of hardware, both mechanical and electrical. The complication arises due to the nature of the problem that these systems are trying to control. The forces controlling the closure of all of these systems are usually very non-linear. As the air gap between the magnetic materials contained in these systems decreases, the force exerted by a constant current in the coil increases. In most on/off systems of this type the result is runaway motion, with the moving element accelerating until it runs into a stop. The resultant impact can create significant noise, vibration and wear.
Many previous approaches have been tried with varying degrees of success and complexity. One example is shown by Jayawant U.S. Pat. No. 5,467,244 wherein a system is built that not only allows control of the runaway; it allows the system to control the position of the object. Other systems, such as Stupak U.S. Pat. No. 4,659,969, have also succeeded at some measure of control, with Stupak adding a Hall sensor to the system in order to monitor the magnetic flux contained in the system. An even earlier system, Gingrich U.S. Pat. No. 4,368,501 shows us how to derive the flux signal from a second winding on the device. Other systems have attempted feedback control with a variety of position sensors looking at the moving element. Seale U.S. Pat. No. 5,635,784 describes another approach to such control.
The sensing technique used is of great importance in many systems. Often the device is located in an environment that is hostile to certain types of sensors. Hall effect sensors, for example, are semiconductor integrated circuits that have a limited temperature range. The addition of extra coils to perform the sensing can have an impact on the physical size, not to mention the complexity of the additional wires. Jayawant describes a system that uses just the two wires of the existing coil, but surrounds that coil with a complicated electronic circuit to extract the signals needed for control.
What has not been realized in these previous systems is that the system they are trying to control has been overdetermined. More information is available than is needed to adequately control the motion of the moving element. Systems have attempted to drive a coil with a known signal, while at the same time measuring two parameters in the driven system. Typically the coil is driven with a voltage, and the current and the flux in the coil are measured. As noted above there are a variety of techniques used to measure both the current and the flux.
Jayawant describes measuring the inductance rather than flux in order to have two independent measurements of the system, which he felt, was necessary to compute the position of the moving element.
SUMMARY OF THE INVENTION
The present invention provides for the control of the position of, or force on an electromagnetic actuator using the minimum possible amount of information from the system. In a typical example of
FIG. 1
, this is a system driving a coil with a voltage (a duty cycle or pulse width modulated (PWM) driven system) and measuring just the current that flows through the coil. But that is not the only possible configuration. There are a number of physical parameters that can be either known or measured: voltage across the coil, current through the coil, magnetic flux in the coil, and inductance of the coil. A number of different combinations of these can be combined to calculate the position of the moving element or the magnetic force that is applied to it. Traditionally the current through or voltage across the coil, along with the flux, has been used to calculate the position. What will be shown here is that by knowing the drive to the coil (either voltage or current) and then measuring only one of the other parameters (voltage, current, or flux), it is possible to calculate the position. This minimizes the hardware required, and in some systems the actuator can be reduced to just the coil itself. Alternatively, magnetic flux, and therefore force instead of position, can be controlled.
First the background equations concerning the system to be controlled need to be developed. Referring to
FIG. 7
, we see a system composed of:
a coil L
1
a moving element M
1
a Hall effect sensor H
1
a sense coil Ls
a current sensor S
1
(Not all of these elements will be present in all systems.)
A detailed description of the above follows:
L
1
—this coil is composed of a number of optional elements. It is first broken electrically down into two components: an ideal inductance L in series with the dc resistance of coil RL. The coil can then be either an air core, wound on a permanent magnet, or more commonly wound on a ferromagnetic material (e.g. soft iron) to enhance its effect. Some systems might even include both the ferromagnetic material and permanent magnet. The coil is composed of n turns of wire.
M
1
—the moving element can also be composed of a number of different kinds of material. Both a ferromagnetic material and/or a permanent magnet can be used as the active moving component. Other materials can be present, and will move along with the active material, but they will not contribute to the motion other than to add mass to the moving element. The relative motion between M
1
and L
1
is considered to be x, the motion of M
1
. We choose the coil L
1
to be the stationary element just for a frame of reference for this discussion. In practice either component or both may move with respect to the observer.
H
1
—an optional Hall effect sensor mounted in a location that allows it to measure the magnetic flux in the gap and output a voltage proportional to the flux.
Ls—an optional sense coil wound with m turns of wire so that it is coupled to coil L
1
. This coil needs to be wound closely enough to L
1
so that the flux passing through it is either substantially the same as the flux passing through L
1
or proportional to the flux.
S
1
—an optional current sensor used to detect the current flowing through the coil L
1
. This would commonly be a small valued resistor. A small value is usually used to reduce the voltage drop, and therefore power dissipated, in the resistor. In a typical system an amplifier is often needed to raise the small voltage generated by this small valued resistor to a level convenient for the rest of the system.
Equations relating these items to each other can now be written:
The voltage V
L
across the ideal inductor L in a typical system (as in
FIG. 7
) is:
V
L
=V
applied
−I*RL−V
sense
Equation 1
This simply states that the voltage across the ideal inductance i
Atwood Pierce
Caseiro Chris A.
Leja Ronald W.
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