Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-03-31
2001-05-01
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S017000, C363S132000
Reexamination Certificate
active
06226195
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to providing a current to a load. More particularly, this invention relates to a system that changes the polarity of voltage applied to a load from a single power source. Still more particularly, this invention relates to circuitry that provides power to a drive system of a Coriolis flowmeter.
PROBLEM
Some loads require that the polarity of the voltage of current applied to the load be periodically reversed. The reversal of polarity of voltage changes the direction of current flowing through the load. This change in direction of current flow may achieve a certain function performed by a load. One example of a load requiring a change in the polarity of applied voltage is a drive system for a Coriolis flowmeter.
A Coriolis mass flowmeter measures mass flow and other information of materials flowing through a conduit in the flowmeter. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524 of Aug. 29, 1978, 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional or coupled type. Each conduit is driven to oscillate at resonance in one of these natural modes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit or conduits, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, all points along the conduit oscillate due to an applied driver force with identical phase or small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the conduit to have a different phase. The phase on the inlet side of the conduit lags the driver, while the phase on the outlet side of the conduit leads the driver. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between two pick-off sensor signals is proportional to the mass flow rate of material through the conduit(s).
The drive system of a Coriolis flowmeter is affixed to the conduit(s) and oscillates the conduit(s) in response to a signal from driver control circuitry. A conventional driver for a Coriolis flow meter has a magnet mounted in opposition to a coil. The driver control circuitry applies an electric current or drive signal to the coil of the driver. The current flowing through the coil generates electromagnetic forces between the coil and the magnet. The coil is alternately attracted and repelled by the magnet. The attraction and repulsion causes the flow tubes to vibrate.
In order to alternately attract and repel the magnet, the polarity of the voltage of current flowing through the driver is reversed. This allows the driver to apply force to the conduit(s) through both halves of a cycle of oscillation.
It is a problem that two separate supply rails to the driver control circuitry are required to reverse the polarity of voltage with respect to ground. This increases the complexity and the cost of manufacture of the drive control circuitry.
A second problem particular to the drive system of a Coriolis flowmeter is that the output voltage of the power supply is controlled. However, the conversion of electrical energy to kinetic energy or force applied to the conduit(s) is dependent upon current as shown by Faraday's law. The relationship between applied voltage and force imparted on the conduit is indirect. Therefore, the current may not be in phase with the motion of the conduits when voltage is controlled. This reduces the efficiency of power conversion to force for vibrating the conduit(s).
A third problem that is also particular to a drive system of a Coriolis flowmeter is maintaining intrinsic safety of the drive circuit while maximizing power transfer. Intrinsic safety requirements place a limit on the maximum instantaneous voltage and current applied to a load, such as the driver system. However, mechanical motion of the conduit(s) is dependent upon average voltage and current applied to the driver system. Therefore, the drive signal must minimize the difference between peak values and averages values to maximize the efficiency of the drive system.
SOLUTION
The above and other problems are solved and an advance in the art is made by circuitry for supplying a controlled square wave to a drive system of this invention. The circuitry of this invention allows a single power supply to supply voltage of alternating polarity to a load. This reduces the cost and complexity of the circuitry. This circuitry also allows the amount of current applied to a load to be controlled instead of the amount of voltage. The circuitry of this invention also provides current in the form of a square wave which maximizes the average voltage and current applied to the load by minimizing the difference between peak and average volumes for the voltage and current.
The circuitry of this invention includes an H-bridge. H-bridges are used commonly in fixed amplitude applications to reverse polarity of voltage through a load. An H-bridge has two sets of switches connected to terminals connecting the load to the circuit. The sets of switches are alternatively opened and closed to reverse the flow of current to the load. When a first and second switch of the first set of switches are closed, current flows in a first direction over the h-bridge and through the load. When a second and a third switch of the second set of switches is closed, current flows over the h-bridge and through the load in a second direction that is opposite of the first direction.
In order to adjust the amplitude of current applied to the load, the h-bridge is connected to a power source that can adjust the amplitude of current applied to the h-bridge and delivered to the load.
An aspect of this invention is circuitry that provides alternating current to a load from a unipolar power supply in the following manner. A current source controls the amplitude of current applied to the load. A first switch and a second switch are connected between the load and the current source and allow current to flow from the current source to the load in first direction responsive to the first switch and the second switch being closed. A third switch and a fourth switch are also connected between the load and the current source and allow current to flow from the current source to the load in a second direction responsive to the third switch and the fourth switch being closed.
Another aspect of this invention is control circuitry that opens and closes the first switch, the second switch, the third switch, and the fourth switch to change direction of the flow of current between the first and the second direction.
Another aspect of this invention is that the control circuitry comprises a comparator that receives a feedback signal from the load and determines which switches to close.
Another aspect of this invention is that the comparator is a zero crossing comparator.
Another aspect of this invention is that amplitude control circuitry adjusts the amplitude of the current applied to the load.
Another aspect of this invention is circuitry for providing a drive signal to a drive system that vibrates at least one conduit in a Coriolis flowmeter having the following components. A current source that controls current applied to the load. A first switch and a second switch connected between the drive system and the current source and that allow current to flow from the current source to the drive system in a first direction responsive to the first switch and
Duft, Graziano & Forest, P.C.
Micro Motion Incorporated
Riley Shawn
LandOfFree
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