Abstract:
Circuitry that provides alternating current to a load from a unipolar power supply. A current source in the circuitry controls 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 a first direction responsive to the first switch and the second switch being closed. A third switch and a fourth switch connected between the load and the current source 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.

Description:
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&#39;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 the second switch being closed. A third switch and a fourth switch connected between the drive system and the current source and allow current to flow from the current source to the drive system 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 said flow of current between the first direction and the second direction. 
     Another aspect of this invention is that the control circuitry comprises a comparator that receives a feedback signal from pick-off sensors connected to said at least one conduit and determines which of the switches to close responsive to said feedback signal. 
     Another aspect of this invention is that the comparator is a zero crossing comparator. 
     Another aspect of this invention is that amplitude control circuitry in the power source controls the amplitude of said current applied to said drive system. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above and other features of this invention can be seen from the detailed description and the following drawings: 
     FIG. 1 illustrating a Coriolis flowmeter having drive circuitry that includes circuitry in accordance with the present invention; 
     FIG. 2 illustrating a prior art circuit for supplying a controlled square wave to a load; and 
     FIG. 3 illustrating a circuit for supplying a controlled square wave to a load in accordance with this invention. 
    
    
     DETAILED DESCRIPTION 
     This invention relates to providing an alternating controlled square wave from a power source to a load. FIG. 1 illustrates a Coriolis flowmeter having a drive circuit that incorporates circuitry that operates in accordance with the present invention. Coriolis flowmeter  100  includes a flowmeter assembly  110  and meter electronics  150 . Meter electronics  150  are connected to a meter assembly  110  via leads  120  to provide for example, but not limited to, density, mass-flow-rate, volume-flow-rate, and totalized mass-flow rate information over a path  175 . A Coriolis flowmeter structure is described although it should be apparent to those skilled in the art that the present invention could be practiced in conjunction with any apparatus having loads requiring currents of alternating voltage. 
     A Coriolis flowmeter structure is described although it should be apparent to those skilled in the art that the present invention could be practiced in conjunction with any apparatus having a vibrating conduit to measure properties of material flowing through the conduit. A second example of such an apparatus is a vibrating tube densitometer which does not have the additional measurement capability provided by a Coriolis mass flowmeters. 
     Meter assembly  110  includes a pair of flanges  101  and  101 ′, manifold  102  and conduits  103 A and  103 B. Driver  104 , pick-off sensors  105  and  105 ′, and temperature sensor  107  are connected to conduits  103 A and  103 B. Brace bars  105  and  105 ′ serve to define the axis W and W′ about which each conduit oscillates. 
     When Coriolis flowmeter  100  is inserted into a pipeline system (not shown) which carries the process material that is being measured, material enters flowmeter assembly  110  through flange  101 , passes through manifold  102  where the material is directed to enter conduits  103 A and  103 B. The material then flows through conduits  103 A and  103 B and back into manifold  102  from where it exits meter assembly  110  through flange  101 ′. 
     Conduits  103 A and  103 B are selected and appropriately mounted to the manifold  102  so as to have substantially the same mass distribution, moments of inertia and elastic modules about bending axes W—W and W′—W′, respectively. The conduits  103 A- 103 B extend outwardly from the manifold in an essentially parallel fashion. 
     Conduits  103 A- 103 B are driven by driver  104  in opposite directions about their respective bending axes W and W′ and at what is termed the first out of phase bending mode of the flowmeter. Driver  104  may comprise any one of many well known arrangements, such as a magnet mounted to conduit  103 A and an opposing coil mounted to conduit  103 B and through which an alternating current is passed for vibrating both conduits. A suitable drive signal is applied by meter electronics  150  to driver  104  via path  112 . 
     Pick-off sensors  105  and  105 ′ are affixed to at least one of conduits  103 A and  103 B on opposing ends of the conduit to measure oscillation of the conduits. As the conduit  103 A- 103 B vibrates, pick-off sensors  105 - 105 ′ generate a first pick-off signal and a second pick-off signal. The first and second pick-off signals are applied to paths  111  and  111 ′. The driver velocity signal is applied to path  112 . 
     Temperature sensor  107  is affixed to at least one conduit  103 A and/or  103 B. Temperature sensor  107  measures the temperature of the conduit in order to modify equations for the temperature of the system. Path  111 ″ carries temperature signals from temperature sensor  107  to meter electronics  150 . 
     Meter electronics  150  receives the first and second pick-off signals appearing on paths  111  and  111 ′, respectively. Meter electronics  150  processes the first and second velocity signals to compute the mass flow rate, the density, or other property of the material passing through flowmeter assembly  10 . This computed information is applied by meter electronics  150  over path  175  to a utilization means (not shown). It is known to those skilled in the art that Coriolis flowmeter  100  is quite similar in structure to a vibrating tube densitometer. Vibrating tube densitometers also utilize a vibrating tube through which fluid flows or, in the case of a sample-type densitometer, within which fluid is held. Vibrating tube densitometers also employ a drive system for exciting the conduit to vibrate. Vibrating tube densitometers typically utilize only a single feedback signal since a density measurement requires only the measurement of frequency and a phase measurement is not necessary. The descriptions of the present invention herein apply equally to vibrating tube densitometers. 
     In Coriolis flowmeter  100 , the meter electronics  150  are physically divided into 2 components a host system  170  and a signal conditioner  160 . In conventional meter electronics, these components are housed in one unit. 
     Signal conditioner  160  includes drive circuitry  163  and pick-off conditioning circuitry  161 . One skilled in the art will recognize that in actuality drive circuitry  163  and pick-off conditioning circuitry  161  may be separate analog circuits or may be separate functions provided by a digital signal processor or other digital components. Drive circuitry  163  generates a drive signal and applies an alternating drive current to driver  104  via path  112  of path  120 . The circuitry of the present invention may be included in drive circuitry  163  to provide an alternating current to driver  104 . 
     In actuality, path  112  is a first and a second lead. Drive circuitry  163  is communicatively connected to pick-off signal conditioning circuitry  161  via path  162 . Path  162  allows drive circuitry to monitor the incoming pick-off signals to adjust the drive signal. Power to operate drive circuitry  163  and pick-off signal conditioning circuitry  161  is supplied from host system  170  via a first wire  173  and a second wire  174 . First wire  173  and second wire  174  may be a part of a conventional 2-wire, 4-wire cable, or a portion of a multi-pair cable. 
     Pick-off signal conditioning circuitry  161  receives input signals from first pick-off  105 , second pick-off  105 ′, and temperature sensor  107  via paths  111 ,  111 ′ and  111 ″. Pick-off circuitry  161  determines the frequency of the pick-off signals and may also determine properties of a material flowing through conduits  103 A- 103 B. After the frequency of the input signals from pick-off sensors  105 - 105 ′ and properties of the material are determined, parameter signals carrying this information are generated and transmitted to a secondary processing unit  171  in host system  170  via path  176 . In a preferred embodiment, path  176  includes 2 leads. However, one skilled in the art will recognize that path  176  may be carried over first wire  173  and second wire  174  or over any other number of wires. 
     Host system  170  includes a power supply  172  and processing system  171 . Power supply  172  receives electricity from a source and converts the received electricity to the proper power needed by the system. Processing system  171  receives the parameter signals from pick-off signal conditioning circuitry  161  and then may perform processes needed to provide properties of the material flowing through conduits  103 A- 103 B needed by a user. Such properties may include but are not limited to density, mass flow rate, and volumetric flow rate. 
     FIG. 2 illustrates a prior implementation of drive circuitry  163  including a prior art system for applying an alternating current to a load which is driver  104 . A sinusoidal signal is received by multiplier  204  from sensors  105 - 105 ′ (FIG. 1) via path  162 . The multiplier adjusts the drive amplitude. The adjusted signal from multiplier  204  is applied to amplifier  201 . Amplifier  201  boosts the sinusoidal signal to a proper level to cause driver  104  (FIG. 1) to oscillate. A supply voltage is applied to amplifier  201  from current limiter  202  or  203 . Current limiters  202  and  203  assure against excessively low impedance in a load such as driver  104  (FIG.  1 ). 
     The polarity of the applied voltage is periodically reversed with respect to ground which is connected to driver  104 . The reversal of polarity allows driver  104  (FIG. 1) to impart energy to flow tubes  103 A and  103 B during both halves of each cycle of oscillation. The reversal of voltage polarity requires to separate supply rails Vcc and Vee. Supply rails Vcc and Vee have opposite voltage polarities. 
     The use of separate supply rails Vcc and Vee increase complexity of the circuit and increases power consumption. Power consumption is increased because simple amplifiers  201  typically used in drive circuit  162  drive an out close but not equal to a supply rail. This requires additional voltage overhead to provide a certain voltage to driver  104  (FIG.  1 ). 
     A second problem is that output voltage of drive circuit  162  is controlled. However, the conversion of electrical energy to kinetic energy in driver  104  is dependent upon current according to Faraday&#39;s law. Even though applied voltage results in applied current, the relation between force applied and voltage applied is indirect and is dependent upon other factors. For example, the inductance of the coil and motion of conduits  103 A and  103 B effect the applied force applied. Therefore, it is desirable to control current rather than voltage. 
     Another problem with drive circuit  163  shown in FIG. 2 is the ability to maximize power delivered to driver  104  while constrained by intrinsic safety standards. Intrinsic safety standards are set by various regulating agencies to assure that a spark or heat from a circuit does not ignite volatile material in an environment. Intrinsic safety standards place limits on the maximum instantaneous voltage and current that may be delivered to a load such as driver  104  (FIG.  1 ). However, the force applied to conduits  103 A and  103 B is dependent upon the average value of current applied. Thus, maximum efficiency is achieved by minimizing the difference between average current levels and a peak current level. Since driver  104  (FIG. 1) utilizes sinusoidal current and the electro-mechanicaI force generated is also a sinusoidal. The product of sinusoidal current and the electro-mechanical force generated is also a sinusoidal and is the useful power of the system. Since a square current multiplied by a sinusoidal voltage produces more average power than the product of two sinusoids, a square wave current will allow lower peak values of current for the same average power. 
     FIG. 3 illustrates a drive circuit  163  that provides a constant square wave alternating current using a single power supply. In drive circuit  163  there is a single current source  333 . The polarity of voltage applied to a load, such as driver  104  (FIG.  1 ), is determined by two sets of switches in H-bridge circuit  350 . When a first set of switches including switch  301  and  302  are closed current flows in a first direction to driver  104  (FIG.  1 ). When the first set of switches is open and a second set of switches switch  303  and  304 , is closed, voltage is applied to driver  104  in a second opposite direction. 
     When switches  301  and  302  are closed and switches  303  and  304  are open, current flows through driver  104  in the following manner. Supply rail Vcc applies current over path  314  to closed switch  301  and open switch  303 . Current flows through switch  301  to path  315  and to driver  104  via path  315 . Current then is flows to the driver and returns via path  316 . The current flows through closed switch  302  and over path  317  to current source  333 . Current source  333  is connected to ground. 
     When switches  303  and  304  are closed and switches  302  and  301  are open, current flows to driver  104  in the following manner. Supply rail Vcc applies current over path  314  to switch  303 . Current flows through switch  303  and is applied via path  316  to driver  104 . Current returns via path  315  and flows through closed switch  304  to path  317 . This is a direction that is opposite of the path provided by switches  301  and  302 . Control circuitry  320  opens and closes switches  301 - 304  to change the polarity of voltage applied to driver  104 . A feedback signal is received by control circuitry  320  via path  162 . From the feedback signal, the control circuitry changes the direction of flow. In a preferred embodiment, control circuitry  320  includes a zero comparator. Zero comparator includes a delay  321  and an invertor  322  that receive signals and alternately apply opposite signals to switches  301 - 304  to open and close the switches. Delay  321  applies signals to switches  301  and  302  via paths  312  and  313 . Invertor  322  applies signals to switches  303  and  304  via paths  310  and  311 . 
     Switches  301 - 304  are set for a constant impedance since changing the impedance of switches dynamically is difficult. Amplitude is controlled in well known and conventional manners in current source  333  which receives an amplitude signal from path  163  via path  331 . This works because H-bridge  350  is essentially part of the load connected to the current source. Since switches  301 - 304  are either completely opened or completely closed, the output appears as a square waveform. 
     The above is a description of a preferred of circuitry for supplying a controlled square wave to a load. It is expected that those skilled in the art can and will design alternative circuits that infringe this invention as set forth in the claims below literally or through the Doctrine of Equivalents.