The present invention relates generally to mass flow rate and density measuring apparatus, and more particularly to an improved flow rate sensor having improved sensitivity.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information pertaining to materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and U.S. Pat. No. Re. 31,450 to J. E. Smith of Feb. 11, 1982. Flowmeters have one or more conduits of a straight, curved or irregular configuration. Each conduit has a set of natural vibration modes which may be of a simple bending, torsional, or twisting type. Each material filled conduit is driven to oscillate at resonance in one of these natural modes. The natural vibration modes are defined in part by the combined mass of the flow conduits and the material within the flow conduits. If desired, a flowmeter need not be driven at a natural mode.
Material flows into the flowmeter from a connected material source on the inlet side. The material passes through the conduit or conduits and exits the outlet side of the flowmeter.
A drive mechanism applies force to oscillate the conduit. When there is no material flow, all points along a conduit oscillate with an identical phase in the first bending mode of the conduit. With material flow, Coriolis accelerations cause each point on the conduit to have a different phase with respect to other points on the conduit: the phase on the inlet side of the conduit lags the driver; the phase on the outlet side leads the driver. Pickoffs are placed on the conduit to produce sinusoidal signals representative of the motion of the conduit. The phase difference between two sensor signals is divided by the frequency of oscillation to obtain a delay which is proportional to the mass flow rate of the material flow.
The drive mechanism of the Coriolis flowmeter is affixed to the conduit(s) and oscillates the conduit(s) in response to a signal from driver control circuitry. A conventional drive mechanism for a Coriolis flow meter has a magnetic circuit comprising a keeper, magnet and pole piece mounted in opposition to a coil. The driver control circuitry applies an electric current or drive signal to the coil of the drive mechanism. The current flowing through the coil generates electromagnetic forces between the drive coil and the magnet thereby causing the conduits to vibrate.
The design and implementation of a drive mechanism is important because the greater the amount of power a drive mechanism can produce, the better the performance of the flow meter in high damping applications.
Past drive mechanism designs have focused on reducing cost and mass while doing very little to increase the power output. This design focus, coupled with the industry""s desire to lower the cost and size of Coriolis flow meters, magnifies the difficulty in drive system design.
A typical drive design is developed based on the following two equations:
Pdisapated=xe2x88x922*xcfx89*"xgr"*K*A2xe2x80x83xe2x80x83(1) 
where:
xcfx89=angular velocity of the system
"xgr"=critical damping ratio of system
K=system stiffness
A=system amplitude
P=system power
and
Pdelivered=2*xcfx89*I*B*L*Axe2x80x83xe2x80x83(2) 
where:
xcfx89=angular velocity of the system
I=available current
B=total flux
L=length of wire on coil
A=system amplitude
P=system power
Equation (1) represents the power dissipated by the Coriolis flow meter and Equation (2) represents the power delivered to the flow meter by the drive mechanism. In some cases, depending on the application and location that the flow meter will be placed in, the amount of power delivered to the flow meter is limited by area approval agencies (i.e. UL, CENELEC, TIIS).
In normal operation, frequency and conduit amplitude are pre-defined resulting in equations (1) and (2) being equal. However, many factors can cause a flow meter to deviate from normal operation. Such factors include entrained air, high viscosity fluids, and material flow comprising large amounts of solids. The deviation from normal operation results in damping of the vibrational characteristics of the system, thus requiring an increase in the power supplied to the flow meter to return the meter to normal operation. In order to ensure continued operation of the sensor during occasions when a flow meter deviates from normal operation, designers design in xe2x80x9coverheadxe2x80x9d or xe2x80x9creserve powerxe2x80x9d. xe2x80x9cOverheadxe2x80x9d is defined as the maximum power available to the sensor divided by the power needed to drive the system during normal operation.
In order to generate the overhead needed by a sensor, a drive mechanism designer must strive to increase the power available to the sensor. However, of the variables comprising equations (1) and (2), variables K, xcfx89 and "xgr" are determined by the geometry of the sensor and I is limited by the area approval agencies, leaving only B, L and A available to the designer.
From equations (1) and (2), it is clear that increasing conduit amplitude, A, would result in power being dissipated faster than power being supplied. Increasing the length of wire on a coil would increase power, however, an increase in the length of wire would increase the resistance and thereby reduce the delivered power. Additionally, there are additional safety restraints imposed by the approval agencies on the relationship between a coil""s inductance and resistance. However, the flux, B, can be increased without impacting the power dissipated nor affecting those variables constrained by an approval agency.
The total flux, B, represents how closely packed (i.e. the xe2x80x9cdensityxe2x80x9d) the flux lines are that compose the magnetic field. In order to efficiently utilize the magnetic field, a xe2x80x9ckeeperxe2x80x9d is placed around the magnet. The keeper is a piece of ferromagnetic material, such as carbon steel, that acts as a conductor for the lines of flux. The flux lines are concentrated in the steel keeper, as a ferromagnetic material will support a greater concentration than will air. In addition to serving as a conductor for the flux lines, the keeper also channels the lines of flux so as to create the maximum flux density in an air gap where work will be done. In the case of a magnet/coil driver, the coil is positioned in the air gap and orientated to maximize the cross product between the flux and current vectors.
One prior art design uses a strip of metal bent into an open channel (FIG. 1). The channel design is relatively inexpensive to build but surrounds only a small portion of the magnet, failing to xe2x80x9ccapturexe2x80x9d a large number of flux lines. Another prior art design utilizes a cup-shaped keeper (FIGS. 2A and 2B). This cup-shaped keeper design maximizes the flux lines that are captured, due to its 360 degree conductive area, however, the design is costly to produce and extremely weight prohibitive.
The object of the invention is a linear actuator that maximizes the flux density in the air gap where work is to be done by increasing the lines of flux that are captured while keeping the cost of production and mass relatively low. The object is achieved by an improved linear actuator, which is characterized by a keeper comprised of a cross-shaped piece of ferromagnetic material bent such that the four ends of the cross are located perpendicular to the longitudinal axis of the magnet.
The keeper increases the total flux available to the drive mechanism without negatively impacting other variables in the system. In addition, the keeper is light weight and easy to manufacture.
One possible preferred exemplary embodiment of the linear actuator according to the invention is characterized by the keeper being composed of a ferromagnetic material, preferably steel. The keeper is manufactured by forming a piece of material into a cross-shape with end portions of the legs contoured to closely match the exterior contour of the magnet. The legs are bent to form two 90 degree angles resulting in the end portions being perpendicular to the longitudinal axis of the magnet.
Another possible preferred embodiment of the linear actuator according to the invention is characterized by a mounting bracket composed of a ferromagnetic material. In some cases the flux density generated by the magnet exceeds the keeper""s capacity to carry, resulting in flux saturation of the keeper. When the keeper becomes saturated, any additional flux generated by the magnet goes through the air. In order to capture the additional flux, a ferromagnetic mounting bracket is located at a pole of the magnet.
The invention allows a majority of the lines of flux to be captured by the linear actuator and channeled through the coil at a preferential angle. The invention also significantly reduces the gap between the magnet while nearly surrounding the magnet.