It is generally known to use Coriolis effect mass flow meters to measure mass flow and other information for materials flowing through a conduit in the flow meter. Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524, U.S. Pat. No. 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at resonance in one of these natural modes. Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter, is directed through the conduit or conduits, and exits the flow meter through the outlet side of the flow meter. 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 flow meter, all points along the conduit oscillate due to an applied driver force. The points can oscillate with identical phase or a small initial fixed phase offset, which can be corrected. As material begins to flow through the flow meter, Coriolis forces cause each point along the conduit to have a different phase. For example, the phase at the inlet end of the flow meter lags the driver, while the phase at the outlet 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 the two or more pick-off sensors is proportional to the mass flow rate of material through the conduit(s).
Although there are many pick-off arrangements, one particularly popular driver and pick-off arrangement comprises a magnet-coil assembly. Typically in a dual flow tube arrangement, a magnet is affixed to one flow tube and a coil is affixed to the other flow tube and positioned proximate the magnet. In this arrangement, the driving coil is supplied with an alternating current which induces the flow tubes to vibrate. The pick-off sensor magnet-coil assembly then produces an induced voltage which is proportional to the motion of the flow tubes. Typically, there is one pick-off sensor at an inlet end of the flow tubes and another pick-off sensor positioned at the outlet end. Therefore, each flow tube includes at least a driver component and two pick-off components. The operation of the magnet-coil assembly is generally known in the art.
One problem with the above arrangement is connecting the wires to the coils on the moving flow tubes. In the past, this has been dealt with in a number of ways. The first way is to attach the wires to the flow tubes using some sort of tape or adhesive. Another approach, particularly in smaller tube diameter flow meters, is to use thin flexible conductors (flexures). Both of these approaches have drawbacks. The use of tape or adhesives provides an unsatisfactory solution because the tape or adhesive is generally a high damping material, with the damping changing unpredictably with both time and temperature. These changes can result in erroneous flow signals and errors in the meter's performance. While the flexures have little damping, they typically have very distinct natural frequencies and exciting them at a natural frequency can result in rapid failure. In addition, they are extremely fragile due to their size.
A number of patents have disclosed proposed solutions to the problems outlined above. For example, U.S. Pat. No. 4,756,198 discloses welding a coil mount to the flow meter housing. The coils are then mounted to the coil mount with only the magnets being attached to the flow tubes. A problem with the '198 patent's solution is that the wires hang freely from the coils. Thus, while this proposed solution provides an improvement over attaching the coils to the flow tube, the same problems are encountered with loose wires.
Another proposed solution is disclosed in U.S. Pat. No. 5,349,872. The '872 patent discloses removing the coils from the flow tubes and mounting them on two printed circuit boards (PCB), one positioned above the flow tubes and another positioned below the flow tubes. While this solution solves the loose wire situation of the '198 patent, it incorporates an excess of components and creates the possibility of errors caused by inconsistent gaps.
Therefore, there is a need to provide a flow meter that does not require the coils to be attached to the flow tubes and at the same time uses a minimum number of components. The present invention overcomes these and other problems and an advance in the art is achieved.