Zero setting arrangement in a measuring device

Abstract: The invention relates to a method for zero setting of a measuring device in which particles to be measured are subjected to tangential acceleration in a whirling impeller (1), and the torque exerted on the vanes of the impeller (1) in reaction to the Coriolis force of accelerated particles is being measured as an indication of a mass flow rate, and wherein the flow (4) of material is fed to the central part of the impeller and thereafter by the centrifugal force thrown towards the circumference of the impeller (1) and the flow being measured by means of an electronic device (12, 14) including a microcomputer (12). When the flow (4) is absent the mean value of the torque during a chosen time or a number of revolutions of the impeller (1) is memorised, and this memorised torque value is used as a tare value until next time a new tare value can be memorised.

Description: The present invention relates to an apparatus for measuring a mass flow, in particular of bulk material or of a particle-containing fluid, and the invention especially relates to a zero setting arrangement used in a measuring device in which the mass flow rate is measured by using the Coriolis measuring principle. The arrangement according to the invention concerns both a method for zero setting and a device for implementing the method. Such a prior measuring device is described in our copending European patent application No. 96850202.1, filed on 29 November 1996 and not made available to the public before the date of filing of the present application, describing the main principles for such a mass flow meter, and to which application herewith is referred. In such a measuring device the particles to be measured are subjected to tangential acceleration in a whirling impeller, and the torque exerted on the vanes of the impeller in reaction to the Coriolis force of accelerated particles is being measured as an indication of the mass flow rate. A prior example of a measuring device using the Coriolis principle is known from WO-A-9208958, which is intended for packing by weight. The impeller is here driven by a motor underneath the impeller, the motor including an encoder for measuring its rotational speed. A measurement of the current to the motor is considered as an indication of the torque exerted on the impeller. From these measurements the mass flow through the impeller is calculated by a signal processing device. This prior device is intended for weighing up batches, During periods when no material is flowing, and due to a command signal received from an input terminal, a zero setting is accomplished by memorizing an averaged output built up over several cycles of the motor. This memorized averaged output is then used as a tare. The object of our above European patent application was to avoid measurement errors, due to uneven distribution of the material to be measured. Even though the accuracy in the measurements has been greatly improved by our previous device, it is still necessary to adjust the device eliminating/reducing measurement errors due to variations in bearing friction in the bearings supporting the impeller shaft. The object of the present invention is therefore to provide a method and a device for the zero setting of a measuring device. The above object has been obtained in accordance with the independent claims 1 and 4. According to the invention, when the flow of material into the impeller is temporarily interrupted, either naturally interrupted or mechanically interrupted, during the interruption period the torque on the drive motor for the impeller is measured, and the mean value of the torque is used as the zero mass flow value. An automatic procedure for setting the zeropoint of a gas flowmeter is known from Japanese patent application No. 7-333017, according to which is introduced a separate judging means, a zero flow meter (10), to sense when the flow through the meter is zero. When the flow is sensed to be zero a separate isolation valve (30) in the flow channel is closed, and thereafter the zero setting of the meter is accomplished. The present invention also provides for a device for the zero setting, which can include a controllable valve. The invention will now be described in the form of a non-limiting embodiment, shown on the enclosed drawings, in which Fig. 1 shows a schematic block diagram of a zero setting device according to the invention. In Fig. 1 is shown an impeller 1 intended to be a part of a complete device for measuring a mass flow of the kind described in our copending European patent application No. 96850202.1. The other parts relating to the measuring device can have the same, but also alternative, designs as those shown in our previous application. The impeller 1 is mounted for rotation around a vertical rotatable shaft 2, driven by an electric motor 3. The material 4 to be measured is fed to the impeller 1 via an inlet device, ending with an inlet tube 5 closest to the impeller 1, the tube 5 reaching into a housing 6 enclosing the impeller 1. The housing 6 can as shown be formed as a hopper, at the bottom ending with an outlet opening 7 for the material measured. The inlet device shown comprises a controllable flap valve 8, or another type of quickly movable valve, which can stop the flow of material into the inlet tube 5 and the impeller 1. The flap valve 8 is rotatable by means of a shaft 9, journalled in bearings 10 and its rotation is controlled by a turning device 11. The turning device 11 is in its turn controlled by a control device 12, e.g. a measuring and controlling computer. The turning device can be an electric or hydraulic operated turning device. To the vertical rotatable shaft 2 a tachometer 13 is attached, for registering the rotation speed of the shaft 2, and transmitting the information to the control device 12. The torque on the electric motor 3 is monitored by a torque measuring device 14, connected to a signal processor 15, in which the output signal from the torque measuring device 14 is converted to an input signal to the control device 12, for transmitting a torque input signal to the control device 12. Further, a static frequency changer 16 is connected between the control device 12 and the electric motor 3, so that the control device 12 can control also the speed of the electric motor 3. When no material then enters the impeller 1, the torque on the electric motor 3 is measured by the torque measuring device 14, and the signal from this torque measuring device 14 is via the signal processor 15 transmitted to the control device 12 in which the mean value of the torque during a chosen time or during a number of revolutions is decided. The mean value is memorised in the control device 12, and is thereafter considered as the zero torque value, the tare value, when no material is fed through the impeller 1. For the zero setting of the measuring device the flow of material in the inlet device is sensed by the control device 12, and the mean value of the torque can then be calculated and memorised when the flow is absent. Alternatively the material in the inlet device can by the rotatable valve 8 be automatically stopped when the rotatable valve 8 is rotated to its closed position by means of the shaft 9 and the turning device 11 controlled by the control device 12. The zero setting in the later form can preferably be made at regular intervals. It can also be done at regular intervals and when the flow of material simultaneously is absent. In arrangements where the rotational speed of the rotatable shaft 2 can be varied, a zero setting of the above described type is made and memorized for a number of chosen discrete rotational speeds, which are regulated by the control device 12. During variation of the rotational speed the influence of change in the moment of inertia of the impeller on the indication of material totalisation and flow may also be compensated for.

Claims: A method for zero setting of a measuring device in which particles to be measured are subjected to tangential acceleration in a whirling impeller (1), and the torque exerted on the vanes of the impeller (1) in reaction to the Coriolis force of accelerated particles is being measured as an indication of a mass flow rate, and wherein the flow (4) of material is fed to the central part of the impeller and thereafter by the centrifugal force thrown towards the circumference of the impeller (1) and the flow being measured by means of an electronic device (12, 14), wherein the electronic control device automatically continuously senses when the flow is absent, and when the flow (4) is absent the mean value of the torque during a chosen time or a number of revolutions of the impeller (1) is calculated and memorised, and this memorised torque value is used as a tare value until next time a new tare value can be memorised. A method according to claim 1, characterised in that a valve (8) is arranged to close the inlet to the impeller (1) during a time corresponding to a chosen number of revolutions of the impeller and thereafter caused to open the flow (4) of material. Method according to claim 1 or 2, characterised in that the zero setting values are memorized for a number of chosen discrete rotational speeds of the impeller (1). Zero setting device for a measuring device in which particles to be measured are subjected to tangential acceleration in a whirling impeller (1), and the torque exerted on the vanes of the impeller (1) in reaction to the Coriolis force of accelerated particles is being measured as an indication of a mass flow rate, and wherein the flow (4) of material is fed to the central part of the impeller and thereafter by the centrifugal force thrown towards the circumference of the impeller (1) and the flow being measured by means of an electronic device (12, 14), wherein the electronic control device (12) is arranged to continuously sense the flow, and when the flow is absent the electronic control device (12) is arranged to calculate and memorise the mean value of the torque during a chosen time or a number of revolutions of the impeller (1) as a zero torque value. Zero setting device according to claim 4, characterised in that a valve (8), e.g. a flap valve or another type of quickly movable valve, is arranged in the inlet to the impeller (1) to prevent the flow of material into the impeller when the flow has been decided to be absent, and that the electronic device (12, 14) is arranged to measure the torque on the impeller when the valve (8) is closed. Zero setting device according to claim 5, characterised in that the valve (8) is activated by the electronic device (12, 14).

Inventors: SOEDERHOLM ARNE, SOEDERHOLM OLLE, SOEDERHOLM, ARNE, SOEDERHOLM, OLLE, Söderholm, Arne, Söderholm, Olle
Applicants: SEG MEKANIK AB, S.E.G. MEKANIK AB
Application year: 1997
Main classification: G01F1
IPC codes: G01F1, G01F25