WO2013132156A1 - Method for determining a bending moment of a drive shaft of a mixing apparatus, and a mixing system - Google Patents

Method for determining a bending moment of a drive shaft of a mixing apparatus, and a mixing system Download PDF

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Publication number
WO2013132156A1
WO2013132156A1 PCT/FI2013/050240 FI2013050240W WO2013132156A1 WO 2013132156 A1 WO2013132156 A1 WO 2013132156A1 FI 2013050240 W FI2013050240 W FI 2013050240W WO 2013132156 A1 WO2013132156 A1 WO 2013132156A1
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WO
WIPO (PCT)
Prior art keywords
load cell
mixing apparatus
mixing system
tank
mixing
Prior art date
Application number
PCT/FI2013/050240
Other languages
French (fr)
Inventor
Jouko PERÄAHO
Timo LINDSTRÖM
Jukka Lekkala
Jarmo Verho
Timo SALPAVAARA
Markus KARJALAINEN
Heimo Ihalainen
Jouko HALTTUNEN
Original Assignee
Uutechnic Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
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Publication date
Application filed by Uutechnic Oy filed Critical Uutechnic Oy
Publication of WO2013132156A1 publication Critical patent/WO2013132156A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
    • G01L5/0061—Force sensors associated with industrial machines or actuators
    • G01L5/0071—Specific indicating arrangements, e.g. of overload
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01F—MIXING, e.g. DISSOLVING, EMULSIFYING OR DISPERSING
    • B01F27/00—Mixers with rotary stirring devices in fixed receptacles; Kneaders
    • B01F27/80—Mixers with rotary stirring devices in fixed receptacles; Kneaders with stirrers rotating about a substantially vertical axis
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
    • B29B7/00—Mixing; Kneading
    • B29B7/02—Mixing; Kneading non-continuous, with mechanical mixing or kneading devices, i.e. batch type
    • B29B7/22—Component parts, details or accessories; Auxiliary operations
    • B29B7/28—Component parts, details or accessories; Auxiliary operations for measuring, controlling or regulating, e.g. viscosity control
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/16—Measuring force or stress, in general using properties of piezoelectric devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L5/00—Apparatus for, or methods of, measuring force, work, mechanical power, or torque, specially adapted for specific purposes
    • G01L5/0004—Force transducers adapted for mounting in a bore of the force receiving structure
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
    • B29B7/00—Mixing; Kneading
    • B29B7/02—Mixing; Kneading non-continuous, with mechanical mixing or kneading devices, i.e. batch type
    • B29B7/06—Mixing; Kneading non-continuous, with mechanical mixing or kneading devices, i.e. batch type with movable mixing or kneading devices
    • B29B7/10—Mixing; Kneading non-continuous, with mechanical mixing or kneading devices, i.e. batch type with movable mixing or kneading devices rotary
    • B29B7/12—Mixing; Kneading non-continuous, with mechanical mixing or kneading devices, i.e. batch type with movable mixing or kneading devices rotary with single shaft
    • B29B7/16—Mixing; Kneading non-continuous, with mechanical mixing or kneading devices, i.e. batch type with movable mixing or kneading devices rotary with single shaft with paddles or arms

Definitions

  • the present invention relates to a method for determining a bending moment of a drive shaft of a mixing apparatus and to a mixing system according to the preambles of the appended independent claims.
  • a conventional mixing apparatus comprises a drive mechanism which is coupled to a drive shaft having an impeller thereon.
  • the drive mechanism comprises a motor for rotating the impeller via the drive shaft, so that the impeller can be used to mix a fluid that is contained in a tank of a mixing system.
  • a mixing apparatus is susceptible, during use, to forces which may damage the mixing apparatus.
  • the drive shaft of the mixing apparatus may fatigue, permanently bend or even break due to excessive forces.
  • the main forces acting on a mixing apparatus are fluid forces. They are generated at the impeller perpendicular to the drive shaft axis and produce deflection on the drive shaft and on the drive mechanism. Fluid forces are especially large at draw-off or when the impeller operates near the fluid surface.
  • the rotating impeller is also subject to buoyancy in the fluid which produces a force in the longitudinal direction of the drive shaft, and thus tends to elevate the mixing apparatus.
  • the state of a mixing apparatus is monitored by measuring the deflection of a drive shaft, which deflection correlates with forces acting on the mixing apparatus.
  • the deflection of the drive shaft is measured with strain gauges which are attached to the drive shaft.
  • the state of the mixing apparatus can be evaluated either directly from the measured strain values or from a bending moment which is calculated from the strain values.
  • bending moment is the moment basically caused by unbalanced horizontal forces on the impeller (blades) multiplied by the drive shaft length. The moment is defined and it is on its maximum where the bearings are located.
  • a problem associated with the known method is that the measuring equipment must be arranged inside the tank, whereby the equipment may be contaminated from a fluid being mixed. Another problem is that at least some parts of the measuring equipment must be attached to the rotating drive shaft. Still another problem is that electrical connections must be arranged into and out of the tank, for example by using slip rings on the drive shaft.
  • a typical method for determining a bending moment of a drive shaft of a mixing apparatus which is attached to a tank at a plurality of attachment points comprises providing at least two of the plurality of attachment points with a load cell, measuring an electrical signal produced by each load cell, the electrical signal being indicative of a force acting on the attachment point, and calculating the bending moment from the measured electrical signals.
  • a mixing apparatus that is used for mixing a fluid is susceptible to forces which may damage the mixing apparatus. Especially the drive shaft of the mixing apparatus may easily be damaged.
  • the main forces acting on the mixing apparatus are fluid forces. A perpendicular component of these forces bends the drive shaft and thus causes a bending moment and torque to the mixing apparatus.
  • a good indicator of a state of the mixing apparatus is the bending moment of its drive shaft. It has now been found that such bending moment can be easily and accurately determined from the forces acting on the attachment points of the mixing apparatus. In other words, the bending moment of the drive shaft can be calculated from forces which are measured at points in which the mixing apparatus is fastened to the tank. This is possible because the bending forces from the mixing apparatus are transformed to longitudinal forces at the attachment points.
  • the calculation of the bending moment depends on the number of load cells and their locations. When only two load cells are used, the load cells are preferably located on the opposite corners of the mixer mounting.
  • the bending moment is obtained from the following equation:
  • the load cells are preferably arranged in an equilateral triangle on a round base plate.
  • the bending moments in two perpendicular directions are obtained from the following equations:
  • the load cells are preferably attached to four corners of a base plate.
  • the bending moments in two perpendicular directions are obtained from the following equa- tions:
  • Fi, F 2 , F 3 and F are forces measured by the load cells and L is the distance between the load cell and the mixer centre.
  • the vertical force can be calculated by summing the force signals together.
  • Different types of load cells such as cells based on a piezoelectric, resistive, inductive and capacitive measurement principle, can be used in the method according to the invention. Because each load cell type relies on a different measurement principle, electrical signals produced by the load cells may substantially differ from each other. However, the electrical signals produced by the different load cells can be easily scaled if needed so that they are compatible with each other.
  • An advantage of measuring the forces at attachment points of a mixing apparatus is that the measurement can be made outside a tank of a mixing system whereby the contamination of measuring equipment from a fluid being mixed can be avoided. Another advantage is that a measurement at an attachment point of a mixing apparatus provides an accurate value of forces acting on the drive shaft of the mixing apparatus.
  • two, three, four, at least three or at least four of the plurality of attachment points is provided with a load cell.
  • each of the plurality of attachment points is provided with a load cell.
  • the load cell is a piezoelectric load cell.
  • a piezoelectric load cell comprises a piezoelectric sensing element which produces a voltage signal in response to an applied dynamic force.
  • the method comprises controlling the operation of the mixing apparatus based on one or more of the measured electrical signals and/or the calculated bending moment. For example, the speed of a motor of a drive mechanism may be varied according to the bending moment. If the bending moment exceeds a predefined threshold value, the motor is stopped in order to prevent the mixing apparatus from overloading.
  • the invention also relates to a mixing system.
  • a typical mixing system according to the invention comprises a tank for holding a fluid, and a mixing apparatus attached to the tank at a plurality of attachment points.
  • the mixing apparatus comprises a drive mechanism, a drive shaft and an impeller. A first end of the drive shaft is coupled to the drive mechanism and a second end of the drive shaft is coupled to the impeller.
  • a typical mixing system further comprises at least two load cells, each being arranged in connection with one of the plurality of attachment points, and configured to produce an electrical signal which is indicative of a force acting on the attachment point, and a control unit configured to calculate a bending moment of the drive shaft from the measured elec- trical signals.
  • the mixing apparatus and the tank are separate units which are coupled to each other through the plurality of attachment points.
  • the number and position of the attachment points may vary depending on a specific type of the mixing apparatus and the tank.
  • the mixing apparatus may be attached to the tank, for example, by means of two, three, four or even more attachment points. Different kinds of fastening means may be used in the attachment points for attaching the mixing apparatus to the tank.
  • the mixing apparatus may have been attached, for example, to walls of the tank or to a cover of the tank. Preferably, the mixing apparatus is attached in connection with an opening of the tank.
  • the mixing system according to the invention is used for mixing fluids.
  • a fluid to be mixed is arranged into the tank and the mixing apparatus is attached to the tank in such a manner that the impeller is in contact with the fluid.
  • the drive mechanism rotates the drive shaft whereby the impeller that is coupled to the drive shaft mixes the fluid that has been arranged into the tank.
  • the drive mechanism may comprise a motor and a gear-box.
  • the control unit controls the operation of the mixing apparatus.
  • the control unit comprises means for measuring the electrical signals of the load cells, means for processing data related to the electrical signals, and means for controlling the motor.
  • the control unit may comprise, for example, a processor configured to calculate the bending moment from the values of the electrical signals, and a memory for storing data such as the values of the electrical signals and the calculated bending moments.
  • the control unit is connected to the load cells with wires.
  • the mixing apparatus is preferably attached to the tank in such a manner that the drive mechanism is outside the tank, and the drive shaft, the first end of which is coupled to the drive mechanism, extends through an opening of the tank into the tank.
  • the impeller which is coupled to the second end of the drive shaft is inside the tank.
  • the size and shape of the tank may vary.
  • the tank may have a circular, square or rectangular cross-sectional shape.
  • the volume or capacity of the tank may vary, for example, from a few hundreds of litres to several thousands of litres.
  • the volume of the tank may be even hundreds or thousands of cubic metres.
  • the tank may comprise a cover for closing the tank, to which cover the mixing apparatus may be attached at the plurality of attachment points.
  • the cover comprises a through hole through which the drive shaft passes into the tank.
  • the cover is releasably attached to the tank so that it can be removed in order to facilitate filling and emptying of the tank.
  • At least three of the plurality of attachment points is provided with a load cell.
  • the mixing apparatus is attached to the tank at said plurality of attachment points by means of bolts.
  • the bolt may be screwed into the structure of the tank or the bolt may be tightened with a nut.
  • Other fastening means can alternatively be used for attaching the mixing apparatus to the tank.
  • the load cell comprises a through hole, through which the bolt passes.
  • the load cell can be arranged directly under the bolt head.
  • the load cell can be arranged at the attachment point between the mixing apparatus and the tank.
  • a load cell which comprises a through hole can be easily assembled while assembling the bolts. Replacing a load cell is also easy.
  • the mixing apparatus comprises a base plate with which the mixing apparatus is attached to the tank.
  • the base plate is preferably part of the structure which also comprises the drive mechanism.
  • the base plate may have been dimensioned in such a manner that it covers the opening of the tank whereby it works as a cover to the tank.
  • the load cell is arranged between the bolt head and the base plate.
  • the mixing apparatus is typically attached to the tank by using a base plate which is mounted horizontally.
  • the drive shaft is typically aligned vertically.
  • the base plate is rectangular and an attachment point is located at each corner of the base plate.
  • the base plate can alternatively have other shapes such as circular. In a circular base plate the attachment points are preferably distributed evenly on the perimeter of the base plate.
  • the mixing apparatus is attached to a cover of the tank. If the mixing apparatus comprises a base plate, the mixing apparatus is attached to the tank by attaching the base plate to the cover at a plurality of attachment points.
  • the load cell can be arranged between the base plate and the cover.
  • the control unit is configured to control the operation of the mixing apparatus based on one or more of the measured electrical signals and/or the calculated bending moment.
  • the control unit may, for example, change the speed of a motor of the drive mechanism according to the electrical signal(s) received from the load cell or according to the calculated bend- ing moment.
  • the control unit may be configured to control the mixing apparatus continuously or at predetermined time intervals.
  • the load cell is a piezoelectric load cell.
  • Other types of load cells such as capacitive load cells, can alternatively be used in a mixing system according to the invention.
  • the piezoelectric load cell comprises a first support plate and a second support plate fastened together, said plates being perforated by a through hole, and a ring-shaped piezoelectric sensing element arranged between the first support plate and the second support plate, a first side of said piezoelectric sensing element being provided with a ring-shaped electrode.
  • the piezoelectric load cell further comprises a ring-shaped resilient element arranged between the first support plate and the first side of the piezoelectric sensing element.
  • the piezoelectric sensing element When a force is applied to the load cell, the piezoelectric sensing element is strained and due to its piezoelectric properties a voltage is generated between the opposing sides of the piezoelectric sensing element. This voltage is sensed and measured between the electrode and the second support plate.
  • the support plates are fastened together, for example, using screws or bolts.
  • the purpose of the resilient element is to electrically insulate the electrode from the support plates.
  • Another purpose of the resilient element is to attenuate forces which compress the piezoelectric sensing element in order to prevent it from damaging.
  • the second support plate comprises a circular recess for receiving a second side of the piezoelectric sensing element.
  • the piezoelectric load cell comprises an electrical connector connected to the electrode and the second support plate. An electrical signal produced by the piezoelectric load cell is measured through the electrical connector.
  • the load cell may comprise a buffer amplifier for amplifying the electrical signal.
  • the piezoelectric sensing element is made of polyvinylidene fluoride. Also other piezoelectric sensing materials, such as piezoelectric crystalline materials (for example quartz) or piezoelectric ceramic materials (for example PZT, lithium niobate, or zinc oxide), can be used to realize the load cell.
  • the resilient element is made of poly- dimethylsiloxane.
  • the resilient element can be made of poly- vinylidene fluoride.
  • Fig. 1 illustrates a mixing system according to an embodiment of the invention
  • figs. 2a-2b illustrate an example load cell.
  • Fig. 1 illustrates a mixing system according to an embodiment of the invention.
  • the mixing system comprises a tank 1 10 which contains a fluid 120 to be mixed, and a mixing apparatus 130 for mixing the fluid 120.
  • the mixing apparatus 130 comprises a drive mechanism 131 , a drive shaft 132 and an impeller 133. A first end of the drive shaft 132 is coupled to the drive mechanism 131 and a second end of the drive shaft 132 is coupled to the impeller 133.
  • the mixing apparatus 130 is arranged in connection with the tank 1 10 in such a manner that the drive mechanism 131 is outside the tank 1 10, and the drive shaft
  • the impeller 133 which is coupled to the second end of the drive shaft 132 is inside the tank 1 10.
  • the impeller 133 is arranged in contact with the fluid 120 so that the impeller 133 which is driven by the drive mechanism 131 is capable of mixing the fluid 120.
  • the drive mechanism 131 comprises a motor 134 and a gear-box 135 for driving the impeller 133.
  • the mixing apparatus 130 comprises a base plate 136 with which the mixing apparatus 130 is attached to the tank 1 10.
  • the drive mechanism 131 is coupled to the base plate 136.
  • the base plate 136 is attached to the tank 1 10 at four attachment points 140 (only two of them are shown). At each of the attachment points 140 the base plate 136 is attached to the tank 1 10 by means of a bolt 150.
  • the base plate 136 has been dimensioned in such a manner that it covers the opening of the tank 1 10 whereby it works as a cover to the tank 1 10.
  • the base plate 136 may be separated from the tank 1 10 in order to facilitate filling and emptying of the tank 1 10.
  • Two of the attachment points 140 are provided with a load cell 160 which produc- es an electrical signal indicative of a force acting on the attachment point 140.
  • the load cell 160 comprises a through hole, through which the bolt passes.
  • the load cell 160 is arranged directly under the bolt head, that is to say, between the bolt head and the base plate 136.
  • the drive mechanism 131 comprises a control unit 137 for controlling the opera- tion of the mixing apparatus 130.
  • the control unit 137 which receives the electrical signals from the load cells 160, is configured to calculate the bending moment of the drive shaft 132 from the measured electrical signals.
  • the operation of the drive mechanism 131 may be controlled based on the electrical signal(s) and/or the calculated bending moment.
  • the control unit 137 may, for example, vary the speed of the motor 134 based on the measured electrical signal(s) and/or the calculated bending moment in order prevent to the mixing apparatus 130 from damaging.
  • Fig. 2a illustrates an example piezoelectric load cell that may be used in a mixing system according to the invention.
  • An exploded view of the piezoelectric load cell is illustrated in fig. 2b.
  • the piezoelectric load cell 260 of figs. 2a and 2b comprises a first support plate 261 and a second support plate 262 which are fastened together by four bolts 263.
  • the support plates 261 , 262 are perforated by a through hole 264, through which an attachment bolt of a mixing apparatus may be arranged to pass.
  • the piezoelectric load cell 260 comprises a ring-shaped piezoelectric sensing element 265 which is arranged between the first support plate 261 and the second support plate 262.
  • the piezoelectric sensing element 265 is made of polyvinyli- dene fluoride.
  • the first side of the piezoelectric sensing element 265 is provided with a ring-shaped electrode 266.
  • the piezoelectric load cell 260 further comprises a ring-shaped resilient element 267 which is arranged between the first support plate 261 and the first side of the piezoelectric sensing element 265.
  • the resilient element 267 is made of polydime- thylsiloxane. The resilient element 267 electrically insulates the electrode 266 from the support plates 261 , 262.
  • the second support plate 262 comprises a circular recess 268 for receiving a se- cond side of the piezoelectric sensing element 265.
  • the purpose of the circular recess 268 is to reduce the compression of the piezoelectric sensing element 265 in an unloaded state of the piezoelectric load cell 260.
  • the piezoelectric load cell 260 comprises an electrical connector 269 which is connected to the electrode 266 and the second support plate 262.
  • the electrical signal produced by the piezoelectric load cell 260 is measured through the electrical connector 269.
  • the electrical signal provides a voltage difference between the electrode 266 and the second support plate 262.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Mixers Of The Rotary Stirring Type (AREA)
  • Accessories For Mixers (AREA)

Abstract

The present invention relates to a method for determining a bending moment of a drive shaft of a mixing apparatus (130) which is attached to a tank (110) at a plurality of attachment points (140). In the method at least two of the plurality of attachment points (140) are provided with a load cell (160, 260), an electrical signal produced by each load cell is measured, and the bending moment is calculated from the measured electrical signals. The invention also relates to a mixing system.

Description

METHOD FOR DETERMINING A BENDING MOMENT OF A DRIVE
SHAFT OF A MIXING APPARATUS, AND A MIXING SYSTEM
TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for determining a bending moment of a drive shaft of a mixing apparatus and to a mixing system according to the preambles of the appended independent claims.
BACKGROUND OF THE INVENTION
Mixing apparatuses are widely used in processing industry for mixing fluids in vari- ous stages of chemical processes. A conventional mixing apparatus comprises a drive mechanism which is coupled to a drive shaft having an impeller thereon. The drive mechanism comprises a motor for rotating the impeller via the drive shaft, so that the impeller can be used to mix a fluid that is contained in a tank of a mixing system. A mixing apparatus is susceptible, during use, to forces which may damage the mixing apparatus. For example, the drive shaft of the mixing apparatus may fatigue, permanently bend or even break due to excessive forces. The main forces acting on a mixing apparatus are fluid forces. They are generated at the impeller perpendicular to the drive shaft axis and produce deflection on the drive shaft and on the drive mechanism. Fluid forces are especially large at draw-off or when the impeller operates near the fluid surface. The rotating impeller is also subject to buoyancy in the fluid which produces a force in the longitudinal direction of the drive shaft, and thus tends to elevate the mixing apparatus.
It is known to monitor forces acting on a mixing apparatus and to control the oper- ation of the apparatus based on this information. Especially the forces which produce resonance vibrations in the structure of the mixing apparatus are monitored. Typically, the rotation speed of the impeller is varied according to the measured forces; when excessive forces are detected the rotation speed of the impeller is reduced, thereby reducing the forces and preventing any damage to the mixing apparatus which might result if such forces would become too large.
In a known method the state of a mixing apparatus is monitored by measuring the deflection of a drive shaft, which deflection correlates with forces acting on the mixing apparatus. The deflection of the drive shaft is measured with strain gauges which are attached to the drive shaft. The state of the mixing apparatus can be evaluated either directly from the measured strain values or from a bending moment which is calculated from the strain values. In vertical mixing apparatuses bending moment is the moment basically caused by unbalanced horizontal forces on the impeller (blades) multiplied by the drive shaft length. The moment is defined and it is on its maximum where the bearings are located.
A problem associated with the known method is that the measuring equipment must be arranged inside the tank, whereby the equipment may be contaminated from a fluid being mixed. Another problem is that at least some parts of the measuring equipment must be attached to the rotating drive shaft. Still another problem is that electrical connections must be arranged into and out of the tank, for example by using slip rings on the drive shaft.
OBJECTIVES OF THE INVENTION It is the main objective of the present invention to reduce or even eliminate prior art problems presented above.
It is an objective of the present invention to provide a method for determining a bending moment of a drive shaft of a mixing apparatus. In more detail, it is an objective of the invention to provide a method enabling to determine, from the out- side of a tank, the bending moment of a drive shaft. It is a further objective of the invention to provide a method enabling to determine the bending moment of a drive shaft easily and accurately.
It is also an objective of the present invention to provide a mixing system which is capable of determining a bending moment of a drive shaft of a mixing apparatus. In order to realise the above-mentioned objectives, the method and system according to the invention are characterised by what is presented in the characterising parts of the appended independent claims. Advantageous embodiments of the invention are described in the dependent claims.
DESCRIPTION OF THE INVENTION A typical method according to the invention for determining a bending moment of a drive shaft of a mixing apparatus which is attached to a tank at a plurality of attachment points, comprises providing at least two of the plurality of attachment points with a load cell, measuring an electrical signal produced by each load cell, the electrical signal being indicative of a force acting on the attachment point, and calculating the bending moment from the measured electrical signals.
A mixing apparatus that is used for mixing a fluid is susceptible to forces which may damage the mixing apparatus. Especially the drive shaft of the mixing apparatus may easily be damaged. The main forces acting on the mixing apparatus are fluid forces. A perpendicular component of these forces bends the drive shaft and thus causes a bending moment and torque to the mixing apparatus.
A good indicator of a state of the mixing apparatus is the bending moment of its drive shaft. It has now been found that such bending moment can be easily and accurately determined from the forces acting on the attachment points of the mixing apparatus. In other words, the bending moment of the drive shaft can be calculated from forces which are measured at points in which the mixing apparatus is fastened to the tank. This is possible because the bending forces from the mixing apparatus are transformed to longitudinal forces at the attachment points.
The calculation of the bending moment depends on the number of load cells and their locations. When only two load cells are used, the load cells are preferably located on the opposite corners of the mixer mounting. The bending moment is obtained from the following equation:
M bi ending L wherein
Figure imgf000005_0001
and F3 are forces measured by the load cells and L is the distance between the load cell and the mixer centre. The vertical forces can be separated by summing up the forces from the load cells.
When three load cells are used, the load cells are preferably arranged in an equilateral triangle on a round base plate. The bending moments in two perpendicular directions are obtained from the following equations:
M bi ending x-axis 0.87L and
Fi - (F2 + F3)
M bi ending y-axis L wherein
Figure imgf000006_0001
F2 and F3 are forces measured by the load cells and L is the distance between the load cell and the mixer centre. With three load cells there are three independent variables and so three components of force can be separated. The drive shaft is subjected to bending forces in two directions and pushed and pulled in the third direction, i.e. the vertical direction. The vertical force can be calculated by summing the force signals together.
When four load cells are used, the bending moment can be measured accurately. The load cells are preferably attached to four corners of a base plate. The bending moments in two perpendicular directions are obtained from the following equa- tions:
= F1 + F1 - F1 - F±
^bending x-axis ^ and
^bending y-axis ^ wherein Fi, F2, F3 and F are forces measured by the load cells and L is the distance between the load cell and the mixer centre. The vertical force can be calculated by summing the force signals together. Different types of load cells, such as cells based on a piezoelectric, resistive, inductive and capacitive measurement principle, can be used in the method according to the invention. Because each load cell type relies on a different measurement principle, electrical signals produced by the load cells may substantially differ from each other. However, the electrical signals produced by the different load cells can be easily scaled if needed so that they are compatible with each other.
An advantage of measuring the forces at attachment points of a mixing apparatus is that the measurement can be made outside a tank of a mixing system whereby the contamination of measuring equipment from a fluid being mixed can be avoided. Another advantage is that a measurement at an attachment point of a mixing apparatus provides an accurate value of forces acting on the drive shaft of the mixing apparatus.
According to an embodiment of the invention two, three, four, at least three or at least four of the plurality of attachment points is provided with a load cell. The greater the number of attachment points at which a force is measured, the more accurate the method is. It also provides accurate information on the distribution and directions of the forces acting on the mixing apparatus which information may be exploited in controlling the operation of the mixing apparatus. According to an embodiment of the invention each of the plurality of attachment points is provided with a load cell.
According to an embodiment of the invention the load cell is a piezoelectric load cell. A piezoelectric load cell comprises a piezoelectric sensing element which produces a voltage signal in response to an applied dynamic force.
According to an embodiment of the invention the method comprises controlling the operation of the mixing apparatus based on one or more of the measured electrical signals and/or the calculated bending moment. For example, the speed of a motor of a drive mechanism may be varied according to the bending moment. If the bending moment exceeds a predefined threshold value, the motor is stopped in order to prevent the mixing apparatus from overloading. The invention also relates to a mixing system. A typical mixing system according to the invention comprises a tank for holding a fluid, and a mixing apparatus attached to the tank at a plurality of attachment points. The mixing apparatus comprises a drive mechanism, a drive shaft and an impeller. A first end of the drive shaft is coupled to the drive mechanism and a second end of the drive shaft is coupled to the impeller. A typical mixing system according to the invention further comprises at least two load cells, each being arranged in connection with one of the plurality of attachment points, and configured to produce an electrical signal which is indicative of a force acting on the attachment point, and a control unit configured to calculate a bending moment of the drive shaft from the measured elec- trical signals.
In the mixing system according to the invention the mixing apparatus and the tank are separate units which are coupled to each other through the plurality of attachment points. The number and position of the attachment points may vary depending on a specific type of the mixing apparatus and the tank. The mixing apparatus may be attached to the tank, for example, by means of two, three, four or even more attachment points. Different kinds of fastening means may be used in the attachment points for attaching the mixing apparatus to the tank. The mixing apparatus may have been attached, for example, to walls of the tank or to a cover of the tank. Preferably, the mixing apparatus is attached in connection with an opening of the tank.
The mixing system according to the invention is used for mixing fluids. A fluid to be mixed is arranged into the tank and the mixing apparatus is attached to the tank in such a manner that the impeller is in contact with the fluid. The drive mechanism rotates the drive shaft whereby the impeller that is coupled to the drive shaft mixes the fluid that has been arranged into the tank. The drive mechanism may comprise a motor and a gear-box. The control unit controls the operation of the mixing apparatus. The control unit comprises means for measuring the electrical signals of the load cells, means for processing data related to the electrical signals, and means for controlling the motor. The control unit may comprise, for example, a processor configured to calculate the bending moment from the values of the electrical signals, and a memory for storing data such as the values of the electrical signals and the calculated bending moments. The control unit is connected to the load cells with wires.
The mixing apparatus is preferably attached to the tank in such a manner that the drive mechanism is outside the tank, and the drive shaft, the first end of which is coupled to the drive mechanism, extends through an opening of the tank into the tank. The impeller which is coupled to the second end of the drive shaft is inside the tank.
The size and shape of the tank may vary. For example, the tank may have a circular, square or rectangular cross-sectional shape. The volume or capacity of the tank may vary, for example, from a few hundreds of litres to several thousands of litres. The volume of the tank may be even hundreds or thousands of cubic metres.
The tank may comprise a cover for closing the tank, to which cover the mixing apparatus may be attached at the plurality of attachment points. The cover comprises a through hole through which the drive shaft passes into the tank. Preferably, the cover is releasably attached to the tank so that it can be removed in order to facilitate filling and emptying of the tank.
According to an embodiment of the invention at least three of the plurality of attachment points is provided with a load cell. According to an embodiment of the invention the mixing apparatus is attached to the tank at said plurality of attachment points by means of bolts. The bolt may be screwed into the structure of the tank or the bolt may be tightened with a nut. Other fastening means can alternatively be used for attaching the mixing apparatus to the tank.
According to an embodiment of the invention the load cell comprises a through hole, through which the bolt passes. The load cell can be arranged directly under the bolt head. Alternatively, the load cell can be arranged at the attachment point between the mixing apparatus and the tank. A load cell which comprises a through hole can be easily assembled while assembling the bolts. Replacing a load cell is also easy.
According to an embodiment of the invention the mixing apparatus comprises a base plate with which the mixing apparatus is attached to the tank. The base plate is preferably part of the structure which also comprises the drive mechanism. The base plate may have been dimensioned in such a manner that it covers the opening of the tank whereby it works as a cover to the tank. Preferably, the load cell is arranged between the bolt head and the base plate. The mixing apparatus is typically attached to the tank by using a base plate which is mounted horizontally. The drive shaft is typically aligned vertically. According to an embodiment of the invention the base plate is rectangular and an attachment point is located at each corner of the base plate. The base plate can alternatively have other shapes such as circular. In a circular base plate the attachment points are preferably distributed evenly on the perimeter of the base plate. According to an embodiment of the invention the mixing apparatus is attached to a cover of the tank. If the mixing apparatus comprises a base plate, the mixing apparatus is attached to the tank by attaching the base plate to the cover at a plurality of attachment points. The load cell can be arranged between the base plate and the cover. According to an embodiment of the invention the control unit is configured to control the operation of the mixing apparatus based on one or more of the measured electrical signals and/or the calculated bending moment. The control unit may, for example, change the speed of a motor of the drive mechanism according to the electrical signal(s) received from the load cell or according to the calculated bend- ing moment. The control unit may be configured to control the mixing apparatus continuously or at predetermined time intervals.
According to an embodiment of the invention the load cell is a piezoelectric load cell. Other types of load cells, such as capacitive load cells, can alternatively be used in a mixing system according to the invention.
According to an embodiment of the invention the piezoelectric load cell comprises a first support plate and a second support plate fastened together, said plates being perforated by a through hole, and a ring-shaped piezoelectric sensing element arranged between the first support plate and the second support plate, a first side of said piezoelectric sensing element being provided with a ring-shaped electrode. The piezoelectric load cell further comprises a ring-shaped resilient element arranged between the first support plate and the first side of the piezoelectric sensing element.
When a force is applied to the load cell, the piezoelectric sensing element is strained and due to its piezoelectric properties a voltage is generated between the opposing sides of the piezoelectric sensing element. This voltage is sensed and measured between the electrode and the second support plate.
The support plates are fastened together, for example, using screws or bolts. The purpose of the resilient element is to electrically insulate the electrode from the support plates. Another purpose of the resilient element is to attenuate forces which compress the piezoelectric sensing element in order to prevent it from damaging.
According to an embodiment of the invention the second support plate comprises a circular recess for receiving a second side of the piezoelectric sensing element. According to an embodiment of the invention the piezoelectric load cell comprises an electrical connector connected to the electrode and the second support plate. An electrical signal produced by the piezoelectric load cell is measured through the electrical connector. The load cell may comprise a buffer amplifier for amplifying the electrical signal. According to an embodiment of the invention the piezoelectric sensing element is made of polyvinylidene fluoride. Also other piezoelectric sensing materials, such as piezoelectric crystalline materials (for example quartz) or piezoelectric ceramic materials (for example PZT, lithium niobate, or zinc oxide), can be used to realize the load cell.
According to an embodiment of the invention the resilient element is made of poly- dimethylsiloxane. In some applications the resilient element can be made of poly- vinylidene fluoride. By using different materials for the resilient element the thermal properties and stiffness of the load cell can be adjusted.
The exemplary embodiments of the invention presented in this text are not interpreted to pose limitations to the applicability of the appended claims. The verb "to comprise" is used in this text as an open limitation that does not exclude the exist- ence of also unrecited features. The features recited in the dependent claims are mutually freely combinable unless otherwise explicitly stated.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Fig. 1 illustrates a mixing system according to an embodiment of the invention, and figs. 2a-2b illustrate an example load cell.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
Fig. 1 illustrates a mixing system according to an embodiment of the invention. The mixing system comprises a tank 1 10 which contains a fluid 120 to be mixed, and a mixing apparatus 130 for mixing the fluid 120. The mixing apparatus 130 comprises a drive mechanism 131 , a drive shaft 132 and an impeller 133. A first end of the drive shaft 132 is coupled to the drive mechanism 131 and a second end of the drive shaft 132 is coupled to the impeller 133.
The mixing apparatus 130 is arranged in connection with the tank 1 10 in such a manner that the drive mechanism 131 is outside the tank 1 10, and the drive shaft
132 extends through an opening of the tank 1 10 into the tank 1 10. The impeller
133 which is coupled to the second end of the drive shaft 132 is inside the tank 1 10. The impeller 133 is arranged in contact with the fluid 120 so that the impeller 133 which is driven by the drive mechanism 131 is capable of mixing the fluid 120. The drive mechanism 131 comprises a motor 134 and a gear-box 135 for driving the impeller 133. The mixing apparatus 130 comprises a base plate 136 with which the mixing apparatus 130 is attached to the tank 1 10. The drive mechanism 131 is coupled to the base plate 136. The base plate 136 is attached to the tank 1 10 at four attachment points 140 (only two of them are shown). At each of the attachment points 140 the base plate 136 is attached to the tank 1 10 by means of a bolt 150. The base plate 136 has been dimensioned in such a manner that it covers the opening of the tank 1 10 whereby it works as a cover to the tank 1 10. The base plate 136 may be separated from the tank 1 10 in order to facilitate filling and emptying of the tank 1 10.
Two of the attachment points 140 are provided with a load cell 160 which produc- es an electrical signal indicative of a force acting on the attachment point 140. The load cell 160 comprises a through hole, through which the bolt passes. The load cell 160 is arranged directly under the bolt head, that is to say, between the bolt head and the base plate 136.
The drive mechanism 131 comprises a control unit 137 for controlling the opera- tion of the mixing apparatus 130. The control unit 137, which receives the electrical signals from the load cells 160, is configured to calculate the bending moment of the drive shaft 132 from the measured electrical signals. The operation of the drive mechanism 131 may be controlled based on the electrical signal(s) and/or the calculated bending moment. The control unit 137 may, for example, vary the speed of the motor 134 based on the measured electrical signal(s) and/or the calculated bending moment in order prevent to the mixing apparatus 130 from damaging.
Fig. 2a illustrates an example piezoelectric load cell that may be used in a mixing system according to the invention. An exploded view of the piezoelectric load cell is illustrated in fig. 2b.
The piezoelectric load cell 260 of figs. 2a and 2b comprises a first support plate 261 and a second support plate 262 which are fastened together by four bolts 263. The support plates 261 , 262 are perforated by a through hole 264, through which an attachment bolt of a mixing apparatus may be arranged to pass. The piezoelectric load cell 260 comprises a ring-shaped piezoelectric sensing element 265 which is arranged between the first support plate 261 and the second support plate 262. The piezoelectric sensing element 265 is made of polyvinyli- dene fluoride. The first side of the piezoelectric sensing element 265 is provided with a ring-shaped electrode 266. When a force is applied to the piezoelectric load cell 260, the piezoelectric sensing element 265 is strained and due to its piezoelectric properties a voltage is generated between the opposing sides of the piezoelectric sensing element 265.
The piezoelectric load cell 260 further comprises a ring-shaped resilient element 267 which is arranged between the first support plate 261 and the first side of the piezoelectric sensing element 265. The resilient element 267 is made of polydime- thylsiloxane. The resilient element 267 electrically insulates the electrode 266 from the support plates 261 , 262.
The second support plate 262 comprises a circular recess 268 for receiving a se- cond side of the piezoelectric sensing element 265. The purpose of the circular recess 268 is to reduce the compression of the piezoelectric sensing element 265 in an unloaded state of the piezoelectric load cell 260.
The piezoelectric load cell 260 comprises an electrical connector 269 which is connected to the electrode 266 and the second support plate 262. The electrical signal produced by the piezoelectric load cell 260 is measured through the electrical connector 269. The electrical signal provides a voltage difference between the electrode 266 and the second support plate 262.
Only advantageous exemplary embodiments of the invention are described in the figures. It is clear to a person skilled in the art that the invention is not restricted only to the examples presented above, but the invention may vary within the limits of the claims presented hereafter. Some possible embodiments of the invention are described in the dependent claims, and they are not to be considered to restrict the scope of protection of the invention as such.

Claims

Claims
1 . A method for determining a bending moment of a drive shaft of a mixing apparatus which is attached to a tank at a plurality of attachment points, characterised in that the method comprises: - providing at least two of the plurality of attachment points with a load cell,
- measuring an electrical signal produced by each load cell, the electrical signal being indicative of a force acting on the attachment point, and
- calculating the bending moment from the measured electrical signals.
2. The method according to claim 1 , characterised in that at least three of the plu- rality of attachment points is provided with a load cell.
3. The method according to claim 1 or 2, characterised in that the load cell is a piezoelectric load cell.
4. The method according to any of the preceding claims, characterised in that the method comprises controlling the operation of the mixing apparatus based on the calculated bending moment.
5. A mixing system, comprising:
- a tank for holding a fluid, and
- a mixing apparatus attached to the tank at a plurality of attachment points, the mixing apparatus comprising: - a drive mechanism,
- an impeller, and
- a drive shaft, a first end of the drive shaft being coupled to the drive mechanism and a second end of the drive shaft being coupled to the impeller; characterised in that the mixing system comprises:
- at least two load cells, each being arranged in connection with one of the plurality of attachment points and configured to produce an electrical signal which is indicative of a force acting on the attachment point, and - a control unit configured to calculate a bending moment of the drive shaft from the measured electrical signals.
6. The mixing system according to claim 5, characterised in that at least three of the plurality of attachment points is provided with a load cell.
7. The mixing system according to claim 5 or 6, characterised in that the mixing apparatus is attached to the tank at said plurality of attachment points by means of bolts.
8. The mixing system according to claim 7, characterised in that the load cell comprises a through hole, through which the bolt passes.
9. The mixing system according to any of the claims 5-8, characterised in that the mixing apparatus comprises a base plate with which the mixing apparatus is attached to the tank.
10. The mixing system according to claim 9, characterised in that the base plate is rectangular and an attachment point is located at each corner of the base plate.
1 1 . The mixing system according to any of the claims 5-10, characterised in that the mixing apparatus is attached to a cover of the tank.
12. The mixing system according to any of the claims 5-1 1 , characterised in that the control unit is configured to control the operation of the mixing apparatus based on the calculated bending moment.
13. The mixing system according to any of the claims 5-12, characterised in that the load cell is a piezoelectric load cell.
14. The mixing system according to claim 13, characterised in that the piezoelectric load cell comprises:
- a first support plate and a second support plate fastened together, said plates being perforated by a through hole,
- a ring-shaped piezoelectric sensing element arranged between the first support plate and the second support plate, a first side of said piezoelectric sensing element being provided with a ring-shaped electrode, and
- a ring-shaped resilient element arranged between the first support plate and the first side of the piezoelectric sensing element.
15. The mixing system according to claim 14, characterised in that the second support plate comprises a circular recess for receiving a second side of the piezoelectric sensing element.
16. The mixing system according to claim 14 or 15, characterised in that the pie- zoelectric load cell comprises an electrical connector connected to the electrode and the second support plate.
17. The mixing system according to any of the claims 14-16, characterised in that the piezoelectric sensing element is made of polyvinylidene fluoride.
18. The mixing system according to any of the claims 14-17, characterised in that the resilient element is made of polydimethylsiloxane.
PCT/FI2013/050240 2012-03-05 2013-03-05 Method for determining a bending moment of a drive shaft of a mixing apparatus, and a mixing system WO2013132156A1 (en)

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