FENTON RICHARD STANLEY MICHAEL (NZ)
| WO1989002701A1 | 1989-04-06 |
| WHAT WE CLAIM IS:
1. A method for mixing dough in a dough mixer comprising:
(a) mixing the dough mixture in a mixing bowl of a dough mixer comprising at least one mixing element driven via a variable speed electric motor, said mixing element transferring energy to the dough mixture;
(b) during the mixing measuring an instantaneous energy value transferred to the dough mixture;
(c) during the mixing comparing the instantaneous energy value with a desired energy value to obtain an energy deviation value; and
(d) varying the speed of the variable speed electric motor in response to the energy deviation value to maintain the instantaneous energy value to be transferred to the dough mixture at a substantially constant value.
2. A method for mixing dough according to claim 1 wherein the instantaneous energy value is an instantaneous current value drawn by the variable speed motor.
3. A method for mixing dough according to claim 1 or claim 2 wherein the desired energy value is a desired current value.
4. A method for mixing dough according to any one of claims 1 to 3 wherein the energy deviation value is a current deviation value. 5. A dough mixer comprising: a mixing bowl for receiving a dough mixture; a variable speed electric motor to drive a mixing element; at least one mixing element for transferring energy from the variable speed electric motor to the dough mixture; and a control system adapted to control said variable speed electric motor such that the energy transferred to the dough mixture during mixing remains substantially constant. 6. A dough mixer according to claim 5 wherein the control system comprises a control circuit for:
(a) ascertaining an instantaneous energy value transferred to the dough mixture; (b) comparing the instantaneous energy value with a desired energy value to obtain an energy deviation value; and
(c) selectively varying the speed of the variable speed electric motor in response to the energy deviation value to substantially maintain the instantaneous energy value equal to the desired energy value.
7. A dough mixer according to claim 5 wherein the control circuit further measures force or torque on the mixing element.
8. A dough mixer according to claim 5 or claim 6 wherein the dough mixer includes a display for displaying the speed and/or current drawn by the variable speed electric motor, said display being controlled by said control circuit. |
"DOUGH MIXER AND METHOD FOR CONTROLLING A DOUGH MIXER"
FIELD OF THE INVENTION
This invention relates to a dough mixer and a method for controlling a dough mixer.
BACKGROUND OF THE INVENTION
It is known to use dough mixers for mixing or kneading dough used in food such as bread products, biscuits, or pastry. The dough is typically made from a combination of dry ingredients such as flour, salt, sugar, and leavening agents, and liquid ingredients, such as water, and oil.
A typical mixer has a mixing bowl, a mixing or kneading tool that is rotatable within the mixing bowl, and an electric motor for driving the mixing tool. The dry and liquid ingredients are added to the mixing bowl and the mixing elements mix or knead the ingredients to form dough. It is bakery industry practice to use a fixed speed motor to impart the required amount of mechanical work or energy to the dough mixture to achieve the necessary rheological qualities of the dough. The optimum work input levels for dough are typically from about 8 to about 16 Watt hours per kilogram of dough.
With a fixed speed motor, the dough rheology and erratic movement of the dough during mixing cause inconsistencies in the loading of the motor and inconsistencies in the transfer of mechanical work. As mixing and/or kneading occur, the dough begins to develop and the torque on the mixing tool increases. Also, the dough will often move out of a high energy zone around the mixing tool to areas of lower energy that will decrease the torque on the tool. It is an object of at least preferred embodiments of the invention to provide an improved dough mixer and method of controlling a dough mixer and/or to at least provide the public with a useful alternative.
SUMMARY OF THE INVENTION In one aspect, the invention consists in a method for mixing dough in a dough mixer, comprising:
(a) mixing the dough mixture in a mixing bowl of a dough mixer comprising at least one mixing element driven via a variable speed electric motor, said mixing element for transferring energy to the dough mixture;
(b) during the mixing measuring an instantaneous energy value transferred to the dough mixture;
(c) during the mixing comparing the instantaneous energy value with a desired energy value to obtain an energy deviation value; and (d) varying the speed of die variable speed electric motor in response to the energy deviation value to maintain the instantaneous energy value to be transferred to die dough mixture at a substantially constant value.
Preferably, die instantaneous energy value is an instantaneous current value drawn by the variable speed motor. Preferably, the desired energy value is a desired current value.
Preferably, die energy deviation value is a current deviation value. In anodier aspect, die invention consists in a dough mixer comprising: a mixing bowl for receiving a dough mixture; a variable speed electric motor to drive a mixing element; at least one mixing element for transferring energy from the variable speed electric motor to die dough mixture; and a control system adapted to control said variable speed electric motor such that die energy transferred to die dough mixture during mixing remains substantially constant.
Preferably, die control system comprises a control circuit for: (a) ascertaining an instantaneous energy value transferred to die dough mixture;
(b) comparing die instantaneous energy value widi a desired energy value to obtain an energy deviation value; and
(c) selectively varying die speed of die variable speed electric motor in response to the energy deviation value to substantially maintain die instantaneous energy value equal to die desired energy value.
Preferably, die control circuit furdier measures force or torque on die mixing element. Preferably, the dough mixer includes a display for displaying die speed and/or current drawn by die variable speed electric motor, said display being controlled by said control circuit. It has been found diat inconsistencies in die loading of die motor and die transfer of mechanical work leads to longer time periods required to reach optimum work input levels. Widi the dough mixer and mediod of die invention, die optimum time to mix dough can be reduced relative to die mixing time widi a dough mixer comprising a fixed speed motor.
To diose skilled in die art to which die invention relates, many changes in construction and widely differing embodiments and applications of die invention will suggest diemselves widiout departing from die scope of die invention as defined in die appended claims. The
disclosures and the descriptions herein ate purely illustrative and are not intended to be in any sense limiting.
The term 'comprising' as used in this specification and claims means 'consisting at least in part of, that is to say when interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present.
The invention consists in the foregoing and also envisages constructions of which the following gives examples only.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described with reference to the accompanying drawings in which:
Figure 1 shows a flow chart of a method for controlling a dough mixer.
Figure 2 shows a graphical output of tests undertaken using an industrial dough mixer using a fixed speed electric motor.
Figure 3 shows a graphical output of tests undertaken using an industrial dough mixer using a variable speed electric motor.
DETAILED DESCRIPTION A preferred embodiment dough mixer includes a mixing bowl or chamber into which a number of ingredients used to form a dough are placed. In this specification "bowl" understood as including any vessel or container within which dough may be mixed. Rotatably mounted above the mixing bowl and extending into the mixing bowl is a dough arm having a mixing or kneading tool attached that is used to mix the ingredients placed in the mixing bowl together. The dough arm and mixing tool is driven by a variable speed electric motor which rotates the dough arm in a substantially circular pattern around the mixing bowl to mix the dough ingredients and form dough. A controller interfaced with the variable speed electric motor is programmed to drive the variable speed electric motor within certain operating parameters that will be discussed later. The dough mixer and control system is used for mixing, kneading, or mixing and kneading dough. The dough mixer and control system is used in commercial bakery applications. Alternatively, the dough mixer and control system can be used in other bakery applications.
The accompanying Figure 1 illustrates the dough mixer and method for controlling the dough mixer 7 via a feedback loop 1. As an initial step, the electronic motor controller 6 is set up with a number of operating parameters one of which will include the instantaneous energy level
that is to be trans ferred to the dough. It is known that the torque of the mixing tool is directly proportional to mixer motor current 3 drawn by the mixer motor 2. In the preferred embodiment, the instantaneous current drawn by the motor 2 is measured which gives an instantaneous energy value. Alternatively, the instantaneous energy value can be measured or determined by measuring the current supplied to the motor 2. This is achieved, by configuring one of the closed loop controller's 4 variable speed drive analogue outputs to represent the motor current 3. Alternatively, this can be achieved by measuring the force exerted on the mixing tool, for example by using strain gauge attached to the mixing tool.
The instantaneous current value is continuously compared with a desired current value or a specific pre-defined set point, to obtain a current deviation value. The speed of the motor 2 is varied in response to the current deviation value to maintain the instantaneous current value provided by the motor 2 at a substantially constant level. If the instantaneous motor current value is higher than the pre-defined set point, the controller 4 will output a motor drive signal 5 to the electronic motor controller 6 which then drives the motor speed to be decreased until the instantaneous motor current value becomes equal to the pre-defined set point. If the instantaneous motor current value is lower than the pre-defined set point, the controller 4 will output a motor drive signal 5 that causes the motor speed to be increased until the instantaneous motor current value becomes equal to the pre-defined set point.
A programmable logic controller (PLC) 4 is used to achieve either proportional-integral- derivative control (PID) or a simple tracking algorithm.
A PID controller 4 calculates a motor drive signal 5 control output with high precision but considering fast changes in the process dynamics and a short time period within which the mixing process must be complete, a smooth control is very hard to achieve using a PID type controller 4. Tracking control operates by continuously monitoring the motor 2 using either a current transformer-current transducer arrangement or variable speed drive data configured in a way acceptable to be processed by PLC 4.
Operation The dough is formed by adding dry and liquid ingredients in required amounts into the mixing bowl. The mixing tool rotates repeatedly within the mixing bowl so that the ingredients mix together and form a mass of dough. During the mixing process, the dough begins to develop and the torque on the mixing tool increases. Also, during the dough development process, the dough will often move between a high energy zone around the mixing tool to lower energy zones away from the mixing tool. As the dough moves between these areas, the torque
experienced by the mixing tool will decrease and increase in an inconsistent manner. The preferred embodiment dough mixer and method of controlling the dough mixer ensures that the energy transferred to the dough remains substantially constant and equal to the desired value. This will reduce the time required to mix the dough to the required rheology. The preferred embodiment dough mixer and method for controlling the dough mixer 7 bring the mixing time required by some mixing processes below the ideal three minute mix time, as shown in the following trial results. This results in higher water absorption and a reduction in the quantities of other ingredients such as yeast and dough improvers plus a reduction of finished dough temperature. The invention is further illustrated by the following description of trial results.
Trials
The dough mixer 7 used for the first test was equipped with 22kW, 42.5 A FLC motor controlled via a DANFOSS VLT 5032 variable speed drive. The drive via a serial communication port was connected to a computer for data acquisition and monitoring of the mixing process. A temperature probe was inserted into the mixer bowl to provide the dough temperature readings.
A force-measuring probe was also inserted into the mixer bowl to monitor the forces applied by the mixing tool on the dough during the mixing process.
An ECS kilowatt-hours controller was incorporated to provide an automatic vacuum and mixing control. A digital speed display and an analogue ammeter (not shown) were used to display the motor speed and a current drawn.
A first set of tests used the proportional-integral-derivative control built in the variable speed drive motor. The drive was set up as follows: analogue output at terminal 42 was configured to 0-2OmA /0- lmax (0-70.4A) analogue input at terminal 60 was configured to 0-2OmAIO- 60Hz parameter 215 'Preset reference 1" — set point, 42.6% of Full Load Current (30A) parameter 314 "Analogue Input on terminal 60" — FEEDBACK parameter 319 "Analogue Output on terminal 42" - 0-2OmA /O-Imax (0-70.4A) parameter 414 "Minimum Feedback" — 0 parameter 415 "Maximum Feedback" — 70.4 parameter 416 "Reference/Feedback Unit" - % parameter 437 "Process PID" — NORMAL [0] parameter 440 "Process PID Proportional Gain" — 0.8 parameter 441 "Process PID Integral Time" — 5 sec parameter 442 "Process PID Differentiation Time" — 0 sec
parameter 444 "Process PID Lowpass Filter Time" — 0.1 sec Parameter 455 "Frequency Range Monitor" — DISABLE [O].
This setup achieved a substantially constant current value, but tuning of the PID loop was not easily achieved due to the structure of parameters of the variable speed drive. A decision was made to use a SHIMADEN SR9I PID controller with readily accessible parameters and a inbuilt AUTO TUNE function. An output of the RPA series current transducer was used as a process variable for the controller. The transducer input was connected to the existing 60/5A current transformer installed to the red phase on the variable speed drive power supply. The current reading on the variable speed drive input was around 6A which corresponded with the motor current of approximately 2OA.
The SHIMADEN controller was set up as follows:
Set Point 6A
Proportional Gain 50
Integral Time 3 Differentiation Time Off
Parameter 1-6 44.0
Parameter 1-7 0.40
Parameter 1-8 0
Parameter 1-9 100% Parameter 1-20 Hd
Parameter 1-21 0.5
Parameter 1-22 1
Parameter 1-23 Id
Parameter 1-24 0.5 Parameter 1-25 1
Parameter 1-42 rA
Parameter 1-43 Off
Parameter 1-44 0.0
Parameter 1-45 60.0 Parameter 1-46 0
Parameter 1-47 0
Parameter 1-48 92
Parameter 1-50 0
Parameter 1-51 60 Parameter 1-52 0.0
Two 20 kg doughs were mixed with the above setup. The kW hour controller was set at 16W/kg work input. A reasonably smooth control was achieved at the beginning of the mixing cycle, current deviation from the set point was +I-0.5A and the motor speeded up and slowed down between the range of 50Hz to 55Hz. At the last stage of the dough mixing cycle, current deviation from the set point was + 1-2.5 to 3A. This round of tests proved that it was possible to control the motor load current by changing the motor speed.
The following changes were made as a result of the first round of tests: The dough size was increased to 40kg in order to increase the motor loading for better and more conclusive observation and control. The set up of the controller remained unchanged.
It was determined that driving the motor 2 by monitoring the current drawn by the motor 2 as opposed to the supply current to the variable speed motor drive will provide more accurate control of the motor output. The current transformer and the current transducer were moved to the variable speed drive output. Alternatively, one of the variable speed drive analogue outputs can be configured to represent the motor current and connected to the SHIMADEN controller input as a process variable.
The second round of tests started with an automatic adaptation of the motor 2 (parameter 1-7) to improve the drive and the motor performance. Some doubts arose about the performance of the kWatt hours controller and a decision was made to use computer data and manual mixer controls.
The dough mixer 7 was started by a user pushing "START" button. The work input figure was monitored and the dough mixer stopped when the work input reached a desirable level, the time in which the dough mixer was monitored and noted. A work input of 16 Wh/kg was selected for all the test mixes in order to extend the mixing time.
The recipe selected for the test mixes was as follows: Flour 24kg
Water 61% (14.64kg) at 14.7°C Salt 2% (0.48kg) Total 39.12kg
Two control doughs were mixed with the following results recorded:
Hence, with the motor 2 running at a fixed motor speed of 50Hz, during the dough mixing process the current drawn by the motor 2 varied from between 2OA to 25A due to the increased resistance of the dough as it formed on the mixing tool. Hence, the energy level at the mixing tool decreased while the motor speed remained constant.
A further dough was mixed widi the motor 2 running at a fixed speed of 60Hz with the following results recorded.
TRIAL DOUGHS 16WhIkg
Mix Time Motor Current, A Motor Speed, Hz Temperature, 0 C
Dough #1 2mm 34sec 25-32 60 34.3 39.12kg
With the motor 2 running at a higher speed, increase current was drawn by the motor 2. As the dough formed the current drawn varied from between 25A to 32A as the dough resistance increased. When compared to die motor 2 running at 50Hz, the energy being supplied to the mixing tool was greater at 60Hz dian that being supplied to the mixing tool at a slower motor speed of 50Hz. As a result, it can be seen from the tables above that there is a substantial reduction in the time taken to mix the dough (35% reduction in mix time) if higher motor speed and hence an increased amount of energy being input to die mixing tool.
It is desirable to drive the mixing tool to ensure the energy input to the mixing tool remains substantially constant throughout the dough mixing process. Hence, the amount of current drawn by the motor 2 needs to be varied to compensate for the increased resistance developed at the mixing tool as the dough is formed. By maintaining a substantially constant energy level input to the mixing tool will reduce the time taken to mix the dough to the required
rheology. As will be seen by the results below, this is achievable using a variable speed electric motor 2.
As the instantaneous drive current provides an indication of the instantaneous energy at the mixing tool, a high instantaneous current equates to a high resistance provided by the dough which causes the motor speed to decrease.
The drive's analogue output was configured to represent the motor current. The SHIMADEN controller was set up as recorded above with a set point of 3OA. Two doughs were mixed with the above setup with the following results recorded:
Hence, the power of the motor 2 is used to provide a constant energy input to the mixing tool to mix the dough regardless of the dough batch size.
The drive's analogue output was configured to represent the motor kW. The SHIMADEN controller was set up as recorded above with the set point of 15kW.
Two doughs were mixed with the above setup with the following results recorded:
The above tests were undertaken using small dough mix batch sizes. Industrial dough mixers generally mix dough batches that weigh in the order of 250kg. For batches of this size using large dough mixing machines in an industrial environment, it has been found to it is not viable to use motors that run at a high speed to effect the reduced dough mix time as provided by the present invention.
Figures 2 and 3 shows the results of tests undertaken using industrial size dough mixers using a dough mix batch size of 256.4kg. Figure 2 is a graphical output of motor current output over time using a fixed speed electric motor while Figure 3 is a graphical output of motor current output over time using a variable speed electric motor. On comparing the two outputs, along with reviewing test data 10, 11, it can be seen that the mix time reduced from 232.73 seconds using a fixed speed motor down to 166.24 seconds using a variable speed motor. As such the current drawn by the motor averaged 75A for a fixed speed motor. However, the current drawn using a variable speed motor has increased to approximately 95A. Hence, as instantaneous current equates to the instantaneous energy input to the mixer tool, it is evident that more energy is being provided to the mixer tool using a variable speed motor as compared to a fixed speed motor. Furthermore, the time taken to mix the dough has been substantially reduced when a variable speed motor drive is used. Hence, using a variable speed electric motor as provided by the present invention provides a viable system and method for reducing dough mix times and as such provide a means of increasing production.
Preferred embodiments of the invention have been described by way of example only and modifications may be made thereto without departing from the scope of the invention.