GB2171326A - Controlling mixing of slurries by power consumption - Google Patents

Abstract

A method of mixing slurry, e.g. concrete, comprises mixing at least one of the ingredients using a motor, and adding at least one other ingredient, e.g. water, in dependence upon the power required by the motor. The motor may be an electric motor, and valves controlling the flow of water may be controlled by a microprocessor connected to means for sensing the current drawn by the motor.

Description

SPECIFICATION Mixing methods and apparatus The invention relates to mixing methods and particularly to methods of mixing materials the consistency or viscosity of which varies dependent upon the relative quantities of ingredients present in the mixture, for example slurries. All such materials will be referred to hereinafter as "slurries".

The invention also relates to apparatus for mixing slurries.

Considering one slurry in particular, namely concrete, problems arise in the mixing of concrete to a desired consistency in automatic machinery. It is easy to arrange for predetermined quantities of cement, aggregate, sand and water to be mixed together, but the initial moisture content of the sand and aggregate may vary, particularly if the sand and aggregate are stored outside where they may be exposed to rain. Consequently, if batches of concrete are to be produced having a consistently repeatable consistency, some method must be found of adjusting the amount of water which is added to the mixture in dependence upon the initial moisture content of the other ingredients.

The known method of dealing with this problem involves measuring or calibrating the initial moisture content of the solid ingredients and reducing accordingly the quantity of water which is added.

This method is complicated and not entirely accurate.

We have realised that if the mixture is mixed using a motor, the power required by the motor will vary in dependence upon the consistency of the mixture. Clearly the mixing of a thin, watery mixture will require less power than the mixing of a thick viscous mixture.

Accordingly the invention provides a method of mixing a slurry comprising mixing at least one of the ingredients using a motor, and adding at least one other ingredient in dependence upon the power required by the motor.

The said other ingredient will normally be a liquid, e.g. water, and it is preferred that at least one valve is provided to control the flow of liquid into the mixture, the valve being controlled by control means which monitors the power required by the motor.

There may be two valves, the first arranged to admit liquid to the mixture at a lower rate than the second, so that the bulk of the liquid can be added relatively quickly using the second valve or both valves, but fine control of the total quantity of liquid added can be finally completed by using the first valve only.

The purpose of the method, particularly when mixing batches of concrete, is to be able to ensure that each batch has the desired consistency. However the physical parameters of mixing apparatus can vary, particularly when mixing abrasive material such as concrete. A variation of the physical parameters may, itself, bring about a variation in the power required by the motor.

Consequently, a preferred method comprises sensing at periodic intervals, e.g. before the mixing of each batch, the power required by the motor when no mixture is present, and using this information for calibration or re-setting purposes.

Preferably the motor is an electric motor.

Preferably the control means comprises a microprocessor.

The invention includes apparatus for carrying out the method.

The invention includes the products of the method and apparatus according to the invention.

By way of example, a specific embodiment of the invention will now be described, with reference to the single accompanying Figure, which comprises a graph showing the variation of the current of a mixer motor during a mixing cycle according to the invention.

The embodiment of the invention will be described by particular reference to the mixing of concrete. Mixing apparatus is used which comprises a container for the ingredients, means, e.g. a hopper, for feeding a measured charge of solid ingredients to the container, a mixing device driven by an electric motor, and means for eventually discharging the mix from the container. The apparatus described thus far is conventional and will not therefore be described in detail.

In addition, two pipes are provided for supplying water to the container, one pipe being controlled by a first valve and the other pipe being controlled by a second valve. The valves can be opened or closed under the control of a microprocessor. The first valve admits water to the container at a substantially lower rate than the second valve.

The microprocessor, in addition to being connected to the valves, is connected to means which senses the current drawn by the electric motor.

The microprocessor is also connected to the hopper or other means for supplying a weighed batch of solid ingredients.

A mixing cycle will now be described in detail, with reference to the Figure.

Batches of concrete are prepared fairly rapidly, say every 45 seconds, and so the mixer motor is run continuously and is not switched off between batches.

When a batch of solid ingredients has been weighed and is ready for discharge into the container, an electrical signal is passed to the microprocessor. The microprocessor then monitors the current drawn by the motor with no ingredients present. The current drawn by the motor at this stage is represented by the point 10 on the graph.

The vertical axis of the graph measures motor current and the horizontal axis measures time. Based upon this level of current represented by the point 10 the microprocessor calculates a switching level 11 for the first valve. The switching level 11 comprises the current 10 when empty plus a predetermined percentage of the current when empty. It may be desired to vary the percentage so that different consistencies of concrete can be produced for different purposes and so an appropriate control device is provided, for example a thumbwheel operated control, the thumbwheel bearing the nu merals 1 to 100. Thus if the thumbwheel is set at 50 the switching level 11 will be 150% of the current when empty.

The microprocessor also calculates a switching level 12 for the second valve. The level 12 comprises the level 11 plus a predetermined percentage of the current when empty. This percentage is again individually controllable by means of a thumbwheel operated control device.

It may be that some slight residue from a previous batch, for example clinging to a mixer blade, may cause an initial false current reading. Consequently the microprocessor is arranged to take several readings, for example three readings over a set period of time, for example one second.

These readings are then averaged and compared with the current when empty which was recorded when mixing the last batch. If the average reading is within a predetermined percentage of the reading for the last batch then the reading is accepted by the microprocessor as an accurate reading. If however the average reading is outside the previous reading by a predetermined percentage then the microprocessor takes a fresh set of readings.

Once the microprocessor has accepted a reading for the current when empty and has calculated the switching levels 11 and 12, the microprocessor transmits a signal to the hopper or like apparatus causing the discharge of the solid ingredients into the container. If desired this signal may be used to generate a visual or audible warning to a supervisor of the apparatus.

As the solid ingredients enter the mixer the current taken by the motor will rise, as indicated by the solid line 13 on the graph. When the current rises to the switching level 11 the microprocessor causes the first valve to be switched on and water starts to flow at a relatively low rate into the container to moisten the ingredients. When the current rises further to the switching level 12 the second valve is opened so that approximately the desired quantity of water can be fed to the container as quickly as possible with a view to increasing production rates. Eventually the quantity of water added is such that the consistency or viscosity of the mixture starts to drop and so the current drawn by the motor also drops. When the current drops to the switching level 12 the second valve is shut off.However a small quantity of water continues to be fed into the mixture by the first valve until the current drops to level 11 whereupon the first valve is also shut off.

If only one valve were used, providing a large throughput of water with a view to increasing production time, it would be difficult to control the quantity of water added with any great accuracy.

Since valves have a finite closing time a significant quantity of water could pass through the valve during the time that the valve is signalled to close and the time when it finally closes. Since however the final quantity of water is provided only by the first valve, and the flow through this valve is small, very little error will result from any time delay in the closing of this valve.

The required consistency or "stirrability" will be reached shortly after the first valve has closed, at the point 14 on the graph. The time required for the desired stirrability to be reached after the valves have been closed can be predetermined and programmed into the microprocessor. Thus at the point 14 the microprocessor sends a signal to indicate that the mixture is ready for discharge from the container. As the container discharges the current drawn by the motor gradually drops until it reaches the current level when empty, at the point 15.

Since switching level 12 comprises switching level 11 plus a predetermined percentage of the current when empty, there is no possibility of the second valve being switched on before the first valve, or being switched off after the first valve.

The flow of water through the first valve is chosen by trial and error to give a rate of addition of water that can be absorbed into the mixture without creating any ponding on top of the mixture.

The flow through the second valve is chosen to give an acceptable mixing time provided that the water added through the second valve must be fully absorbed before the first valve is turned off. If so much water is being added through the second valve that it cannot be absorbed before the first valve is turned off then either the flow rate through the second valve should be reduced or the thumbwheel setting for the second valve should be increased thus raising the switching level 12 and causing the second valve to be closed earlier.

The invention is not restricted to the details of the foregoing embodiments. For example although the invention has been described with particular reference to concrete, the invention is equally applicable to the mixing of any materials the consistency or viscosity of which varies dependent upon the relative quantities of ingredients present in the mixture. The invention may for example be applicable to the mixing of clays for use in pottery making, or the making of paints.

It is not essential that an electric motor be used.

A hydraulic motor can for example be used, with a transducer to sense the pressure of the hydraulic supply and convert it to an electrical signal.

Claims (10)

1. A method of mixing a slurry comprising mixing at least one of the ingredients using a motor, and adding at least one other ingredient in dependence upon the power required by the motor.

2. A method as claimed in claim 1, in which one of the ingredients is a liquid, at least one valve being provided to control the flow of liquid into the mixture, the valve being controlled by control means which monitors the power required by the motor.

3. A method as claimed in claim 2, in which two valves are provided, the first arranged to admit liquid to the mixture at a lower rate than the second, so that the bulk of the liquid can be added relatively quickly using the second valve or both valves, but fine control of the total quantity of liquid added can be finally completed by using the first valve only.

4. A method as claimed in any one of the preceding claims, in which, at periodic intervals, the power required by the motor when no mixture is present is sensed, and this information is then used for calibration or resetting purposes.

5. A method as claimed in any one of the preceding claims, in which an electric motor is used.

6. A method as claimed in claim 5, in which the electric motor is controlled by a microprocessor.

7. A method of mixing a slurry substantially as hereinbefore described, with reference to the accompanying drawing.

8. Apparatus for mixing a slurry, comprising a motor for mixing at least one of the ingredents, means to detect the power required by the motor, and means to add at least one other ingredient in dependence upon the power required by the motor.

9. Apparatus for mixing a slurry, constructed and arranged substantially as hereinbefore described, with reference to the accompanying drawing.

10. A product when produced from a slurry mixed by the method claimed in any one of claims 1 to 7 or by the apparatus claimed in claim 8 or claim 9.