Weigh batching systems for concrete production are not as straight forward as first meets the eye. There is almost no variation in the weighing of the cement as its properties do not change to any real extent and there is no problem weighing it. The dynamics of the aggregate, however, do change from batch to batch. Slight changes in the moisture content alter the flow properties of the material and the results are variations of the amount of material weighed due to variations in the feed rate. The problem is further exacerbated by the fact that, even if accurately weighed, the percentage moisture in the material has altered the quantity of dry aggregate in the scale. This in turn alters the pigment (if used), cement and water requirements further into the process. The user can, to a certain extent, circumvent this problem by designing the batch ratios to cater for average moisture content for that day. As new aggregates are delivered the moisture content will change and new batch designs should be recalculated to maintain the correct ratios. This is impracticable in most cases and does not happen. This is particularly apparent in systems where a star type configuration is used to batch the aggregates. This form of batching almost ensures large variations in the moisture content of the aggregates. To compensate, the cement component is increased to maintain the strength of the batch and the result is an increase in the cement costs, which in turn raises the price to the consumer. If pigment is used for colouring the product, colour variations will be observed from batch to batch.

Our company, Rovinco C.C., has gained valuable experience relating to these plants over a number of years and have systems that can cater for these types of problems in the form of moisture compensation on the aggregates batched.

The System.

The system generally comprises three sections. The hardware, the software and the plant being controlled.

1) The plant being controlled.

The plant generally consists of an aggregate scale with several components, a cement scale with several components, a water scale, a skip and a mixer. The aggregate is normally fed into a scale sequentially by gravity according to a preset recipe. Once complete, the scale is discharged into the skip, which, in turn, is raised to the mixer and discharged. The skip is then lowered to the aggregate scale to collect the next batch. The cement is batched in a similar manner to the aggregates and once the skip has discharged into the mixer, the cement is discharged. If no moisture compensation is fitted, the water scale is batched as per the other scales and will be discharged at the correct time during the process. Pigment is generally added in pre-batched containers. The batch is mixed for a time and then discharged from the mixer.

2) The hardware.

1. A suitable PLC from any supplier.
2. A MMI unit which must be compatible with the PLC system and is generally purchased from the same vendor as the PLC.
3. Sensing devices such as

o        Limit switches or proximity switches purchased from any vendor.

o        Loadcells and amplifiers purchased from any vendor.

o        Microwave moisture sensors from a preferred vendor.

3) The software.

Rovinco C.C. has developed software over a number of years, which can cater for almost any type of batching system. Consider a plant required for concrete tile manufacturing. This type of plant requires two aggregate components, a single cement component and a single water component. The two aggregates are to be blended in a specific ratio and weighed into the aggregate scale. Here the aggregates could be blended mechanically prior to being placed into a holding hopper and batched as a single component, or as individual components into the scale. When batched as individual components, a moisture sensor must be fitted in the aggregate flow of each component. It is highly desirable that the feed mechanisms for the aggregate components be short conveyors as this enables very accurate moisture sensing. If pre-blended, only a single moisture sensor is required. Dosing aids are fitted to cater for any material holdup and are activated should the flow rate fall below a selected value. To achieve accurate batches the feed rate should be able to be altered during the batching process. The cement scale would be similar to the aggregate scale except that moisture sensing is not required and a screw feeder is used to feed the material. The water scale does not normally require a dosing aid but a booster pump could be used for this purpose.

Weighing the cement.

The software will load the required value (a), subtract the coarse to fine change value (b), subtract the material in flight value (c) and both subtract and add the tolerance value (d and e). The software now has five setpoints for the component being batched. A rapid feed now takes place until the point (b) is reached and the feed rate is altered. Feed continues until point (c) is reached and the feed is halted. After the system has determined that the scale has settled, the actual value in the scale is checked against setpoint (a). The difference is added to, or subtracted from the material in flight value, which is stored in the PLC for the next batch. The system now checks the actual value to setpoints (d) and (e). If the cement is under setpoint (d) fine feed will again commence until setpoint (a) is reached. At this point the actual value in the scale is logged within the PLC. If the cement is over setpoint (e) an alarm is generated and further batching into this scale is stopped. The operator must now decide to accept or reject the batch.

 

Example of calculations below.

Weighing the aggregate with moisture compensation.

The software will load the required value (a), subtract the coarse to fine change value (b), subtract the material in flight value (c) and both subtract and add the tolerance value (d and e). The software now has five setpoints for the component being batched. A rapid feed now takes place. While feeding, the moisture content of the aggregate is measured and the average moisture is computed in the probe or amplifier. This value is input to the PLC and based on the value all the setpoints are continually recalculated. Feed continues until the point (b) is reached and the feed rate is altered. Feed continues until point (c) is reached and the feed is halted. After the system has determined that the scale has settled, the actual value in the scale is checked against setpoint (a). The difference is added to, or subtracted from the material in flight value, which is stored in the PLC for the next batch. The system now checks the actual value to setpoints (d) and (e). If the aggregate is under setpoint (d) fine feed will again commence until setpoint (a) is reached. At this point the actual value in the scale is logged within the PLC. If the aggregate is over setpoint (e) an alarm is generated and further batching into this scale is stopped. The operator must now decide to accept or reject the batch. The difference between the material batched and that required by the recipe is the water already in the sand and this value is subtracted from the required value for the water scale (g).

Weighing the water with moisture compensation.

The software will load the required value (a), subtract the value batched into the aggregate scale (f) and use the result (g) to calculate the water scale setpoints as described above. Batching the water is the same as the cement batching.

 

Features

The software contains the following,

1. Automatic zero taring at the onset of batching each component.
2. Automatic material in-flight correction adjusted after each component feed.
3. Automatic moisture compensation.
4. Coarse and fine feeding.
5. Tolerance control.
6. Sequence control of the mixing process.
7. Scale empty control.
8. Recipe storage for 5 recipes. More on request.
9. Operator interface for all plant parameters and variables.
10. Manual override for all functions.