Precise measurement of bulk materials

Bulk materials are, more or less, finely powdered solids with a certain ability to flow. They are stored and handled in many fields of technology. However, their tendency to form inclined surfaces or solidify over time – even at rest – can lead to the formation of bridges or shafts which can hamper (homogenous) discharge from the silo. What is more, collapsing bridges in the silo can even lead to the destruction of the container. To design the silo or pipelines appropriately, the designer has to depend on well-grounded data regarding the flowability of the respective material.

Even in the era of computer-supported calculation processes, results are only as good as the data on which they are based. Established measurement values are therefore essential, especially in face of the countless different bulk materials with what may be extremely different fluidity properties. A new method for determining the material properties can now provide assistance. The ring shear tester developed by Dr.-Ing. Dietmar Schulze Schüttgutmesstechnik relies on a process that is largely oriented on the standards of the theoretical basics of bulk materials. Electronically commutated DC servomotors with an integrated motion controller from the miniature drive spe-cialist FAULHABER ensure precise mechan-ical implementation of the control specification in the measurement instrument.

Automatically reproducible measurement values

Current measurement processes for determining the properties of bulk materials are based on the measurement of angles of slope. Depending on the system used, this can result in (widely) differing values. A comparison of the data is therefore not possible but still provides good guidelines for assessing loads in practice. In contrast, the new method relies on sample prep-aration that is practically independent of the operator. The actual measurement then runs fully automatically and is easy to reproduce. For this, a material sample is filled into a circular shear cell and the surface is smoothed off. A cover ring, toothed on the lower surface, now presses on the sample in the rotatable bearing shear cell from above. This so-called normal tension, which is computer-controlled, is sensitively applied up to 20 kPa by an EC servomotor. The ring shear cell is then also driven by an EC motor with digital control in the direction of rotation and speed. The fixed cover on which the normal force acts is fixed via a beam system. In this way the shearing forces that occur in the sample can be deter-mined with great precision. By varying the normal force and determining the associated shear forces, this provides a precise picture of the bulk material properties. Depending on the sample material, shear cells of 9 to 70 ml can be used with the same measurement tester. The servomotors supply the required drive power for this.

Focus on sustainable precision

Any precision measurement instrument naturally places exceptional demands on the drive. As well as excellent control of the RPM and power, a long service life is also important – with excellent long-term stability. Developer Dietmar Schulze (Dr. Ing.) stresses an additional point: "Measurement instruments of this type are only manufactured in relatively low numbers. It is essential that the drive can be quickly and easily incorporated in the instrument. Thanks to the integrated motion controller, the servomotor is quickly connected to the internal computer of the instrument. Separate devel-opment is no longer required for the controller. The compact dimensions and sensitive control of the sine-wave motor plus the well-graduated gears available for this solution are further advantages for the deployment of the measurement instrument."
In the case in hand, the RST-XS measurement instrument is equipped with two EC motors. One takes over control of the rotational movement of the shear measurement cell, the other is responsible for setting the normal force. With a diameter of only 35 mm and a length of 64 mm, the motors generate an output power of up to more than 90 W. Thanks to sinusoidal commutation, the drives can be sensitively controlled in compliance with application requirements in the RPM range of 5,000 to 10,000 rpm – an essential prerequisite for this application. As the EC motors are not subject to wear (except for the bearings), the reproducible long-term stability of the measurements is assured. Integrated Hall sensors allow precise positioning and the motion controller takes on communication with the instrument computer. This relieves the computer of the actual motor control, allowing it to process signals already evaluated. State-of-the-art EC motors with integrated control electronics are suitable for a wide variety of tasks – no matter whether high-precision positioning and superior long-term stability is required or it’s a question of extended full-load service life for deployment in automation technology. Thanks to the electronics, integration in existing control environments is simple and special know-how of drive technology is not required, thus reducing the development costs and shortening the "time to market".

<b>Measurement method using the ring shear tester</b>
Closely related to the theoretical basics of the mathematical con-sideration of bulk materials is the measurement with a ring shear tester. Here the sample is preloaded vertically with a force and then
exposed to a shear deformation by turning the measurement cell. During this process, the force required for shear deformation is measured. As all parameters are specified to be reproducible, the values that are obtained are also reproducible and can be used to calculate the stiffness of the silo hopper walls as well as the diameter of the discharge outlet in order to avoid the formation of bridges.