The present invention relates to a method and an apparatus for directly measuring the mass of an object. Such an object for example could be an object that is weighted while it is moved by a robot from one place to another, or an object suspending in the hook of a hoisting element. Another example of such an object is a material being present in a container, such as the grab of a crane which is applied in transfer of dry bulk goods in a harbour. Examples of such bulk goods are, iron core, scrap, agribulk, coal, mineral products, fertilizer base materials and the like.

It is a requirement in transfer of material from a container that the amounts of transferred product are determined very carefully in view of settlement purposes.

Also for planning purposes in transfer of material from a container (e. g. use of number of people and equipment) it is desirable that at each moment in time the amounts of transferred product are known.

It is an object of the present invention to provide a method for measuring the mass of an object which gives accurate results and actual information on each moment of time.

The invention therefore provides a method for directly measuring the mass of an object comprising the steps of a) exciting the said object with a vibration by applying a non-constant but known force which changes as a function of time, said changing force being applied with a certain frequency; b) measuring the accelerations of the said object which are the results of the said non-constant force- application of step a) and obtaining a plurality of acceleration-measurements; and c) selecting from said plurality of acceleration-measurements the acceleration- component having the said certain frequency and deriving the mass of the said object.

The method of the invention is substantially based upon the use of the well-known force law: F = m. a. wherein F is the force applied on a certain quantity of matter, m is the mass of said matter and a is the acceleration resulting from the applied force.

In a direct measurement of mass the force and the acceleration are measured and the mass is determined as the quotient of these quantities.

In particular, according to the invention, force and acceleration can be considered as a function of frequency, if a changing force having a certain frequency is applied. If no other forces having this frequency are effective, then the acceleration-component having this frequency is merely caused by the force of this frequency. From acceleration- measurements the said acceleration-component is to be selected e. g. by (analogue or digital) filtering and subsequently the mass can be determined. In this manner a measurement can be carried out over a certain time period and has the advantage that its accuracy is enhanced because statistical errors are averaged.

The invention further provides an apparatus for carrying out the said method for directly measuring the mass of an object, comprising means for exciting the object with a vibration by applying a non-constant force which changes as a function of time, said changing force being applied with a certain frequency; means for measuring the accelerations of the said object which are the results of the said non- constant force-application and means for obtaining a plurality of acceleration-measurements; and means for selecting from said plurality of acceleration-measurements the acceleration-component having the same certain frequency and deriving the mass of the said mass of the said object.

The invention will now be described in more detail by way of example by reference to the accompanying drawings, in which

fig. 1 represents the vertical displacement of an object as a result of the changing force applied according to an advantageous embodiment of the invention; and fig. 2 represents the acceleration of the said object as a result of the changing force applied according to an advantageous embodiment of the invention.

Referring now to fig. 1 it is shown that the applied vibrations in fact are superposed on the"normal"accelerations of the object such as gravity and accelerations which are a result of the hoisting process itself and in fig. 1 the vertical displacement (vertical axis) (in mm) is represented as a function of time (horizontal axis) (in seconds).

On the sine-shaped vibration which is related to the natural frequency of the equipment a higher frequency vibration is superposed, which is the result of the excitation applied according to the invention.

In fig. 2 the acceleration (which is the second derivative of the position signal) has been represented. It is appreciated that the high-frequency excitation is shown clearly.

The horizontal axis of fig. 2 represents time (in seconds) whereas the vertical axis represents the grab acceleration (in mm/s2). By application of any filter suitable for the purpose the signal of interest having the correct frequency can be measured.

It is an advantage of the method according to the invention that it is not necessary to measure the force at a plurality of locations.

Advantageously, the mass of the object is determined by the following steps: d) exciting the object with a vibration by applying a non-constant force which changes as a function of time, said changing force being applied with the said predetermined frequency; e) measuring the accelerations of the object which are the results of the said non-constant force-application of step d) and obtaining a plurality of

acceleration-measurements; and f) selecting from said plurality of acceleration-measurements the acceleration-component having the said predetermined frequency and deriving the mass ouf'thé said empty container.

In case of an object being present in a container, the mass m of interest is derived as follows from the plurality of acceleration-measurements: m = ml-m2 = Fln/aln-F2n/a2n wherein ml is the mass of the filled container, m2 is the mass of the empty container, Fln is the force applied with a frequency of n Hz (wherein n > 0) for the filled container, F2n is the force applied with a frequency of n Hz for the empty container and aln and avare the acceleration- components in the direction of the force and having the same frequency of n Hz for the filled container and the empty container respectively. Since Fln, F2n are known and aln, a2n are selected from the plurality of acceleration-measurements, ml and m2 and in this manner m can be determined. When the object is not present in a container, the mass m of the object can be calculated in an analogue way from m = Fn/an, as will be clear to those skilled in the art.

The excitation of the object with a vibration is generated by an actuator. Such an actuator comprises advantageously a rotating mass having an excentric gravity centre is connected to the container and excites the container. More advantageously two such masses are applied in anti-phase to generate a force in one direction.

It will be appreciated by those skilled in the art that any suitable actuators such as electromagnetic actuators, piezo-material based actuators and the like could be used. The actuator has to be coupled to the object with some bias, such a bias for example can be caused by the weight of the object itself. In case of measurement of the

mass of an object in a container, the actuator is advantageously part of the container itself. After having applied the changing force it is only needed to measure the object response with a suitable sensor and to filter the acceleration-component from it, which has the same frequency as the force applied.

The sensor should be sensitive due to the small accelerations of the object. Advantageously e. g. a geophone is used. The sensor advantageously could be located within the housing of the actuator. However, other more suitable positions near the object or on the container may be used.

There is no need for signal processing near the object or on the container itself, the signals obtained e. g. can be transmitted wireless to a receiver that is located near a computer for carrying out the calculations, which computer is at any suitable location.

However, it will be appreciated by those skilled in the art that any location near the object or on the container suitable for the purpose could be used.

Further, it will be appreciated by those skilled in the art that any excitation suitable for the purpose can be applied, e. g. striking the object with a hammer, which generates a jump excitation, which can be considered as a sum of a plurality of sine-shaped wave forms.

It will be appreciated that various modifications of the present invention will be apparent to those skilled in the art from the foregoing description. Such modifications are intended to fall within the scope of the appended claims.