Calibration weight arrangement for an electronic balance

This application is a continuation under 35 USC §120 of PCT/EP2007/056414, filed 27 Jun. 2007, which is in turn entitled to benefit of a right of priority under 35 USC §119 from European patent application 06 116246.7, filed 28 Jun. 2006. The content of each of the applications is incorporated by reference as if fully recited herein.

The invention relates to a calibration weight arrangement for an electronic balance and in particular to a drive mechanism for a calibration weight arrangement.

Electronic balances are in many cases calibrated by means of an internal calibration weight. To perform a calibration, a calibration weight of a defined mass is brought into force-transmitting contact with the force-transmitting mechanism that is arranged in the force-measuring cell of a balance, whereupon a reference weight is determined. Based on this reference value, it is possible to adjust further weighing parameters of the balance. After the calibration has been successfully completed, the contact between the calibration weight and the force-transmitting mechanism is released again and the calibration weight is locked in a rest position. In this process, the calibration weight is moved from a rest position into a calibrating position and back by a transfer mechanism which includes at least one lifting element cooperating with a drive mechanism. In the calibrating position, the calibration weight is in force-transmitting contact with the force-transmitting device, while there is no force-transmitting contact in the rest position.

The known state of the art offers various types of lifting elements and versions of calibration weight arrangements.

A calibration weight which is disclosed in EP 0 468 159 B1 is moved vertically by pairs of wedge blocks sliding horizontally against each other and is thereby brought into force-transmitting contact with the force-transmitting device of the balance. This lifting element is driven by way of a motor and a horizontally oriented spindle which is connected to the wedge blocks.

A device described in EP 0 955 530 A1 likewise effects a vertical lifting and lowering of a calibration weight. The weight rests on a seat which is moved by an electrically driven lifting element.

An arrangement is described in DE 203 18 788 U1, where a monolithically formed calibration weight is lifted and lowered by a ramp-like lifting element, wherein the lifting element is actuated by a linear drive and performs a kind of slanted translatory movement.

In many balances, the calibration weight arrangement and the force-transmitting device are arranged behind one another, as is disclosed in EP 0 955 530 A1. However, the calibration weight can also be split up for example into two calibration weights and can be attached laterally to the force-transmitting device, like the cylindrical calibration weights disclosed in EP 0 789 232 B1. The two identical weights are arranged on two opposite sides of the force-transmitting device. Two different mechanisms for moving the calibration weights are described. In the first case, the calibration weight which is equipped with a guide pin is resting on a calibration weight seat configured as a support. To perform a calibration, the calibration weight seat which is hinged on one side is tilted, whereby the calibration weight is lowered and set onto two calibration weight carriers that are connected to the force-transmitting device and are configured as rods or levers. In a second version, the weight in its rest position is held on a calibration weight seat that is arranged between the calibration weight carriers that are connected to the force-transmitting device. To perform a calibration, the calibration weight is brought into contact with the calibration weight carriers through a vertical downward movement of the calibration weight seat.

A calibration weight arrangement is disclosed in DE 201 19 525 U1 with a lifting device for a calibration mechanism which includes two angled levers with fulcrum mounts fixed in the housing, whose vertical lever arms are coupled to each other by a horizontal slide and on whose horizontal lever arms the calibration weight is seated.

The aforementioned lifting elements are generally driven by servo motors. The disadvantage in using a servo motor is that it uses a comparatively large amount of space in the force-measuring cell of the balance, whereby the force-measuring cell as well as the balance itself is unnecessarily enlarged.

Especially in highly sensitive electronic balances, the weighing result is influenced and even changed by electrostatic charges and interactions. The servo motors which are used to drive the transfer mechanisms contain electrically non-conductive gearbox components which generate electrostatic charges through friction which occurs during operation. The resulting electrostatic fields, but also electromagnetic fields of conventional electric motors, are strong enough to influence the weighing result, in particular in balances of high sensitivity.

Almost always, the calibration weight arrangements of the known state of the art have relatively large drive mechanisms. To make an improvement in the calibration weight arrangement therefore requires in particular an optimization and miniaturization of the drive source of the transfer mechanism. The drive source needs to be very small, compact and flexible to meet different application requirements.

This task is solved by a calibration weight arrangement and by an electronic balance according to the independent claims. The calibration weight arrangement for an electronic balance with a force-transmitting mechanism comprises at least one calibration weight that can be coupled to the force-transmitting mechanism of the balance, and it also comprises a drive source and a transfer mechanism for the guided movement of the calibration weight. The drive source includes an actuator working together with the transfer mechanism and at least one piezoelectric element driving the actuator. The piezoelectric element interacts with a drive wheel which is arranged in the center of the calibration weight arrangement and drives a likewise centrally located shaft, wherein the interaction occurs during the advance in one direction through the repeated engagement and release of a frictional contact force.

An actuator, as the term is used in the present context, encompasses the elements of the drive mechanism which perform a movement, wherein a kinematic behavior of the desired kind and direction often occurs as a result of at least two elements working together.

A calibration weight arrangement which is equipped with a drive mechanism that includes a piezoelectric element has the advantage that only a small amount of space is required to add the drive mechanism to the calibration weight arrangement. The drive mechanism is small and compact and can therefore be placed at any desired location. As a further advantage, the accumulation of electrostatic charges in the drive mechanism or its components is avoided. The drive mechanism further has no magnetic or magnetizable components which could interfere with the operation of a force-measuring cell of a balance that operates according to the principle of electromagnetic force compensation.

The transfer mechanism of the calibration weight arrangement includes a lifting element, a seat for the calibration weight, and a guiding device. The lifting element includes the shaft which produces the advancing movement and a lifting- or guide platform. The latter can be integrally connected with the seat for the calibration weight. One advantage of this arrangement is that due to its rigid connection with the shaft, the seat for the calibration weight cannot tip over. This trait is further enhanced by the guiding device. However, the main function of the guiding device is to prevent a rotation of the lifting- or guide platform during the advance movement.

The effect of this is a guided movement, in particular a vertical movement, of the calibration weight seat and thus also of the calibration weight itself, so that when a calibration is taking place, the calibration weight can be brought into force-transmitting contact with the force-transmitting device of the force-measuring cell of a balance. After the calibration has been successfully completed, this force-transmitting contact has to be released again and the transfer mechanism needs to be returned to its rest position. This task is solved through the especially advantageous way in which the drive mechanism works, as the direction of movement is reversible with this kind of drive mechanism, which means that the upward and downward movements are accomplished with the same elements.