Injection devices are used in broad areas of medicine for administering a medical or pharmaceutical fluid product. Injection apparatuses, such as injection pens, may be used for dispensing insulin, hormone preparations and the like. An injection apparatus comprises various mechanical means, such as an administering or dosing means, in order to precisely dispense a particular product dosage from the apparatus. To monitor the administering process and its accuracy, sensors or probes may be arranged within the apparatus to detect movement of various elements of the mechanical means. From these sensors, the setting of the mechanical means may be measured ascertained, for example, by a microprocessor and may be indicated on the injection apparatus by a mechanical or electronic display.
Because mechanical measurements are susceptible to contamination, moisture and wear, and exhibit large tolerances between the various apparatus components, which affects the accuracy of measurements, non-contact methods for ascertaining settings have been developed. To this end, a number of sensors and measuring devices are arranged at various locations on the apparatus that are suitable for measuring the desired setting without the measured elements coming into contact with the measuring devices or sensors.
An electronic medical administering pen is described in EP 1095668 A1, which measures settings of the administering means by determining the linear position of a helical rod of the administering mechanism or the rotational position of a setting button of a dosing means. To this end, an optical code converter comprising a code disc coupled to the rotational movement of the setting button is provided. The rotational movement of the code disc is measured by an optical receiver. A microprocessor converts the number of rotations by the code disc into a dosage amount corresponding to the setting. Another sensor is provided between the windings of the helical rod of the administering means and registers the movement in the longitudinal direction along the longitudinal axis of the pen. The administered amount of a product is determined from the shift of the helical rod. The two sensors operate independently of each other and determine only one movement direction of a mechanical means of the pen each.
While such contactless measuring means can increase the accuracy in measuring a setting as compared to mechanical scanning, the arrangement of the individual parts of such a measuring means within the apparatus is often complex, such that manufacturing the apparatus is difficult and expensive. In addition, the circuitry of these measuring means are susceptible to moisture, vibrations and other such effects. Accommodating the individual parts of the measuring means, such as the sensors and the counter pieces for the sensors, often requires structural changes in the injection apparatus, making it unnecessarily large or even restricting the other mechanical operations of the apparatus.
Another injection apparatus, which is described in WO 02/064196 A1, is controlled by a closed switch unit comprising integrated sensors that monitor selected parameters of the apparatus. The closed switch unit is fixedly arranged within the injection apparatus. At least two pairs of integrated Hall elements are used as the sensors. The Hall elements cooperate with a magnetised ring that alternately exhibits north and south poles. The ring is arranged within a dosing means and is moved around the longitudinal axis of the injection apparatus in accordance with a rotational movement for setting a product dosage. In order to measure the volume of a dosage setting, it is necessary to determine the rotational movement of the magnetic ring relative to the closed switch unit. To this end, the Hall elements are circularly arranged and are opposing the magnetic ring, in a defined arrangement with respect to each other and the magnetic ring. When movement is started, a start angle is defined and, an end angle is determined once the movement is terminated, based on measurements of the magnetic field during the movement of the magnetic ring relative to the Hall elements. The start and end angles and the measured magnetic field are compared with a stored table and a product dosage set is determined from the comparison.