DOSE SENSING MODULE

Abstract

The present invention provides a sensor module (50) adapted to be arranged in a cartridge based drug delivery device between a rotatable piston rod and a cartridge piston. The sensor module (50) comprises a first sensor structure (52, 152, 252, 352) adapted to be at least substantially rotationally locked with respect to the cartridge piston and comprising a transversal sensor surface (52.2, 152.2, 252.2, 352.2), and a second sensor structure (53, 153, 253, 353) adapted to be rotationally locked with respect to the piston rod and comprising one or more flexibly supported and axially deflectable contact members (53.1, 53.2, 153.2, 253.2, 353.1, 353.2), which are positioned distally of the transversal sensor surface (52.2, 152.2, 252.2, 352.2) and are adapted to apply a proximally directed force thereto. The first sensor structure (52, 152, 252, 352) and the second sensor structure (53, 153, 253, 353) are adapted to undergo relative rotational motion, whereby the one or more contact members (53.1, 53.2, 153.2, 253.2, 353.1, 353.2) sweep the transversal sensor surface (52.2, 152.2, 252.2, 352.2). A processor (52.5, 152.5, 252.5) determines a relative angular displacement between the first sensor structure (52, 152, 252, 352) and the second sensor structure (53, 153, 253, 353) from signals generated as the one or more contact members (53.1, 53.2, 153.2, 253.2, 353.1, 353.2) sweep the transversal sensor surface (52.2, 152.2, 252.2, 352.2).

Claims

1. A sensor module adapted to be arranged in a cartridge based drug delivery device between a rotatable piston rod and a cartridge piston, the sensor module extending along a reference axis from a proximal module portion adapted to interface with the piston rod to a distal module portion adapted to interface with the cartridge piston and comprising: a module housing, and a powered rotary encoder system comprising: a first sensor structure adapted to be at least substantially rotationally locked with respect to the cartridge piston and comprising a transversal sensor surface axially restricted with respect to the module housing, a second sensor structure adapted to be rotationally locked with respect to the piston rod and comprising one or more flexibly supported and axially deflectable contact members, the first sensor structure and the second sensor structure being capable of undergoing relative rotational motion about the reference axis, whereby the one or more contact members sweep the transversal sensor surface, and a processor adapted to determine a relative angular displacement between the first sensor structure and the second sensor structure from signals generated as the one or more contact members sweep the transversal sensor surface, wherein the one or more contact members are positioned distally of the transversal sensor surface and adapted to apply a proximally directed force thereto.

2. A sensor module according to claim 1, wherein the transversal sensor surface comprises a plurality of electrically conductive sensor areas arranged in a pattern, and the one or more contact members are adapted to sweep at least a subset of the plurality of electrically conductive sensor areas as the first sensor structure and the second sensor structure undergo relative rotation, thereby alternately connecting and disconnecting different sensor areas, a current connection being indicative of a current relative angular position of the first sensor structure and the second sensor structure.

3. A sensor module according to claim 1, wherein the transversal sensor surface is a distal surface of a rigid support sheet which extends perpendicularly to the reference axis within the module housing.

4. A sensor module according to claim 3, wherein the proximal module portion comprises an axial pin member comprising a proximal pin end portion and a distal pin end portion, and wherein the rigid support sheet has a central through-going bore, the axial pin member extends through the through-going bore, the second sensor structure is rotationally interlocked with the distal pin end portion, and the proximal pin end portion is configured for rotational interlocking engagement with a distal end portion of the piston rod.

5. A sensor module according to claim 3, wherein the rigid support sheet further comprises a proximal surface carrying the processor.

6. A sensor module according to claim 1, further comprising anti-rotation structure adapted to interface with an interior wall portion of a cartridge in the cartridge based drug delivery device to impede relative angular displacement between the module housing and the cartridge.

7. A sensor module according to claim 2, wherein the plurality of electrically conductive sensor areas are arranged to form a first circular track and a second circular track, the first circular track being a code track and the second circular track being a ground track, and wherein the one or more contact members constitute one code contact member adapted to sweep the first circular track and one ground contact member adapted to sweep the second circular track.

8. A sensor module according to claim 2, wherein the plurality of electrically conductive sensor areas are arranged to form a first circular track and a second circular track, the first circular track being a code track and the second circular track being a ground track, and wherein the one or more contact members constitute two code contact members adapted to sweep the first circular track and one ground contact member adapted to sweep the second circular track.

9. A sensor module according to claim 8, wherein the first circular track comprises 36 evenly distributed code fields, and the code contact members exhibit a 45° angular separation.

10. A sensor module according to claim 2, wherein the plurality of electrically conductive sensor areas form a single circular track comprising 40 evenly distributed fields where every other field is a code field and every other field is a ground field, and wherein the one or more contact members constitute three contact members exhibiting a 120° angular separation from each other.

11. A sensor module according to claim 7, further comprising a battery arranged in the module housing distally of the transversal sensor surface, wherein the proximal module portion comprises an axial pin member comprising a proximal pin end portion configured for rotational interlocking engagement with a distal end portion of the piston rod and a distal pin end portion with which the second sensor structure is rotationally interlocked, wherein the transversal sensor surface is a distal surface of a rigid support sheet which extends perpendicularly to the reference axis within the module housing and has a central through-going bore, and wherein the axial pin member extends through the through-going bore and the distal pin end portion comprises a contact surface which abuts the battery, connecting electrically to a negative battery terminal.

12. A sensor module according to claim 2, further comprising a battery arranged in the module housing distally of the transversal sensor surface, wherein the plurality of electrically conductive sensor areas form a single circular track comprising 36 evenly distributed code fields and the one or more contact members constitute two code contact members exhibiting a 45° angular separation, wherein the proximal module portion comprises an axial pin member comprising a proximal pin end portion configured for rotational interlocking engagement with a distal end portion of the piston rod and a distal pin end portion with which the second sensor structure is rotationally interlocked, wherein the transversal sensor surface is a distal surface of a rigid support sheet which extends perpendicularly to the reference axis within the module housing and has a central through-going bore, and wherein the axial pin member extends through the through-going bore and the distal pin end portion comprises a contact surface which abuts the battery, connecting electrically to a negative battery terminal.

13. A sensor module according to claim 1 in combination with a drug delivery device comprising: a housing accommodating a dose expelling mechanism comprising a piston rod, and a cartridge rotationally fixed with respect to the housing, the cartridge comprising a drug chamber, sealed distally by a self-sealing septum and proximally by a cartridge piston, wherein the proximal module portion is rotationally fixed to the piston rod and the distal module portion abuts the cartridge piston.

14. A sensor module and drug delivery device according to claim 13, wherein the proximal module portion is friction-fitted to an indentation in the piston rod.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In the following the invention will be further described with references to the drawings, wherein

[0038] FIG. 1 shows a dose detection principle according to the prior art,

[0039] FIG. 2 is a perspective longitudinal section view of an injection device with an integrated dose sensing module according to an exemplary embodiment of the invention,

[0040] FIG. 3 is an exploded view of the dose sensing module,

[0041] FIG. 4 is a perspective longitudinal section view of the dose sensing module,

[0042] FIG. 5 is a side view of a wiper assembly used in the dose sensing module,

[0043] FIG. 6 is a distal perspective view of the wiper assembly, and

[0044] FIGS. 7-9 are respective examples of alternative wiper assemblies for use in the dose sensing module.

[0045] In the figures like structures are mainly identified by like reference numerals.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0046] When/If relative expressions, such as “upper” and “lower”, “left” and “right”, “horizontal” and “vertical”, “clockwise” and “counter-clockwise”, etc., are used in the following, these refer to the appended figures and not necessarily to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as their relative dimensions are intended to serve illustrative purposes only.

[0047] FIG. 1 shows a rotary sensor module according to the prior art, arranged between a distal end of a piston rod 1015 and a proximal end of a piston 1022 sealing a drug containing cartridge 1020. The sensor module, which is powered by a coin cell type battery 1075, comprises a first sensor part 1070 in the form of a flexible printed circuit board sheet having a proximally directed sensor surface 1071 on which 24 individual electrically conductive sensor areas 1072 are disposed circumferentially about a centre axis, and a second sensor part 1060 mounted on a distal end portion of the piston rod 1015 opposite the first sensor part 1070 and having contact structures in the form of two electrically connected flexible arms 1061, each terminating in a contact point 1062.

[0048] The first sensor part 1070 is adapted to engage, directly or indirectly, the piston 1022 such that no relative rotation therebetween is possible. The second sensor part 1060 is rotationally fixed to the piston rod 1015, and the contact points 1062 are adapted to engage and electrically connect various individual electrically conductive sensor areas 1072 upon relative rotational motion between the first sensor part 1070 and the second sensor part 1060, experienced as the piston rod 1015 rotates during a dose expelling action. This allows for an estimation of a total angular displacement exhibited by the piston rod 1015 during the dose expelling action and thereby of the amount of drug expelled.

[0049] During the dose expelling the piston rod 1015 undergoes a helical motion, and the axial component of this motion causes an axial advancement of the piston 1022 in the cartridge 1020, as the axial force from the piston rod 1015 is transferred to the proximal surface of the piston 1022 via the sensor module. In connection therewith the second sensor part 1060 is pressed against the first sensor part 1070 and this increases the contact pressure between the contact points 1062 and the sensor surface 1071, thereby reinforcing the electrical contact which generates the signal output. However, it also causes the flexible arms 1061 to deflect against the axial direction of travel of the piston rod 1015, whereby elastic energy is stored therein.

[0050] In the course of the dose expelling the flexible arms 1061 remain so deflected, but when the piston rod 1015 eventually stops and the whole dose expelling system relaxes the elastic energy stored in the flexible arms 1061 is released and transferred to the sensor surface 1071 which is urged axially away from the second sensor part 1060.

[0051] The additional axial movement of the first sensor part 1070 causes an additional axial movement of the piston 1022 which in turn causes a small additional dose to be expelled. Notably, this additional dose is expelled after the piston rod 1015 has stopped its movement and will resultantly require the user to wait a little longer before removing the injection needle from the skin in order to ensure that the entire dose has been received. Furthermore, even though it is advantageous that an increased contact pressure reduces the risk of an accidental loss of contact between the contact points 1062 and the sensor surface 1071 it comes with the cost of an increased friction in the rotational interface between the first sensor part 1070 and the second sensor part 1060, which increases the risk that an angular displacement is introduced to the first sensor part 1070, thereby affecting the accuracy of the dose detection principle.

[0052] FIG. 2 is a perspective longitudinal section view of an injection device 1 having an integrated sensor module 50 according to an exemplary embodiment of the invention. The injection device 1 is of the prefilled autopen injector type, with an elongated housing 2 extending along a reference axis and accommodating a dose expelling mechanism. A cartridge holder 3, holding a cartridge 20 with an interior chamber 25 defined by a cartridge wall 21, a distal penetrable septum 23 and a proximal piston 22, is permanently fixed to the housing 2. The chamber 25 is at least substantially filled with a liquid substance (not visible). In the depicted state of the injection device 1 a needle assembly 40 is attached to a needle mount portion of the cartridge holder 3 in such a manner that an injection needle 45 has penetrated the septum 23 to establish fluid communication to the chamber 25.

[0053] A user operable dose dial 4 is arranged at a proximal end portion of the housing 2 for selective setting of a dose to be ejected from the cartridge 20. The dose dial 4 is operatively coupled with a scale drum 8 which displays a selected dose through a window 9. An injection button 5 is axially depressible to release a windable torsion spring 10. The release of the torsion spring 10 will cause a helical advancement of a piston rod 15 through a nut member 7 in the housing 2 and thereby result in an execution of a dose expelling action.

[0054] Details of the dose setting and the dose expelling mechanisms are irrelevant to the present invention and will accordingly not be provided in the present text. For an example of how such mechanisms may be constructed reference is made to WO 2015/071354, particularly p. 10, I. 21-p. 15, I. 13. What is important is that the rotational movement of the piston rod 15 during dose expelling is correlated with the prompted movement of the piston 22 through the design of the piston rod thread and the nut member 7 such that a predetermined angular displacement of the piston rod 15 relative to the housing 2 corresponds to a predetermined axial displacement of the piston 22 relative to the cartridge wall 21. This relationship may in principle be chosen arbitrarily by the manufacturer, with a view to the dimensions of the cartridge 20. In the present example a 15° angular displacement of the piston rod corresponds to a specific axial displacement of the piston 22 which results in the expelling of 1 IU of the contained substance through the injection needle 45.

[0055] FIG. 3 is an exploded view highlighting the individual elements of the present sensor module 50. The sensor module 50 comprises a first sensor part in the form of a PCB assembly 52 with a rigid support sheet 52.4 having a proximal surface 52.1 carrying various electronic components 52.5, including a processor, and a distal surface 52.2 carrying a plurality of electrically conductive sensor areas (not visible), the configuration of which will be described below. The support sheet 52.4 has an overall circular periphery, but is provided with several notches, some of which resulting in a pair of diametrically opposite radial protrusions 52.3. Furthermore, the support sheet 52.4 has a central through-going bore 52.6.

[0056] The first sensor part is complemented by a second sensor part in the form of a wiper 53 being fixedly mounted to a piston rod connector 54 to ensure joint rotation therewith. The piston rod connector 54 extends axially through the through-going bore 52.6 and is adapted for press-fit engagement with a cavity in a distal end portion of the piston rod 15, as shown on FIG. 2. This provides for a joint movement of the piston rod 15 and the piston rod connector 54. The wiper 53 comprises one ground contact 53.1 and two code contacts 53.2 arranged on respective flexible arms 53.5 and adapted to galvanically connect with the electrically conductive sensor areas on the distal surface 52.2 of the support sheet 52.4, as described in more detail below. Notably, the ground contact 53.1 and the code contacts 53.2 are all proximally directed.

[0057] The two sensor parts, forming a rotary encoder system, are accommodated in a module housing 51 which also accommodates a power source in the form of a battery 55, a retainer 56 also functioning as a positive battery connector, and a rigid (negative) battery connector 57. The retainer 56 has a transversal support surface 56.1 for carrying the battery 55 and two axially extending opposite retainer arms 56.2. Each retainer arm 56.2 is provided with a proximal cut-out 56.3 shaped to receive one of the radial protrusions 52.3, thereby rotationally interlocking the retainer 56 and the PCB assembly 52 and axially restricting the support sheet 52.4. The module housing 51 has a pair of diametrically opposite side openings 51.2 shaped to receive the retainer arms 56.2 so as to rotationally interlock, or at least substantially rotationally interlock, the retainer 56 and the module housing 51, and a plurality of anti-rotation tabs 51.1 spaced apart along its circumference for interaction with an interior surface of the cartridge wall 21. The PCB assembly 52 is thus at least substantially rotationally locked with respect to the module housing 51, which in turn is rotationally frictionally fitted in the cartridge 20, which is rotationally fixed in the cartridge holder 3. The PCB assembly 52 is thereby at least substantially rotationally fixed with respect to the housing 2 and accordingly suitable as reference component for measuring angular displacements of the piston rod 15.

[0058] FIG. 4 is a perspective longitudinal section view of the sensor module 50 in an assembled state. As can be seen the piston rod connector 54 extends through the through-going bore 52.6 in the support sheet 52.4 and is press-fitted with a sleeve 53.6 on the wiper 53. The module housing 51 has a foot 51.3 which rests against the piston 22 (cf. FIG. 2). Furthermore, the figure shows the position of the retainer arms 56.2 in the side openings 51.2 and the arrangement of the radial protrusions 52.3 in the cut-outs 56.3. During a dose expelling action with the injection device 1 the rotation of the piston rod 15 is transferred to the piston rod connector 54 and further on to the wiper 53. The ground contact 53.1 and the code contacts 53.2 thus sweep the sensor areas of the distal surface 52.2 which remains, at least substantially, rotationally stationary due to the engagement between the radial protrusions 52.3 and the cut-outs 56.3, the fitting of the retainer arms 56.2 in the side openings 51.2, the frictional interface between the foot 51.3 and the piston 22, and the frictional interface between the anti-rotation tabs 51.1 and the cartridge wall 21.

[0059] FIG. 5 is a side view of the two sensor parts showing the connection between the ground contact 53.1 and the code contacts 53.2 and the distal surface 52.2 of the support sheet 52.4, and FIG. 6 is a perspective distal view of the same. In the shown exemplary embodiment of the invention the aforementioned plurality of electrically conductive sensor areas on the distal surface 52.2 are arranged such that a single circular ground track 52.7 provides a ground connection for the ground contact 53.1 and 36 individual code fields 52.8 together constitute a code track 52.9 which the code contacts 53.2 are adapted to sweep. A secondary ground connection is provided through a spherical end 54.1 of the piston rod connector 54 contacting the (negative) battery connector 57. The secondary ground connection may be relevant to stabilise the signal output in case the dynamics of the dose expelling mechanism generates vibrations in the sensor module 50.

[0060] As the piston rod connector 54 rotates jointly with the piston rod 15 during a dose expelling action the two code contacts 53.2, which are circumferentially separated by 45°, respectively sweep the code track 52.9, generating signals representative of the angular position of the wiper 53 as different code fields 52.8 get connected to ground. The two sensor parts output a 4-bit Gray code, i.e. eight different codes which for a 360° rotation of the wiper 53 are repeated nine times, giving 72 distinguishing codes. This output thus forms the basis for an estimation, by one or more of the electronic components 52.5 including the processor, of the total angular displacement of the piston rod 15 during a dose expelling action, and thereby for an estimation of the expelled dose.

[0061] For galvanic sensors like the herein described it is crucial that the contact pressure on each physical contact is sufficiently high to ensure a stable signal. This prerequisite is met by the design of the present sensor module 50, where the combination of the flexible arms 53.5 and the sleeve 53.6 and the restricted axial play of the radial protrusions 52.3 in the cut-outs 56.3 enables an arrangement of the wiper 53 on the piston rod connector 54 relative to the support sheet 52.4 which provide a spring reinforced contact between the ground contact 53.1 and the ground track 52.7 as well as between the respective code contacts 53.2 and the code track 52.9. However, importantly, the fact that the wiper 53 is positioned distally of the support sheet 52.4 such that the flexible arms 53.5 are deflected distally and the respective ground and code contacts 53.1, 53.2 thereby provide proximally directed forces to the support sheet 52.4 is advantageous because during a dose expelling action when the piston rod connector 54 applies an axially directed force to the battery connector 57 this will not result in a further deflection of the flexible arms 53.5 as the wiper 53 is not pressed against the support sheet 52.4, i.e. no additional elastic energy is stored in the flexible arms 53.5 which needs to be released during the subsequent relaxation of the dose expelling system, and the problem of prolonged dose expelling is thus solved.

[0062] Furthermore, since the wiper 53 is not being pressed against the support sheet 52.4 as a result of the advancing piston rod connector 54 the contact pressure in the respective ground contact 53.1/ground track 52.7 and code contact 53.2/code track 52.9 interfaces is not increased during dose delivery. The friction in the rotational interface between the two sensor parts is therefore also not increased, which means that the torque applied by the wiper 53 to the support sheet 52.4 is not increased. The risk of angular displacement of the support sheet 52.4 against the rotation prevention mechanism provided by the interaction between the anti-rotation tabs 51.1 and the cartridge wall 21 is resultantly reduced compared to a solution, e.g. like the one shown in FIG. 1, where the flexible arms exhibit further deflection during piston rod advancement.

[0063] FIG. 7 is a perspective distal view of two sensor parts of an alternative rotary encoder system used in a sensor module according to another embodiment of the invention. The sensor parts comprise a wiper 153 and a PCB assembly 152 held in mutual position by the piston rod connector 54 in a manner similar to that disclosed in connection with the previous embodiment. The geometrical configuration of the PCB assembly 152 as well as its interaction with other components of the sensor module is identical to that of the formerly described PCB assembly 52. Particularly, the PCB assembly 152 comprises a rigid support sheet 152.4 having a proximal surface 152.1 which carries various electronic components 152.5, including a processor, and a distal surface 152.2 on which is disposed a plurality of electrically conductive code fields 152.8 arranged side by side to thereby provide a circular code track. However, contrary to the former embodiment the distal surface 152.2 does not comprise a dedicated ground track. Instead, the ground connection is supplied via the spherical end 54.1 of the piston rod connector 54 being in contact with the (negative) battery connector 57, similarly to the above described.

[0064] The wiper 153 comprises a sleeve 153.6 press-fitted onto the piston rod connector 54, to ensure joint rotation of the piston rod 15 and the wiper 153, and two code contacts 153.2, each arranged at an end portion of a flexible arm 153.5 capable of axial deflection. The code contacts 153.2 are angularly separated by 45° and will when rotated relative to the distal surface 152.2 respectively sweep the code fields 152.8 and produce a 4-bit Gray code, similarly to the previous embodiment. The fact that only two wiper contacts sweep the distal surface 152.2 provides for a reduced internal friction and therefore a reduced torque between the two sensor parts, compared to three sweeping contacts. Hence, the risk of angular displacement of the PCB assembly 152 against the rotation prevention mechanism provided by the interaction between the anti-rotation tabs 51.1 and the cartridge wall 21 is reduced even further, while the advantageous containment of the forces from the flexible arms 153.5 between the PCB assembly 152 and the battery 55 is still obtained, eliminating the prolonged dose expelling problem.

[0065] FIG. 8 is a perspective distal view of two sensor parts of another alternative rotary encoder system used in a sensor module according to a third embodiment of the invention. Similarly to the previous embodiments the sensor parts comprise a wiper 253 and a PCB assembly 252 held in mutual position by the piston rod connector 54. The geometrical configuration of the PCB assembly 252 as well as its interaction with other components of the sensor module is identical to that of the formerly described PCB assembly 52. Particularly, the PCB assembly 252 comprises a rigid support sheet 252.4 having a proximal surface 252.1 which carries various electronic components 252.5, including a processor, and a distal surface 252.2 on which is disposed a plurality of electrically conductive sensor areas.

[0066] However, contrary to the former embodiments the distal surface 252.2 carries 40 electrically conductive sensor areas arranged in a circular track pattern where every other sensor area constitutes a ground field 252.7 and every other sensor area constitutes a code field 252.8. A secondary ground connection is supplied via the spherical end 54.1 of the piston rod connector 54 being in contact with the (negative) battery connector 57, as described above in connection with the first embodiment of the invention.

[0067] The wiper 253 is attached to the piston rod connector 54 and is adapted to sweep the 40 electrically conductive sensor areas as the piston rod 15 rotates during a dose expelling action (as described above). The wiper 253 has three flexible arms 253.5, each terminating in a contact point 253.2 which is adapted to galvanically connect with a ground field 252.7 or a code field 252.8, depending on the angular position of the wiper 253 relative to the PCB assembly 252. The three contact points 253.2 are separated 120° from each other such that one contact point 253.2 is always connected to a ground field 252.7 and two contact points 253.2 are always connected to a code field 253.8. The two sensor parts output a 4-bit Gray code and offer a higher resolution than the former two embodiments of the invention, enabling an even more accurate estimation of the total relative angular displacement between the PCB assembly 252 and the wiper 253, and thereby of the total angular displacement of the piston rod 15 relative to the housing 2, during a dose expelling event.

[0068] FIG. 9 is a perspective distal view of two sensor parts of yet another alternative rotary encoder system used in a sensor module according to a fourth embodiment of the invention. Like in the previous embodiments the sensor parts comprise a wiper 353 and a PCB assembly 352 held in mutual position by the piston rod connector 54. The geometrical configuration of the PCB assembly 352 as well as its interaction with other components of the sensor module correspond to that of the formerly described PCB assembly 52. Particularly, the PCB assembly 352 comprises a rigid support sheet 352.4 having a proximal surface 352.1 which carries various electronic components (not visible), including a processor, and a distal surface 352.2 on which is disposed a plurality of electrically conductive sensor areas. The plurality of electrically conductive sensor areas comprises a circular ground track 352.7 and a circular code track 352.9 formed of 72 individual code fields 352.8 which are arranged side by side.

[0069] The wiper 353 is press-fitted onto the piston rod connector 54, to ensure joint rotation with the piston rod 15, and comprises a code contact 353.2 and a diametrically opposite ground contact 353.1, each contact arranged at an end portion of a flexible arm 353.5 capable of axial deflection. During a dose expelling action when the wiper 353 rotates relative to the PCB assembly 352 the code contact 353.2 will sweep at least a subset of the code fields 352.8 while the ground contact 353.1 will sweep at least a subset of the ground track 352.7. This produces a number of signal shifts which can be correlated with a particular angular displacement of the piston rod 15 relative to the housing 2 and thus used to estimate the size of the expelled dose.

[0070] Again, the fact that only two wiper contacts sweep the distal surface 352.2 provides for a reduced internal friction and therefore a reduced torque between the two sensor parts, compared to three sweeping contacts. Hence, the risk of angular displacement of the PCB assembly 352 against the rotation prevention mechanism provided by the interaction between the anti-rotation tabs 51.1 and the cartridge wall 21 is reduced even further, while the advantageous containment of the forces from the flexible arms 353.5 between the PCB assembly 352 and the battery 55 is still obtained, eliminating the prolonged dose expelling problem

[0071] In a variation of the above sequential encoder, the ground track 352.7 and the flexible arm 353.5 carrying the ground contact 353.1 could be omitted and ground connection could be provided solely by the spherical end 54.1 of the piston rod connector 54 contacting the (negative) battery connector 57. This would reduce the internal friction even further as only one wiper contact would sweep the distal surface 352.2. In order to enhance the structural stability of this alternative wiper, it could be considered to introduce an arm to counterbalance the flexible arm 353.5 carrying the code contact 353.2.