Solenoid

Abstract: The system includes a direct-current voltage supply (30), an electric motor (2) for rotating a movable pinion (6), and an electromagnetic operating device (3) including a solenoid (11) and an associated movable core (10). The core (10) moves the pinion (6) towards an operating position in which it can be coupled for rotation with a rotary member (7) of the internal combustion engine. The connection between the electromagnetic operating device (3) and the voltage supply (30) is controlled by an electronic unit (ECU) which measures the current flowing in the solenoid (11) by means of a portion (11d) of the wire forming the solenoid, which portion is bent and preferably twisted, and acts as a resistive shunt.

Description: The invention relates to a solenoid usable, in particular, in an electromagnetic operating device forming part a starter system for an internal combustion engine and, more specifically, to a solenoid consisting in known manner of an electrical wire wound substantially in a helix. Such a solenoid is disclosed in EP-A-0 225 444 wherein a measuring resistor is connected to the solenoid for measuring the current flow. The solenoid according to the invention is characterised in that a portion of the wire is bent so as to be substantially U-shaped, and is disposed in such a manner that, in operation, the magnetic flux linkage with this portion is low, or even essentially zero; the ends of this portion of the solenoid are accessible from the outside to enable the measurement, in use, of the voltage drop in the said portion, which is indicative of the intensity of the current flowing in the solenoid. This portion of the wire forming the solenoid acts as a resistive shunt and, to advantage, is an end portion of the wire, which is preferably twisted and disposed outside the solenoid, and oriented substantially parallel to the axis of the solenoid. Further characteristics and advantages of the invention will become apparent from the following detailed description, with reference to the appended drawings provided purely by way of non-limiting example, in which: Figure 1 is a partially sectioned side view of a starter unit of known type, including a solenoid according to the invention; and Figure 2 is circuit diagram, partially in block form, of a starter system including a starter unit with a solenoid according to the invention. A starter system, for an internal combustion engine for motor vehicles includes a starter unit of the type shown in Figure 1. The unit includes a support casing 1 in which an electric starter motor 2 and an electromagnetic operating device 3 are mounted. In the embodiment shown as an example, an overrunning (free wheel) coupling 4 is fitted to the shaft of the electric motor 2. A sleeve 5 is movable on the shaft of the motor 2, together with the coupling 4. A pinion 6, which is movable axially along an end portion 2a of the shaft of the electric motor 2, is connected to the side of the coupling 4 which faces away from the motor 2. In particular, the pinion 6 can move between a retracted, rest position, shown in continuous outline, and an advanced, operating position, shown in broken outline. In the latter position the pinion 6 can mesh with the teeth of a flywheel 7 of the internal combustion engine (not shown). A rocker lever, indicated 8, can pivot about a fulcrum 9 and acts as a transmission member between the sleeve 5 (which acts as a member for moving the pinion 6) and a movable core 10 of the electromagnet 3. The electromagnet has an operating winding or solenoid 11, supported by a spool 12, inside which the core 10 is movable axially. One end of the core has an axial extension 13, around which a small plate 14 is fixed. The end of the extension has an appendage 15 containing a slot 16, through which one end 8b of the rocker lever extends. In the embodiment shown, the lever 8 is of the leaf spring type, and consists of two substantially Y-shaped metal plates joined together, and having lower ends 8a forming two prongs which engage in seats 5a in the sides of the sleeve 5. A helical spring, indicated 17, reacts against the small plate 14, keeping the core 10 in the position shown, in which it projects partially from the operating winding or solenoid 11. The far end of the core 10 from its extension 13 has a frustoconical recess 18. A fixed core, generally indicated 19, is inserted in the end of the spool 12 of the electromagnet, which faces away from the side opposite the lever 8. The fixed core has a passage 20 coaxial with the spool 12 and the core 10. One end of the passage opens in the centre of a frustoconical protuberance 21 of the fixed core, the protuberance 21 facing the recess 18 of the movable core 10, and being complementary in shape thereto. A rod 22 having one end which extends into a region 23, defined between the fixed core 19 and an essentially cup-shaped insulating body 24, is movable axially in the passage 20. This end of the rod 22 bears a contact member (a mobile contact) 25 which can cooperate with a pair of fixed contacts 26 and 27, in the form of screws, supported by the end wall of the insulating element 24. A spring 28 is interposed between the insulating body 24 and the end head of the rod 22 and - at rest - keeps the latter in the position shown, in which its other end extends towards the movable core 10, projecting beyond the protuberance 21 of the fixed core 19. In this condition the mobile contact 25 is separated from the fixed contacts 26 and 27. This movable contact and the associated fixed contacts together constitute an electrical switch which controls the current supply to the electric motor 2 (in known manner). The switch closes when, as a result of the excitation of the operating solenoid 11, the movable core 10 moves towards the fixed core 19, giving rise (in the final stage of its travel) to the movement of the rod 22 and of the associated movable contact 25 towards the fixed contacts 26 and 27. The movement of the movable core 10 as a result of the excitation of the solenoid 11 also gives rise, by means of the rocker lever 8, to a corresponding movement of the pinion 6 from its rest position to its operating position in which it can mesh with the flywheel 7. With reference to Figure 2, a starter system for the internal combustion engine of a motor vehicle includes a direct-current voltage supply 30, advantageously consisting of the vehicle battery. In the embodiment illustrated the negative pole of the battery 30 is earthed and the positive pole is connected to a junction 31. Two circuit branches indicated as 32 and 33 branch out from this junction. Interposed in the branch 32 is a manually-operated switch 34 incorporated, for example, in a conventional ignition and starting switch operable by means of a key. The solenoid 11 of the electromagnetic operating device 3 is connected in the circuit branch 32, and has one terminal 11a connected to the switch 34, and another terminal indicated 11b. An electronic device 35 with controlled-conduction such as, for example, a MOS transistor, is connected between the terminal 11b and earth. This device has an input or control terminal connected to an electronic control unit indicated ECU. The fixed contacts 26 and 27 are disposed in the circuit branch 33 in the manner illustrated. The electric starter motor 2 is also disposed in this circuit branch, between the fixed contact 26 and earth. The solenoid 11 consists (in known manner) of a wire wound substantially in a helix. According to the invention, a portion of the wire, indicated 11d in Figure 2, is bent so as to be substantially U-shaped, preferably with twisted arms. In the embodiment illustrated as an example, the portion 11d is an end portion of the wire which forms the solenoid 11. To advantage, this portion of the solenoid is disposed outside the solenoid, and is oriented essentially parallel to the axis of the solenoid. The ends of the portion 11d of the solenoid 11 are accessible from the outside by means of the terminal 11b and a further terminal, indicated 11c. The shape and arrangement of the portion 11d of the solenoid 11 are such that, in operation, the magnetic flux linkage with this portion is very low and virtually zero. The terminals 11b and 11c are connected to corresponding inputs of the control unit ECU. A diode 36 is disposed between the terminals 11a and 11b to protect the transistor 35 against overvoltages. In operation, the portion 11d of the solenoid 11 acts essentially as a resistive shunt and the voltage drop between the terminals 11b and 11c is indicative of the intensity of the current flowing in the solenoid. The control unit ECU is arranged to drive the controlled-conduction device 35 in such a manner that the current flowing in the solenoid 11 (measured by the unit by means of the portion 11d of the solenoid) varies according to a predetermined time law, to which a predetermined law of variation of the speeds of movement of the movable core 10 and the pinion 6 corresponds. In particular, the unit ECU is arranged to drive the controlled-conduction device 35 in such a manner that the speeds of the movable core 10 and of the pinion 6 when they reach their respective operative positions are below a predetermined value, for example, of the order of 0.2 m/sec. The kinetic energy with which the movable core 10 strikes the fixed core 19, and the kinetic energy with which the pinion 6 strikes and meshes with the flywheel 7 of the internal combustion engine, are thus reduced considerably. This gives rise to significant reduction in the operating noise. The unit ECU may be formed with the use of conventional analogue circuits or possibly at least partly with digital circuits. The unit actually controls the speed of movement of the core 10 and of the pinion 6 by controlling the current which flows in the solenoid 11, and is measured by means of the portion 11d thereof. The portion 11d of the solenoid, which is advantageously disposed inside the same casing 40 as the solenoid, undergoes the same variations in resistance with variations in the operating temperature as the solenoid itself. This prevents errors in the measurement of the current flowing in the solenoid, which could occur if the portion 11d which acts as a resistive shunt were subjected to temperature conditions different from those of the solenoid 11. Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may vary widely with respect to those described and illustrated, without thereby departing from the scope of the present invention as defined in the appended claims.

Claims: A solenoid, particularly for use in an electromagnetic operating device (3), consisting of an electrical wire wound substantially in a helix, characterised in that a portion (11d) of the wire (11) is bent so as to be substantially U-shaped and is disposed in such a manner that, in operation, the magnetic flux linkage with this portion (11d) is low or essentially zero; the ends (11b, 11c) of the portion (11d) are accessible to enable the measurement, in use, of the voltage drop in the said portion, which is indicative of the intensity of the current flowing in the solenoid (11). A solenoid according to Claim 1, characterised in that said portion (11d) is formed at one end of the solenoid (11). A solenoid according to Claim 1 or Claim 2, characterised in that saidportion (lld) of the wire is twisted. A solenoid according to any one of Claims 1 to 3, characterised in that said portion (11d) of the wire is disposed outside the solenoid (11) and is oriented substantially parallel to the axis of the solenoid (11).

Inventors: BASSI MARIO, FASOLA GIANCARLO, BASSI, MARIO, FASOLA, GIANCARLO, Fasola, Giancarlo, c/o
Applicants: MAGNETI MARELLI SPA, INDUSTRIE MAGNETI MARELLI S.P.A.
Application year: 1993
Main classification: F02N11
IPC codes: F02N15, H01F5, F02N11