GB1522521A - Electric windscreen wiper systems - Google Patents

Abstract

1522521 Automatic control of D.C. motors GENERAL MOTORS Ltd 30 April 1976 [10 May 1975] 19754/75 Heading H2L A windscreen wiper system comprises a pair of closely similar D.C. motors A, B each driving a respective wiper arm and blade assembly and each controlled by an identical transistor amplifier and switching circuit 28, 32, whose transistors 34 are controlled by signals a, b produced by a logic circuit 10 of a quad two input NAND gate type, Fig. 2 (not shown), the inputs to circuit 10 being signals A, B which are produced by respective switches associated with the motors A, B, each switch consisting of the sectored and segmented plate of Fig. 4 which rotates with a gear wheel driven by the motor and engages or disengages a contact (not shown) to produce a 1 or 0 level signal respectively, the circuit of Fig. 1 operating to maintain the motors in synchronism by virtue of the following features:- (1) If signals A, B have the same level (i.e. 0 or 1), signals a, b are both 1 and transistors 50 are switched on to energize the motors. (2) If signal A or B changes from level 0 to 1 before the signal B or A, the signal a or b changes to level 0 to switch off the transistor 50 of the faster motor, which then slows down or stops until the signal of the slower motor changes from level 0 to 1, whereupon the faster motor is re-energized. (3) To prevent the slower motor being switched off when the signal of the faster motor changes from level 1 to 0, which could result in the slower motor being stopped until the faster motor has rotated to cause its plate sector to re-engage its contact, each plate has a segment S positioned as shown and of such length that, if the corresponding motor is de-energized as its contact engages segment 5, the inertia of the motor is sufficient to ensure continued rotation until the contact disengages segment S. Thus, if motor A is the faster the interaction of signals A, B, produced by engagement and disengagement of the respective sectors, segments and contacts, will result in two successive energization/re-energization operations of motor B and one such operation of motor A if its contact is engaging segment S when the contact of motor B disengages its sector. When the aforementioned sequence is completed both motors are running for a further speed correction as in (2) above. Conventional parking switches for each motor ensure that:- (1) signals A and B are at the same level when a switch 20 is closed, so that the motors are simultaneously switched on, and (2) the circuit of Fig. 1 may be applied to control two-speed three-brush motors braked, when switched off, by short circuiting of the armature.