Electronically controlled horn for motor vehicles

A horn comprising a diaphragm, an electromagnet, a transducer to sense the vibrations of the diaphragm and generate a vibration-dependent electrical signal, and a feedback circuit which controls a power supply to the electromagnet. The feedback circuit includes an electronic power circuit (E, IEP) controlled by a control circuit (.mu., F, CCS) arranged to adapt, condition and process the electrical signal from the transducer (S) in such a manner as to automatically determine the frequency and duty cycle for controlling the electronic power circuit (IEP) under the various environmental, electrical feed and constructional tolerance conditions of the horn.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electromechanical devices for sound generation, 
and particularly to high-sounding horns for use in motor vehicles. 
2. Description of Related Art 
Sound generating devices of the electromagnetic excitation type currently 
consist of: 
a resilient steel diaphragm carrying in its centre the mobile part 
(armature) of an electromagnet; 
an electric switch with a normally closed contact connected in series with 
the power feed to the electromagnet; 
an adjustment screw which determines the switch contact opening and 
a diffuser which resonates at the same frequency as the metal diaphragm. 
When the electromagnet is electrically powered, it attracts the armature 
rigid with the resilient diaphragm. When the diaphragm has nearly attained 
its maximum travel, the switch connected in series with the electromagnet 
coil is opened by a push rod operated by the mobile assembly of the 
electromagnet. At this point the elastic energy accumulated by the 
diaphragm is restituted by reaction with the fixed structure to which it 
is connected, so that the diaphragm reverses its direction of movement. In 
this manner it again closes the switch which, again exciting the 
electromagnet, causes the diaphragm to commence a new oscillation cycle at 
a frequency equal to the resonance frequency of the electromechanical 
system. 
These normal switch devices have considerable drawbacks, which can be 
summarized as follows: 
As the sound output of the horn depends on the time at which the switch 
operates, it is difficult to obtain maximum sound output because of the 
difficulty of fixing or adjusting the switch operation point. 
The sound output is subject to considerable fall-off with time due to the 
mechanical instability of the switch operation points. 
The switch contacts are subject to sparking which causes them to wear and 
lead to a variation in their time of operation, with reduction in sound 
output. 
The contact sparking creates electromagnetic waves which can be troublesome 
to the electronic systems increasingly used in modern motor vehicles. 
To obviate these drawbacks, different methods have been conceived for 
controlling the excitation of the electromagnet coupled to the resilient 
steel diaphragm, these still being essential elements for the low-cost 
generation of high-intensity sound at frequencies less than one kilohertz. 
The first alternative to the switch uses electronic oscillators operating 
at a vibration frequency approximately equal to the resonance frequency of 
the electromagnetic system; with this method the oscillator output 
controls an electronic switch connected in series with the coil, thus 
replacing the mechanically operated switch. 
However, this method has certain drawbacks which can be summarized as 
follows: 
the need for an oscillator the frequency of which is stable with varying 
feed voltage and having a frequency-temperature characteristic curve equal 
to that of the mechanical unit; and 
in order to limit to a minimum any differences between the oscillator 
frequency and the diaphragm resonance frequency, the diaphragm production 
tolerances must be restricted or alternatively a selection and coupling 
procedure must be implemented. 
All this results in high production costs which are difficult to accept by 
the user. 
The aforesaid drawbacks can be obviated by linking the electronic 
oscillator frequency to the resonance frequency of the resonance frequency 
of the electromechanical unit which generates the sound. Such a method has 
already been proposed in French patent 1,428,483, which is now in public 
domain. 
FIG. 1 shows the schematic diagram of said patent. In this figure a 
transducer S sensitive to diaphragm vibration is coupled to the diaphragm 
M of a horn X. The transducer S can be a known sensor sensitive to the 
vibration of the resilient diaphragm M of the horn, to generate at its 
output a voltage signal having a frequency corresponding to the vibration 
frequency. The transducer S feeds its signal to the input of an amplifier 
.mu. via a positive feedback circuit .phi., it being thus suitably 
amplified and then fed to the electromagnet E. The resultant vibration of 
the diaphragm M results in the reproduction of a voltage signal in the 
sensor S greater than that which it had generated but of coincident phase 
and frequency. The required oscillator with a resonance frequency the same 
as that of the electromagnetic sound generation system is therefore 
obtained. 
A horn using an electronic circuit based on the above principle has better 
characteristics than a horn incorporating a mechanical switch or a fixed 
frequency electronic circuit, however the characteristics are insufficient 
for a high-sounding horn. To improve the sound output in relation to the 
current absorbed by the electromagnet in horns with a mechanical switch or 
fixed frequency electronic circuit it is already known to use an 
arrangement which exploits to a maximum the greater force of attraction 
which the electromagnet exerts on the armature when the air gap is reduced 
to the allowable minimum. 
This arrangement consists of prolonging the electrical feed to the 
electromagnet beyond 50% of the inherent frequency period of the 
electromechanical system. The mean optimum value of the feed:response 
ratio is 65%:35%. It therefore follows that by applying this electromagnet 
feed concept the diaphragm oscillation is no longer sinusoidal. A sized 
spacer can be provided for each horn positioned along the diaphragm 
support perimeter on the side facing the electromagnet, to raise the 
voltage at which mechanical contact is obtained between the armature rigid 
with the diaphragm and the electromagnet to beyond the maximum voltage 
which can be provided by the battery. 
This makes the arrangement inapplicable to the circuit configuration of 
FIG. 1. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to make the principle of the 
electronic circuit for exciting the electromagnet at the inherent 
resonance frequency of the electromechanical sound generating component, 
this being a characteristic of the circuit of FIG. 1, compatible with the 
concept of asymmetric cycle feed to the electromagnet. 
A further object of the present invention is to automatically control the 
asymmetric cycle in such a manner as to compensate for the constructional 
differences between one horn and another and to improve its operation as 
the output voltage of the vehicle battery varies. 
These and further objects which will be more apparent from the detailed 
description given hereinafter are attained by a horn comprising a 
diaphragm and electromagnet, of the type comprising a transducer to sense 
the vibrations of the diaphragm and feed a vibration-dependent electrical 
signal to a feedback circuit which controls the power supply to the 
electromagnet, said horn being characterized essentially in that the 
feedback circuit comprises an electronic power circuit controlled by means 
arranged to adapt, condition and process the electrical signal from the 
transducer in such a manner as to automatically determine and generate 
both the frequency and duty cycle for controlling the electronic power 
circuit under the various environmental, electrical feed and 
constructional tolerance conditions of the horn. 
The present invention will be more apparent from the description of some 
non-limiting embodiments thereof shown on the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 2, X indicates the actual horn. It comprises a 
casing K to which a metal diaphram M is peripherally clamped by a spacer 
ring D, this being advantageously non-sized so as to result in greater 
constructional economy. In the chamber Z defined by the casing K and 
diaphragm M there is an electromagnet E, the armature A of which is rigid 
with the diaphragm M. Q indicates a resonant diffuser associated with the 
horn. A sensor or transducer S is operationally engaged with the armature 
A. It generates a voltage signal proportional to the oscillation of the 
diaphragm M. The term "operationally engaged" signifies that the 
transducer S can be either connected mechanically to the diaphragm or 
physically separate from it. An example of a physically separate 
transducer is a piezoelectric transducer connected by a spring or piston 
to the centre of the diaphragm to sense its oscillation. 
The voltage signal leaving the transducer S proportional to the oscillation 
of the diaphragm M reaches a low pass filter F which filters the voltage 
signal to eliminate harmonics generated in the transducer by the 
non-harmonic movement of the diaphragm M. At the output of the filter F 
there is therefore a sinusoidal voltage signal of frequency equal to the 
frequency of the fundamental vibration of the diaphragm M and of amplitude 
proportional to said vibration. 
This output signal is fed to a signal conditioning circuit (CCS). From the 
output voltage signal of the filter F the circuit CCS obtains two logic 
signals, the first of duration equal to the half period of the oscillating 
frequency of the diaphragm M and the second of duration inversely 
proportional to the amplitude of said signal, and then recombines these 
signals to provide at its output a logic signal of duration equal to the 
sum of the times of the two signals analogously with pulse-width 
modulation. Various circuit configurations can be proposed for effecting 
the function assigned to the circuit CCS. 
Assuming that, for correct compensation of the phase lag introduced by the 
low pass filter F, the commencement of excitation of the coil of the 
electromagnet E corresponds to the commencement of the negative half 
period of the sinusoidal signal present at the input of the circuit CCS, a 
single comparator will produce a logic signal 1 foar the entire negative 
half period of the signal. 
A second comparator, preset with a positive switching level equal to about 
60% of the peak value of the positive half wave of the signal, will 
produce a logic signal 1 for the period between the commencement of the 
positive half wave and the attainment of the preset switching value. 
If the outputs of the two comparators are connected together in OR 
configuration the result will be a logic signal 1 the duration of which is 
characteristic of the frequency and amplitude of the signal from the 
sensor S. This logic signal is fed to a current amplifier .mu. which 
interfaces the output of the circuit CCS with the input of a solid state 
power switch IEP which provides the current required for controlling the 
electromagnet E. 
Other circuit techniques can be used to provide the function required of 
the circuit CCS. Amplitude limitation of the input signal can be employed 
using circuits which obtain the logic signal inversely proportional to the 
signal amplitude by differentiating the signal itself instead of by 
circuits using fixed thresholds. This can for example be at the discretion 
of the company constructing the custom circuit, the company then using for 
obtaining the function required of the circuit CCS those circuit 
configurations which best match the chosen integration technology. 
To better understand the overall operation of the circuit, it will be 
assumed that a current flows through the electromagnet E of intensity 
equal to the mean value of the battery voltage B for a time of 65% of the 
period corresponding to the resonance frequency of the electromechanical 
sound generation system E, A, M, D, to produce a sound output equal to the 
average output of the device. The transducer S generates a signal of mean 
amplitude proportional to the movement of the diaphragm M and of frequency 
equal to the resonance frequency of the system E, A, M, D. The low pass 
filter F eliminates the harmonics present in the signal and feeds to the 
circuit CCS a sinusoidal signal of mean amplitude and frequency equal to 
the resonance of the system E. A. M, D. The circuit CCS conditions the 
signal present at its input such as to generate at its output a signal of 
65% duty cycle, phase and frequency of the current circulating through the 
electromagnet E which has generated it. 
The amplifier circuit .mu. provides the signal required for the electronic 
power switch (such as a Darlington transistor) IEP to feed to the 
electromagnet E a current of the given value for a mean battery voltage 
for the time predetermined by the circuit CCS. 
It is therefore apparent that when factors occur such as a fall in the 
battery voltage, an increase in the air gap due to constructional 
dimension tolerances, or any condition resulting in a reduction in the 
sound output of the sound generating device, a circuit with the aforesaid 
functions will make an automatic correction by increasing the duty cycle 
by up to about 75%. This correction takes place because if the sound 
signal falls below the mean value a proportional reduction occurs in the 
signal generated by the sensor S. 
Consequently the circuit CCS makes a proportional increase in the duty 
cycle, thus producing an increase in the mean current through the 
electromagnet E with a consequent increase in the sound output of the 
horn. 
In the same manner, if factors which increase the sound output occur such 
as an increase in the battery voltage or a reduction in the air gap, the 
circuit CCS makes a proportional reduction in the duty cycle by up to 
about 50%. 
Thus a circuit composed in this manner will automatically correct the duty 
cycle and frequency so as to compensate for any constructional tolerances 
of the components concerned in the sound generation, to obtain an optimum 
sound level under all feed voltage and environmental conditions. 
The circuit of FIG. 3 represents a modification to the circuit 
configuration of FIG. 2. A characteristic of this circuit is the different 
command for activating the horn. In this respect the power circuits are 
permanently connected to the feed battery whereas the active circuits CCS 
and .mu. are activated by an electronic switch IE which receives a low 
power logic command originating (line H) from a horn operating pushbutton 
or another electronic circuit. 
For the purposes of economical mass production it is advisable to choose a 
piezoelectric transducer S having the additional characteristic of a 
piezoelectric sound generator (buzzer) which, mass produced for commercial 
applications, is of low cost and of high reliability within the working 
temperature range. 
For the electronic circuit, the solution to adopt is to use the technology 
currently available from semiconductor integrated circuit manufacturers, 
which combine both logic and digital functions on a single chip. In 
particular the best solution is to use a single custom device employing a 
technique which enables a single chip to provide not only the logic and 
analog functions required by the blocks F, CCS and .mu. blocks but also 
the power device for providing the function required of the block IEP. The 
complete custom device therefore assumes the appearance of a power 
transistor the heat dissipation element of which, isolated from the 
electronic circuit, can be advantageously fixed to the metal housing of 
the horn without the need for insulation. 
The advantages offered by a custom circuit arrangement can be summarized as 
follows: 
A small number of components making up the horn control unit (custom 
electronic circuit, sensor, armature connecting the sensor to the 
diaphragm). 
A low custom circuit cost for the high quantities foreseeable for the motor 
vehicle market. 
Possible simplification and automation of the horn assembly.