ELECTRONIC HORN FOR MOTOR VIHICLES FIELD OF THE INVENTION:
The present invention relates an electronically controlled horn used in motor vehicles. More particularly, the present invention relates to a vehicle horn with an electronic circuit for supplying a DC pulse train for energizing the coil of the electromagnet to drive the diaphragm. The electronic circuit for supplying the DC pulse train to the coil of the electromagnet is formed as a single chip integrated circuit and is housed inside an electronic circuit casing which is placed essentially outside the housing of the horn.
BACKGROUND OF THE INVENTION:
Sound generating devices of the electromagnetic excitation type currently consist of:
(i) silient steel diaphragm carrying in its centre the mobile part (armature) of an
electromagnet; (ii) an electric switch with a normally closed contact connected in series with the
power feed to the electromagnet; (iii) an adjustment screw which determines the switch contact opening and (iv) 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. The mechanical aspect of the horn is described in further detail in U.S. Pat. No. 4,361,952 issued to James Neese, which is incorporated herein by reference. It is suggested that a person refer to the above mentioned US Patent to understand the basic construction of the horn.
These normal switch devices have considerable drawbacks, which can be summarized as follows:
1. 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.
2. The sound output is subject to considerable fall-off with time due to the mechanical instability of the switch operation points.
3. 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.
4. 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. One of the speakers that have been built using the above technique is covered in US Patent No. 5,293,149.
However, this method has certain drawbacks which can be summarized as follows:
(a) the need to provide 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
(b) 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.
It has been proposed that the aforesaid drawbacks can be obviated by linking the electronic oscillator frequency to the resonance frequency of the electromechanical unit
which generates the sound. A horn using an electronic circuit based on the above principle was found to have better characteristics than a horn incorporating a mechanical switch. However, it has been noticed that despite all efforts, it has not been possible to precisely link the electronic oscillator frequency to the resonance frequency of the electromechanical unit which generates the sound. Further, the characteristics of such a horn 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 it proposed to utilize 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.
In the previous attempts to maximize the force of attraction which the electromagnet exerts on the armature other inventors have tried 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.
Although, prolonging the electrical feed to the electromagnet was able to impart improvements in the output, still, the electrical feed to the electromagnet cannot be prolonged beyond a certain level.
Accordingly, the Inventors felt a need to arrive at an improved construction of the electronic horn which obviates the above described disadvantages.
Now looking into the construction of the conventional electronic horn, the Inventors would like to highlight that the electronic circuit is typically formed upon a circular PCB and is located essentially inside the housing of the horn. More particularly, the electronic circuit is mounted upon the mobile armature. Figures 1 and 2 of US Patent No. 5,293,149 (Indian equivalent not available) teach construction of an electronic horn with the
electronic circuit placed inside the housing. Placement of the electronic circuit inside the housing of the horn was considered to be beneficial in nature because it reduced the cost incurred in preparing a separate housing for the electronic circuit. It was also believed that placing the electronic circuit inside the housing reduced the extent of vibration up to which the electronic circuit was subjected and hence, increased the life of the electronic circuit. However, over a period it has been noticed that despite placing the electronic circuit inside the housing, the most common reason due to which an electronic horn fails to work is damage to the electronic circuit.
The Inventors have noticed the following problems in the above mentioned horn:
1. Since the PCB is mounted upon the mobile armature, the mobile armature has to have substantial height. Increasing the height of the mobile armature increases the weight of the mobile armature. Due to the increased weight of the mobile armature, the coil has to be supplied additional amount of energy to attract the armature. Hence, the energy spent by the battery increases.
2. The thickness of the diaphragm has to be substantially increased to compensate for the additional weight of the mobile armature. Due to the increased thickness of the diaphragm, the quality of the sound produced by the horn decreases drastically. Also, due to the increased thickness, the elasticity of the diaphragm reduces and the diaphragm is more susceptible to cracking.
3. As indicated above, the most common reason due to which the horn fails is the improper functioning of the electronic circuit. It was noticed that the temperature inside the casing of the horn is quiet high. When ever the horn is blown, the current passing through the coil produces huge amount of heat due to which the temperature inside the horn increases tremendously. The maximum temperature up to which the electronic circuit could function accurately is 80°C. Beyond 80°C, the timer chip (IC 555) which was most commonly used in the electronic circuit starts malfunctioning.
4. Since the electronic circuit is located inside the casing, the electronic circuit has to be assembled altogether at the place where the horn is manufactured. Assembling the electronic circuit inside the horn is a very tedious process which requires lot of time and skill. Also, the numbers of components that are wasted during this
process are high. Further, the machinery required to assemble the electronic circuit inside the horn are very costly. As can be noticed from US Patent 5,293,149 for trimming of the resistor thus assembled on the PCB, special technique such as laser trimming are required, which are not possible to be introduced in a horn manufacturing industry. In a country like India, it is not possible also to educate the laborers to assemble a circuit board inside the casing of the horn.
It was found that the temperature inside the housing increases at very fast rate of and within a few minutes of continuous blowing, the temperature inside the casing used to be beyond 80°C (i.e. the temperature beyond which the 555 IC starts malfunctioning). The Inventors would like to highlight that a horn produced in accordance with the teachings of US Patent 5,293,149 will not meet the standards prescribed by the ISO regarding continuous blowing. In order to meet the standards prescribed by the ISO, attempts to cool the interior spacing of the electronic horn by forced cooling were tried. In one of such methods, the casing of the electronic housing was provided holes for the air passage. Although the method was able to marginally reduce the rate at which the temperature inside the casing increased, still the horn prepared in accordance with the above principles was also not able to meet the requirements prescribed by the ISO.
When the casing is provided with holes, to decrease the temperature inside the casing, the holes thus made in the casing have to be filled with special filters (called SVS filters) to avoid spoilage of the horn by moisture present in the atmosphere. The SVS filter is a very expensive filter and use of SVS filters in the construction of the horn increases the manufacturing cost of the horn by Rs. 4 to 8 depending upon the size of the holes and the number of holes provided. It should be noticed that other components of the horn are capable of withstanding a much higher temperature and hence, use of SVS filter is necessitated because of the use of the electronic circuit, which is not capable of withstanding temperatures above 80°C.
8. When the electronic circuit is placed inside the casing of the horn, it is necessary to maintain the temperature of the electronic circuit at a constant temperature. However, due to the excessive heat produced by passage of the current in the coil, it is not practically possible to maintain the temperature of the electronic circuit at a constant value. In addition to the temperature produced in the surrounding, the electronic circuit also produces considerable amount of heat. Due to the excessive amount of heat produced in the surrounding and produced by the circuit, the frequency of the DC pulse train produced by the electronic circuit used to vary over a wide range and it was not possible to control the same once the horn is packed and sealed.
The following table shows the change in the frequency of the DC pulse train produced by the electronic circuit placed inside the horn casing.
Table 1: Time Vs. Frequency Plot

(Table Removed)
It should be noticed that after manufacturing the mechanical components, it is not possible to change its natural frequency of oscillation. Hence, the frequency of the DC pulse train supplied by the electronic circuit should be maintained at a constant value throughout. As it can be noticed from the above table, the frequency of the DC pulse train gets reduced after about 20 seconds of the continuous blowing. Hence, after 20 seconds, there exists a mismatch between the
frequency of the DC pulse train and the natural oscillating frequency of the
mechanical component and this mismatch causes malfunction of the electronic
horn.
9. The excessive heat produced by the coil and that produced by the circuit has to be
dissipated effectively in order to allow the electronic to function properly. Thus, additional and huge heat sinks have to be provided to enable the horn meet the ISO standards. Providing the additional and huge heat sinks is not only costly, but also increases the size of the horn. Thus, the electronic horn usually was much bulkier and bigger than a conventional mechanical horn.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide a novel electronic horn in which the electronic circuit driving the coil is placed outside the housing of the horn.
Another object of the present invention is to provide an electronic horn wherein the electronic circuit is manufactured at a different location and is releasably fitted on to the horn.
Yet another object of the present invention is to provide an electronic horn which overcomes at least some of the disadvantages described above.
DETAILED DESCRIPTION OF THE INVENTION
Accordingly, the present invention relates to an electronically controlled horn comprising:
(a) a resonator for vibrating or resonating at its natural frequency being mounted upon a mobile armature, said mobile armature providing impact excitation to the resonator;
(b) a core placed substantially vertically below the mobile armature and mounted peripherally with an electromagnet for pulling the mobile armature towards the core and thereby impacting with the same under the excitation of the electromagnet;
(c) a diaphragm being centrally mounted upon the mobile armature and affixed peripherally to a housing for pulling the mobile armature away
from the core and back to its natural position when the electromagnet is switched off;
(d) a metal housing encasing all the above-mentioned parts of the horn and being provided with electrical terminals attached with rivets for providing electrical current to the horn; and
(e) characterized in that an electronic circuit casing housing a solid state energizing circuit being click fitted on an exterior surface of the metal casing and above the electrical terminal for providing DC switching current to the electromagnet at natural frequency of the resonator.
In a seashell type horn, there is provided a metal housing secured to a plastic projector at its periphery. A spring steel diaphragm is clamped at its margin between the housing and the projector and is attached at its center to a mobile armature. A core is placed in an end wall of the housing and extends toward the mobile armature. The core is spaced from the mobile armature by a small air gap.
The housing is stepped to define a small end portion including the end wall, and a larger portion terminating in a radial flange for supporting the diaphragm. An intermediate generally planar annular portion interconnects the small end portion and the larger portion. An electromagnetic coil fits within the small end portion and surrounds the core. A terminal block is fitted upon the planar annular portion which interconnects the end portion and the larger portion. The terminal block contains the terminal contacts which connect the electric horn to the external current source.
The diaphragm is mounted on the flange of the housing. The diaphragm and the mobile armature are joined together for movement as a single unit. The combined mass of the diaphragm and the mobile armature along with the spring rate of the diaphragm determine the resonant frequency of the diaphragm assembly. The coil is energized from the vehicle battery by the solid state energizing circuit of this invention which is placed inside an electronic circuit casing and the electronic circuit is located essentially outside the housing. The solid state energizing circuit is formed a single chip integrated circuit and is electrically connected by external horn terminals to the vehicle battery and to the horn switch. The housing is provided with a pair of rivets upon which the electronic
circuit casing is mounted. The electronic circuit can be shaped in any form. However, as a preferable shape, the electronic circuit casing is shaped in the form of an arc which snuggly fits upon the casing between the larger portion and the intermediate generally planar annular portion. More particularly, on the external surface of the casing of a conventional shell type horn, a terminal block containing terminal contact is provided which are used for connecting the coil of the electromagnet to the vehicle battery. The electronic circuit casing is mounted exactly on top of the terminal block. As a person skilled in the art would be aware, usually the portion above the terminal block is used for attaching the horn to the vehicle. Thus, it should be noticed that the area upon which the electronic circuit casing is mounted does not form a heat dissipating surface i.e. a surface which is capable of dissipating the heat generated by the coil to external environment.
The click fitted electronic circuit casing of the present invention containing the solid state electronic circuitry is also capable of being used with a vehicle horn of the vibrator type. This horn is of the same type of construction as the seashell horn except that the plastic projector is omitted and a resonator plate is carried by the diaphragm.
In this vibrator horn, the combined mass of the diaphragm, the mobile armature and the resonator plate along with the spring rate of the diaphragm determine the resonant frequency of the diaphragm assembly. This type of horn operates in such a manner that the mobile armature physically strikes the core once, and once only, during each cycle of vibration of the diaphragm. The force of the striking action is transmitted through the mobile armature to the center of the resonator plate and causes it to vibrate at or near its resonant frequency. The sound output from the horn is that generated by the vibration of the resonator plate, the sound waves being coupled directly from the resonator plate to the surrounding atmosphere.
It was noticed that the when the electronic circuit is placed outside the metal housing and inside the click fitted casing, the frequency of the DC pulse train produced by the electronic circuit is stable for about 75 seconds. Thereafter also, the change in the frequency of the DC pulse produced does not vary over such a broad range and the mismatch between the frequency of the DC pulse train and that of the oscillating
frequency of the mechanical component is negligible. This not only increase the time period for which the electronic horn is being blown. But also the quality of the sound being produced by the horn.
It is very important to note that the placement of the click fitted casing upon the external surface of the metal housing plays a vital role in the performance of the electronic horn. It is very essential that the click fitted casing housing the electronic circuit should be located exactly on top of the terminal block.
The terminal block is usually made of metal and is exactly on top of the terminal block, the means for mounting the horn on to the vehicle are provided. Thus, the surface exactly on top of the terminal block does not act as an effective radiating surface. Hence, the Inventors have for the first time constructed the terminal block by plastic so that it provides additional heat blocking to the click fitted casing mounted on top of the terminal block. By mounting the terminal block on top of the terminal block, the effective radiating surface is also not decreased and this also helps in maintaining the temperature of the electronic horn.
Although the removable casing for housing the electronic circuit is shown in the drawings as being a click fitting casing, other constructions such as screw mountable casing or riveted casing etc are also possible. The click fitting construction is easily manufactured and the time spent to fit such a click fitting casing is considerably lesser.
Brief Description of the Accompanying Drawings:
In the drawings accompanying the specification,
Figure 1 shows the rear perspective view of the metal casing fitted with the terminal
block.
Figure 2 (a) shows the bottom view of the terminal block.
Figure 2 (b) shows the bottom perspective view of the terminal block.
Figure 2 (c) shows the front view of the terminal block.
Figure 2 (d) shows the sectioned view of the terminal block sectioned at D-D' as shown
in Figure 2 (a).
Figure 3 shows the rear perspective view of the metal casing fitted with the terminal
block and the click fitting casing. The casing is shown with the top cover and is shown
empty for the ease of understanding.
Figure 4 shows the top perspective view of the click fitting casing.
Figure 5 shows the bottom perspective view of the click fitting casing.
Although the description of this invention has been given with reference to a particular
embodiment, it is not to be construed in a limiting sense. Many variations and
modifications will now occur to those skilled in the art. For a definition of the invention
reference is made to the appended claims.

We Claim:
1. An electronically controlled horn comprising:
(a) a resonator for vibrating or resonating at its natural frequency being mounted upon a mobile armature, said mobile armature providing impact excitation to the resonator;
(b) a core placed substantially vertically below the mobile armature and mounted peripherally with an electromagnet for pulling the mobile armature towards the core and thereby impacting with the same under the excitation of the electromagnet;
(c) a diaphragm being centrally mounted upon the mobile armature and affixed peripherally to a metal housing for pulling the mobile armature away from the core and back to its natural position when the electromagnet is switched off;
(d) the metal housing encasing all the above-mentioned parts of the horn and being provided with electrical terminals attached with rivets for providing electrical current to the horn; and
(e) characterized in that a removable casing housing a solid state energizing circuit being click fitted on an exterior surface of the metal casing and above the electrical terminal for providing DC switching current to the electromagnet at natural frequency of the resonator.

2. The electronically controlled horn as claimed in claim 1, wherein the metal housing is stepped to define a small end portion including the end wall, a larger portion terminating in a radial flange for supporting the diaphragm and an intermediate generally planar annular portion interconnects the small end portion and the larger portion.
3. The electronically controlled horn as claimed in claim 1, wherein the electromagnetic coil fits within the small end portion and surrounds the core.
4. The electronically controlled horn as claimed in claim 1, wherein a terminal block
is fitted upon the planar annular portion which interconnects the end portion and
the larger portion.
5. The electronically controlled horn as claimed in claim 1, wherein the terminal
block contains the terminal contacts which connect the electric horn to the
external current source.
6. The electronically controlled horn as claimed in claim 1, wherein the removable casing is mounted upon the pair of rivets provided in the metal housing.
7. The electronically controlled horn as claimed in claim 1, wherein the removable casing is a click fitting casing.
8. The electronically controlled horn as claimed in claim 7, wherein the click fitting
casing is shaped in the form of an arc and the casing snuggly fits upon the metal
housing between the larger portion and the intermediate generally planar annular
portion.
9. The electronically controlled horn as claimed in claim 1, wherein the click fitting
casing is mounted exactly on top of the terminal block.
10. The electronically controlled horn substantially as herein defined with reference
to the accompanying drawings.