Controlling and maintaining air-pressure in vehicle tires

Apparatus for controlling and maintaining air pressure in vehicle tires. The vehicle compressor outlet is connected through individual pneumatic tire circuits to each vehicle tire, via rotary couplings in the associated axles. Each tire circuit includes tubing for carrying air and monitoring tire pressure, the circuits supporting pressure sensors and electrovalves housed in a control unit in the dashboard of the vehicle. The control unit also houses electronic circuits interfaced with the sensors and electrovalves. The sensor includes a piston which causes a shutter to move in and out of the light path of a LED/phototransistor device. When it signals low tire pressure to the electronic circuit, the corresponding electrovalve is energized for the compressor to pump the tire up back to its set pressure. A front panel has LEDs and gauges for the driver to keep tabs on events.

FIELD OF THE INVENTION 
The instant invention comprises an electronic and pneumatic apparatus for 
controlling and maintaining air pressure in vehicle tires. The apparatus 
includes a pneumatic circuit for distributing air from an internal 
pressure source, such as the compressor of a pneumatic braking system of 
the vehicle, and an electronic circuit interfacing the pneumatic circuit 
for sensing tire air-pressure and metering air from the air source to the 
different tires as needs be. 
The invention also refers to a novel pressure sensor for use with the above 
apparatus 
BACKGROUND OF THE INVENTION 
A tire may wear unevenly and its life span shorten if left to roll at low 
pressure for substantial mileage. Moreover, when air-pressure in the 
different tires is uneven or inadequate in any way, vehicles are open to 
accident if braked suddenly at high speed or when transporting heavy 
loads. The development of new means of transport, both for goods and 
passengers, and improved road construction have brought lorries of ever 
increasing size and speeds as well as slicker and faster motorcars on the 
roads. It is thus essential for road safety that technologies regarding 
all the different vehicle parts keep pace by providing means preventing 
minimum fault which could lead to catastrophic results. 
Equipment is already known for regulating air-pressure in tires in both 
light and heavy vehicles. It generally comes with a control panel mounted 
to the vehicle dashboard for the driver to keep a check on tire pressure 
status. The object of this sort of equipment is to enhance vehicle safety 
and protect tires from damage through inadvertant use at inadequate 
pressure levels. 
SUMMARY OF THE PRIOR ART 
Mechanical air-pressure regulators comprise a diaphragm means driven by a 
screw-spring mechanism. Although they have proven some worth in protecting 
tires and making vehicles safer, their intrinsic mechanical nature causes 
them to be insensitive to small pressure variations, rendering them 
unprecise for keeping the initial pressure the same. They are also unable 
to restore the exact set (or initial) pressure to a leaked tire or take 
some time to do so. 
Other apparatuses come with automatic electro-mechanical regulators of the 
type designed originally for refrigerators, air conditioners and the like, 
not really adequate constructively for tires pressure regulators. The 
variety and relative complexity of the parts that make them up make them 
difficult to assemble and prone to continuous maintenance because of early 
wear of some parts, particularly the air distribution rotor damaging 
electrical connections and tube joints. In fact, they have not seemed to 
attain the purpose for which they were developed, that is, in extending 
tire-life. 
U.S. Pat. No. 4,782,879 to Le Chatelier et al discloses a hydraulic or 
pneumatic valve arrangement comprising a two-part housing having two 
independant valve systems combined on a single diaphragm mount. The first 
valve system is a two-way valve alternatively connecting the associated 
tires to an air supply system for inflating it and, via the second valve 
system, to an exhaust hole for deflating the tires. The air supply system 
is in fact a dual purpose system since it functions both as a tires 
pressurizing system and a gateable low/high-pressure valve control system. 
In its latter function, it controls the valves by either supplying 
subatmospheric pressure to open the tires to the exhaust hole or high 
pressure to open the first valve and inflate the tire. The patented device 
further includes a microcomputer control unit. 
The device is costly and difficult to manufacture, requiring highly skilled 
control technicians on the assembly line. In addition, it is both 
cumbersome and expensive to use on vehicles; for instance, in the case of 
a two-axle tractor drawing a three-axle trailer, the system would need 
eighteen pressure controllers, twenty three-way solenoid valves, a 
pressure regulator, a central control unit, eighteen manometers or the 
like plus corresponding interfaces for transmitting electrical signals to 
the control unit. 
Moreover, the size and weight of the device make it difficult to mount to 
the wheel and contribute significantly to wheel imbalance. 
As to tire pressure detection, U.S. Pat. No. 4,599,902 to Gray discloses an 
apparatus comprising a moveable or deformable pressure-responsive control 
element for engagement to a tire inlet/outlet. This control element, 
preferably in the form of a Bourdon-tube pressure-gauge, has a vane 
attached to it which switches in and out of a LED/photodiode coupler which 
drives an output transistor to connect a pulse generator to suitable alarm 
or control means responsive to a low-pressure indicative signal to pump 
the tire up. 
This device is by its very nature sensitive to mechanical vibrations and 
jerks, rendering it unsuitable for vehicles subjected to severe travelling 
conditions, such as encountered when transporting heavy loads on rough and 
earth roads or in bad weather conditions. 
A need is therefore felt for robust yet simple means for detecting and 
distributing air and for a apparatus promoting a simple yet reliable 
design. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a apparatus for 
controlling and maintaining the air pressure in land vehicle tires. 
It is also an object to overcome the prior art drawbacks and obtain a 
apparatus which is simpler, robust, more reliable, less costly, and 
particularly adequate for reliable operation under heavy conditions. 
It is another object of the invention to provide a apparatus very sensitive 
to pressure and small pressure variations, wherein tire leakages may be 
compensated with hardly any time lag. 
Yet another object of the invention is a apparatus which may be installed 
quickly and efficiently and for proper operation. 
It is a further object of the invention a apparatus as forementioned 
wherein the control unit is small and compact enough to be placed in the 
vehicle cab near the driver. 
Another object of the invention is to reduce the quantity of tubes and 
cables required in prior art devices for installation of the apparatus, 
particularly for lorries with multiple axles and trailers. 
Yet another object of the invention is to protect the high pressure supply 
(compressor) from depressurizing under big tire leaks, so that it can also 
be reliably used for the pneumatic brake system of the vehicle. 
Still another object of the invention is to provide a apparatus as 
forementioned wherein pressures may be selected from a large range. 
Another object of the invention is a pressure sensor suitable for the 
apparatus of the invention. 
It is also an object of the invention to provide a robust and reliable, yet 
simple to manufacture and service, pressure sensor.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
Refering to FIGS. 1A and 1B, the apparatus of the invention comprises an 
electronic control unit 11 (FIG. 2) housed in a closed box 13 including a 
front panel 15 mounted to a vehicle's dashboard (not shown) in front of 
the driver. The front panel portrays sundry pressure gauges 17 and LED 
indicators 19, and a push-button 21. 
The push-button switch 21 enables the driver to activate or deactivate the 
apparatus. In the deactivated state, tire pressure is not monitored, one 
of the LEDs 19P lighting up to warn the driver of this unprotected mode. 
LED 19P is preferably red and also lights up if the compressor outlet 
pressure is lower than nominal. 
The control box 13 also has a rear (i.e. facing away from the driver 
towards the vehicle's hood) panel 23 holding a plurality of air-tube 
terminals 24 which are connected to the pneumatic circuit of the 
apparatus. 
The control unit 11 comprises an electronic circuit shown schematically in 
FIG. 2. Power is supplied by the vehicle's battery (not shown) via a 
battery terminal 25 and the push-button switch to a voltage regulator 27. 
The red LED 17P is in parallel with the switch and thus permanently 
connected to the battery terminal 25. In the deactivated mode, the switch 
21 connects the red LED 17P to ground voltage so that it lights up. 
Pressing the button 21 activates the apparatus, whereby LED 17P is turned 
off and electric current from terminal 25 is supplied to a voltage 
regulator circuit 27, to a green LED 17Y on the panel 15 and to LEDs 17A, 
17B . . . lighting up a plurality of pressure gauges 19, one for each tire 
28 (FIG. 3). The electronic circuit 11 of FIG. 2 interfaces with the 
pneumatic circuit 30 of FIG. 3 by means of pressure sensors 29 and 
electrovalves 31. 
FIG. 3 shows the pneumatic circuit 30 which is air-pressurized by the 
compressor 33 to maintain and sustain, when required, the tires 28 of a 
vehicle properly pumped up. Only two such tires 28A, 28B are shown for the 
sake of simplicity to illustrate the basic circuit and the add-ons for 
each tire 28. The compressor outlet is connected by tubing 55P to the 
compressor pressure sensor 29P and a hub 56 for distribution, via tubing 
55A and 55B, to the tires 28A, 28B. Each tire circuit 55A, 55B includes a 
check valve 57, the electrovalve 31, the pressure sensor 29 and the 
pressure gauge 17, before joining a rotor or rotary coupling 59. (Suffixes 
A, B, etc. are omitted from the reference numerals when generalizing.) The 
wheel includes a second check-valve 61 connecting the tire 28 to the 
rotary coupling 59. Both check valves 57, 61 protect the tire system from 
leaking should any part of the tubing 55 be damaged or the compressor be 
disconnected (e.g. for servicing). 
Operation is as follows. The pressure sensors 29 are calibrated to a 
threshold value corresponding to normal tire pressure, say 95 psi. The 
compressor outlet pressure is normally much higher of course. In the event 
tire 28A, for instance, should leak air and lose a certain amount of 
pressure, say down to about 90 psi, its corresponding sensor 29A generates 
an electrical signal for the tire monitor circuit 51A (FIG. 2) to open 
electrovalve 31A, in order to let pressurized air through from the 
compressor 33 to the tire 28A. Once the tire has recovered the right 
pressure (95 psi), the pressure sensor 29A signals the monitor circuit to 
close the valve 31A back again. The driver may monitor all tire pressures 
visually by means of the gauges 19 in the dashboard panel 15. 
FIG. 4 depicts, in longitudinal section, a pressure sensor 29 suitable for 
use in the present invention. It comprises a generally cylindrical housing 
71 having a longitudinal bore 73. The bore 73 contains a diaphragm 75 and 
a piston element 77 generally resting on a seat 79. An orifice 81 on one 
side of the housing connects the bore to the tubing 55 (FIG. 3). On the 
side of the diaphragm 75 opposite the piston 77 there is a plunger 83 
supporting a shutter 85 projecting from its end opposite the diaphragm 75. 
At this end of the housing there is a cross-bore 87 for mounting a LED 37 
and a phototransistor 39 on opposite sides of the central bore 73, so as 
to define a light passage 35 therebetween. A spring 89 urges the shutter 
supporting plunger 83 away from the light passage 35, such that light from 
the LED 37 may impinge on the phototransistor 39. 
If there is sufficient pressure at the side orifice 81 from the associated 
tire 28, the piston pushes the plunger 83 so that the shutter 85 crosses 
into the cross-bore 87 blocking the light passage 35. This enables the 
phototransistor 39 to signal normal air pressure in the tire 28. 
The other end of the housing 71 is threaded externally to accomodate an 
adjustable cap 91. The cap 91 is in contact with a static stem 93 
connected to the piston 77 by a spring 95 and may be adjusted by hand, a 
screwdriver or the like to calibrate the set pressure point. 
One such pressure sensor 29P responds to inlet pressure supplied by the 
compressor 33. Refering back again to FIG. 2, inlet pressure being normal, 
that is, over a predetermined threshold, a light passage 35P between a LED 
37P and an associated phototransistor 39P is blocked. A pull-up resistor 
thereby forces the negative (or inverting) input (-) of an operational 
amplifier (op-amp) 41P up and over the voltage at the positive (or 
non-inverting) input (+) which is set by a voltage-divider resistor 
network 43. The output of the op-amp 41 is connected by a feedback 
resistor 44P to enhance hysteresis in the switching response of the op-amp 
41P. The low level normally present at the output of the op-amp 41P is 
twice inverted by a pair of cascaded logic inverter gates 45 and shuts off 
a driver transistor 47P. The op-amp 41P and its associated circuitry 
condition the signal derived from the pressure sensor, in particular to 
sharpen driver transistor 47P switching and introduce hysteresis to avoid 
oscillations when pressure is near threshold levels or from mechanical 
vibrations, which could be encountered if the vehicle travels over rough 
terrain. 
This driver transistor 47P connects the cathode of warning LED 17P to 
ground, in parallel with the push-button switch 21. If the compressor 33 
pressure is too low, the light passage 35P will become unblocked, 
resulting in that the driver transistor 47P turns on and, consequently, so 
does the LED 17P. 
The electronic circuit in FIG. 2 comprises a plurality of tire monitor 
circuits 51. Again, only two such tire monitor circuits 51A and 51B are 
shown for simplicity. For example, the circuits 51A, 51B may correpond to 
the tires 28A, 28B (FIG. 3) of a two-wheel axle of the vehicle. 
If tire pressures are at the right level, the corresponding pressure 
sensors 29A, 29B will block the light passages 35A, 35B formed between 
respective pairs of LED 37A, 39A and phototransistor 39A, 39B assemblies. 
The phototransistors 39A, 39B are in commonemitter configuration, so that 
the associated op-amps 41A, 41B will normally supply high levels to 
respective logic NOR gates 53A, 53B. The resulting logic low levels at the 
ouput of these NOR gates 53 will thus keep driver transistors 47A, 47B in 
the off state. These driver transistors 47A, 47B form solid state switches 
in series with the solenoids of a pair of electrovalves 31A, 31B. The 
electrovalves 31 control high pressure air supply from the compressor 33 
to the respective tires 28 through the pneumatic circuit disclosed 
hereinbefore in connection with FIG. 3. 
If pressure at one (or more) of the tires 28, say tire 28A, drops down low, 
light from the LED 37A will impinge on the phototransistor 39A in the 
corresponding pressure sensor 29A, driving it to saturation. The op-amp 
41A will receive a high voltage at its negative input (-) so that its 
output will go low, the NOR gate output go high, and driver transistor 47A 
saturate to energize the solenoid of the corresponding electrovalve 31A. 
As a result, the valve 31A will open to let pressurized air enter the tire 
28A until the tire has been inflated to a predetermined pressure (plus a 
bit more to allow for hysteresis). The feedback resistors may be 
dimensiones so that the response function has hysteresis in the 90-100 psi 
range. 
The NOR gates 53 have a second input each jointly connected to the output 
from the op-amp 41P in the compressor monitor circuit, to ensure that the 
tire electrovalves 31 are kept closed in the event that the compressor 33 
should become under pressurized. This will protect the tire 28 from 
becoming even more unpumped in such a circumstance and will also ensure 
that compressor pressure is not permanently lost in the event of a big 
tire leak, such as a blow-out. This is important to preserve vehicle 
braking ability. 
In the case of double-wheel mounts on lorry rear axles or trailer axles, 
wherein a second wheel 28' is mounted alongside the first wheel 28 on the 
same side of each axle, as shown in FIG. 5, the rotary coupling 59 is 
provided with an additional orifice for connecting a tube 55' therefrom to 
the tire air inlet. An additional second check-valve 61' is provided in 
that tubing 55'. The second check-valves 61, 61' prevent all tires 28, 28' 
on one axle from going flat when one of them has a puncture. 
Alternatively, double rotary air couplings could be used although they are 
not preferred because they are complicated to manufacture. 
The pressure sensors 29 and electrovalves 31 are all conveniently housed 
inside the cabin box 13. This minimizes electrical wiring and avoids 
having electrical leads other than the power supply leading out of the 
rear panel 23. All tubing 55, including from a trailer hooked on to the 
back of the lorry, is connected straight from the rotors 59 to the tube 
terminals 24 on the rear panel 23. This has several advantages, such as 
simplifying maintenance and shielding devices 29 and 31 from bangs and 
dirt and reducing the chances of an electrical wire being severed or 
having to be layed out in protective shieths. Thus the tube terminals 24 
comprise an inlet terminal 24P from the compressor 33 plus terminals 24A, 
24B, . . . for the tubing 55 to the tires 28. 
Although a preferred embodiment has been brought out in relation to the 
drawings, the invention is by no means limited thereto but rather extends 
to all embodiments within the purview of the appended claims.