April 30, 2022

Invention of Car Wiper | Robert Kearns flash of genius

 The modern wiper technology we all have taken for granted was born because of one man's flash of genius, Prof. Robert W. Kearns created the efficient wiper technology and grabbed inspiration from the human eye. The wiper technologies before his were pretty inadequate and obstructed the driver's eyesight, Mr. Kearns's wiper technology was so original and brilliant that the ford company tried to steal it.

Invention of intermittent windshield wiper | Robert Kearns | Flash of genius


Let's analyze the details of his brilliant invention. First, we need to understand the mistakes professor Kearns made while developing their wiper technology. Wiper blades achieve an oscillating motion with the help of a four-bar linkage mechanism, a dc motor drives a worm the worm gear assembly, which is used for the torque multiplication. This mechanism gives the continuous movement of the wiper but these continuously moving wipers can block the vision of the driver. The continuously moving wipers will greatly affect your vision even during a light rain. Here comes the genius of Mr. Robert Kearns who presented the idea of periodical wiping. He observed that humans blink their eyelids but this blinking never obstructs their vision, the reason we are unconscious of our blinking is that it's intermittent our eyelids take a long rest after each blink. This long rest at the extreme ends is the reason why our eye blinking does not interrupt our eyesight in terms of driving. If we stop the wiper blade for a certain period after each wiping cycle would reduce interference with the driver's vision this moment was professor Kearns's flash of genius.

Cam Arrangement in Bar linkage mechanism

We can achieve this intermittent wiper design using a cam arrangement. The motor's output can be connected to a cam which will move the wiper for only a short period and stop for a dwell period at the bottom of the windshield. we have achieved intermittent wiping or have we can you see the problem with the design. The reality is that a wiper technology's dwell time or rest time should be differed based on the amount of rainfall. In a low rainfall situation, you need a long dwell time whereas in a heavy rainfall situation you need a shorter dwell time-varying the dwell time using a purely mechanical arrangement is not practical. In fact, Prof. Kearns was so smart that he realized the issue of purely mechanical intermittent wipers and he didn't even try this design to achieve a varying dwell time wiper design the solution cannot be purely mechanical it should possess electronics as well.

Adding electronics solutions to it

This realization was Kearn's second flash of genius for this purpose he developed an electronic circuit with all these features embedded in it. To understand the circuit let’s recollect some basic details about transistors. The transistor turns on when its base is forward biased and it is reverse biased when turned off.

He introduced a double-throw switch to make the switching of forward and reverse bias states easier when the switch is in this condition the base is forward biased and the circuit is on when the switch is in state b the base goes for reverse biased condition and the current flow stops in the circuit. He used this transistor circuit to power the wiper motor here the wiper mechanism is directly connected to the motor output. This is obviously a continuous wiping interestingly a cam connected to the motor can easily operate the switch and the circuit goes off the wiper is in the dwelling stage now. However, this is an infinite dwell time this circuit won't be able to turn on the motor again. 

Let's go back to the active stage and try to fix this issue this circuit is an active stage now and introduce a capacitor resistor pair in the circuit, the current flow direction in the resistor, due to this resistance there will be a potential difference between the two terminals of the capacitor and this will make the capacitor get charged. When the cam operates the switch the circuit goes off state as we know earlier now the charged capacitor acts as a hero it's ready for release the potential at point B is always fixed, however, the potential at point a will change during the discharge. Let's assume when the capacitor is fully charged the potential at point a is more than at point B the transistor is obviously reverse-biased however as the capacitor starts to discharge the voltage at the point a drops at some instance voltage at the point A becomes less than at the point B and the transistor becomes active the time duration of discharge of the circuit to activation of the base is the dwell time of this wiper.

The interesting thing is that we can easily adjust this dwell time by altering the resistance value. If the resistance is more then dwell time will be more, this way using clever electronics professor Kearns achieved a variable dwell time wiper mechanism. The heart of this invention is a brilliant electronic circuit but it is driven by a mechanical switch absolute genius right. 

However, during heavy rainfall, we need almost zero dwell time it's not practical to achieve zero resistance, and thus zero dwell time during heavy rainfall the friction between the glass and wiper is very low.

During its dwell or non-active stage the circuit is is connected but the wipers still have a good momentum they will continue the downward movement since the frictional force is low due to the wiper's high momentum they provide the driving force for the rest of the mechanism in the cam rotates this leads to activation of the motor again, in short, the inertia of the wipers helps to skip the dwell period of the mechanism this is a crude method to get continuous wiping from an intermittent mechanism in professor Kearn's patent, he has even elaborated more sophisticated circuits to achieve the continuous wiping modern-day wipers consist of relays instead of cams to drive the wiper motor the dwell time can be accurately measured and changed using timer circuits and microcontrollers also these microcontrollers take input from moisture sensors or rain sensors present on the windshield to automatically wipe the windshield if it's wet after developing such a brilliant wiper design.

What happened to professor Kearns was tragic he had to spend a good part of his life in court battles against ford motor company for infringing his patent finally he won the legal battle.

The fact that simple observations can lead to amazing inventions.

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