In this article I will take you through the journey of invention of the modern car wiper. Did you know the modern wiper technology we all take for granted was born because of one man’s flash of genius? Professor Robert Kearns, who invented this efficient wiper technology, took inspiration from the human eye. The wiper technologies before his were pretty bad and obstructed the driver’s vision. Mr. Kearns's wiper technology was so original and brilliant that the Ford company tried to steal it.
First, I will make you understand the mistakes the engineers before Professor Kearns made while developing their wiper technology. The 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 is used for the torque multiplication. This mechanism gives continuous movement of the wiper. What will happen with this? These continuously-moving wipers can obstruct vision. The continuously moving wipers will greatly affect vision even during a light rain.
Here comes the genius of Mr. Robert Kearns, who introduced the idea of intermittent wiping. He observed that humans blink their eyelids, but this blinking never obstructs our vision (Fig.2A). 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 vision. In terms of driving, if we stop the wiper blade for a certain period after each wipe cycle, it reduces interference with the driver’s vision. This “a-ha moment” was Professor Kearns’s flash of genius (Fig.2B).
One can achieve this intermittent wiper design using a cam arrangement, as shown in (Fig.3B). The motor’s output can be connected to a cam, which will move the wiper for only a short period of time and stop for a dwell period at the bottom of the windshield. But, there is a problem with this design.
The fact is that a wiper technology’s dwell time or rest time should be varied based on the amount of rainfall. In a low rainfall situation, you need a long dwell time. Whereas in a heavy rainfall situation, you obviously need a shorter dwell time. Varying the dwell time using a purely mechanical arrangement is not practical. In fact, Professor Kearns was so smart that he realised 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 include electronics as well. This realisation was Kearns’s second flash of genius. For this purpose, he developed an electronics circuit with all these features embedded in it.
In order to understand the circuit, let's review some basic information about transistors. The transistor turns on when its base is forward-biased (Fig.4A) and turns off when it is reverse-biased (Fig.4B). Let’s introduce a double throw switch to make the switching of forward and reverse biased states easier. When the switch is in State A condition, the base is forward biased and the circuit is on. When the switch is in state B, the base goes for a reverse biased condition and the current flow stops in the circuit.
Here the transistor circuit is used to power the wiper motor. Now 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 dwell stage now. However, this is an infinite dwell time, this circuit won’t be able to turn on the motor again.
Let’s consider the active stage and try to fix this issue. This circuit is in active stage now. I will be introducing a capacitor-resistor pair in the circuit as shown. The current flow direction in the resistor is shown. Due to this resistance, there will be potential difference between the two terminals of the capacitor and this will make the capacitor get charged as shown.
When the cam operates the switch, the circuit goes off state as we have seen earlier. Now, the charged capacitor acts as a hero. It’s ready for discharging. 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 (Fig.7A). 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 (Fig.7B) 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 one can easily adjust this dwell time by adjusting the resistance value. More the resistance, more the dwell time. 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?
So far I have explained the mounting and demounting dwell time mechanism, but what about zero dwell time. This situation arises during a heavy rainfall. It’s not practical to achieve zero resistance and so zero dwell time.
During a heavy rainfall the friction between the glass and wiper is very low. Let’s see how this low friction affects our circuit during its dwell or non-active stage. The circuit is disconnected, 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 driving force for the rest of the mechanism and 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 obviously 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 in 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 Kearn 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. I hope you are inspired by the fact that simple observations can lead to amazing inventions. See you next time.
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