New cars are confusing. With all the computers, sensors, and gadgets, it may seem like there's some sort of magical witchcraft taking place under the hood. We're here to show you how modern automotive computer control systems work. Last week, we looked at carburetors. Today's topic: electronic throttle control.
Back in the day, a car's throttle was attached to its accelerator pedal via a steel Bowden Cable. Today, that mechanical linkage has gone the way of the dodo in favor of electronic throttle control. Let's see how it works. For a lot of you this is review, but if we want a new generation of car enthusiasts to care about cars, it can't hurt to explain how they actually work.
ELECTRONIC THROTTLE CONTROL: FLY BY WIRE
Electronic Throttle Control (ETC) is the automobile industry’s “Fly by Wire” system. In ETC systems, a vehicle's electronic control unit uses information from the throttle position sensor (TPS), accelerator pedal position sensor (APP sensor), wheel speed sensors, vehicle speed sensor and a variety of other sensors to determine how to adjust throttle position.
Let's look at the two main sensors that comprise "Fly by Wire": the accelerator pedal position sensor and the throttle position sensor. While many think of automobile sensors as little black plastic clips that house all sorts of magic, what goes on inside these sensors is pretty simple. The accelerator pedal position sensor and the throttle position sensor work together to translate user input into throttle plate movement. Until recently, these sensors have utilized potentiometers that worked as voltage dividers. Voltage dividers use a resistive element and a wiper arm to "divide" an input voltage (called a reference voltage). They then send this "divided" voltage to a computer, which uses it to adjust the position of the throttle.
The image above helps illustrate the basic principle behind how a voltage divider works. The resistive element, also called a carbon track, is basically a piece of graphite. Moving the arm across the resistive element effectively alters the resistance on either side of the arm (R1 and R2). Moving the wiper clockwise increases R2 and decreases R1 and moving it counterclockwise does the opposite.
Let's show how the APP sensor works as a voltage divider. When you step on the gas pedal, you move the wiper arm closer to the reference voltage end of the resistive element (Vref). What does this do to the output voltage sent to the ECU? Imagine current flowing from positive (Vref) to the wiper arm. By moving the arm closer to the reference voltage, you decrease the "amount of resistance" through which the current must flow before it reaches the wiper arm. This increases the output voltage to the ECU. The exact relationship between the output voltage, the reference voltage, and the position of the wiper arm can be written as an equation:
Deriving this equation is simple. It involves use of Ohm's law (V=IR) and Kirchoff's Current or Voltage Law. We'll forgo this derivation, as the key here is to understand the concept. The ECU provides a reference voltage to the APP sensor. Physical movement of the pedal moves a wiper across a resistance element and alters the output voltage to the ECU. The ECU takes in this signal, and sends an appropriate signal to a throttle actuator, which moves the throttle plate.
The throttle position sensor works in a similar way. The potentiometer wiper is connected to the butterfly valve spindle. As the butterfly valve opens and closes, it varies the output voltage from 0 to the reference voltage. This output voltage is sent to the ECU. This is how the ECU knows the position of the throttle plate.
The problem with potentiometer-based sensors is that, as the wiper arm and the resistive element rub against one another, they eventually wear out. Newer accelerator pedal position sensors and throttle position sensors don't have this problem, as they use Hall effect as their basic operating principle. These sensors contain transducers that convert external magnetic fields into voltage. Using magnets placed on the pedal and throttle shaft as reference points, Hall effect sensors output a different voltage depending on the intensity of the magnetic field. As the pedal or throttle moves, so does the magnet. This movement changes the magnetic field strength and thus alters output voltage from the sensor to the ECU.
Now let's have a look at how these two sensors interact. Electronic Throttle Control is a closed loop system. The throttle opens based on user input (which is transmitted to the ECU via the accelerator pedal sensor), and adjusts based on readings from the throttle position sensor (which measures the position of the butterfly valve spindle).
Consider the feedback loop above. If you suddenly stomp on the accelerator, the accelerator pedal position sensor provides the “reference input”- a voltage between 0 and Vref- to the ECU. The reference input indicates where you truly want your throttle to be. The ECU interprets this signal and activates an actuator (a motor), which opens or closes the butterfly valve.
The measured output is the position of the throttle after the actuator's initial movement. This position is transmitted to the computer via the throttle position sensor's output voltage. The discrepancy between where the user wants the throttle (as indicated by the APP sensor) and the throttle's current position (as indicated by the TPS) is the “measured error.” The computer reads this error, and sends an appropriate new signal to the throttle actuator to get the throttle where the driver wants it. The new position is read via the throttle position sensor, and the process continues in a loop.
A major benefit of “Fly by Wire” systems is that it allows for easy integration of systems such as adaptive cruise control, brake override systems, and electronic stability control. Modern Fly by Wire systems include multiple TPSs and APP sensors, and throw a fault code if there is a discrepancy between redundant sensors.
If you want to see how it all works, check out the video below. Irony alert: it's a Toyota video about throttle control.
Photo Credit: kevint3141
Top Photo Credit: Bruce Fingerhood