Electronic accelerator, brake or clutch pedal: these are the advantages
08/26/2020Everyone is familiar with the classic Bowden cable. A steel strand is guided in a pressure-resistant, flexible sliding tube or flat wire and encased in a waterproof plastic layer. Its job is to transmit mechanical force. Bowden cables are usually used to control the throttle valve via the accelerator pedal or to actuate the release mechanism via the clutch pedal.
The Bowden cable transmits tractive forces and a spring return mechanism returns to its original position mechanically. A simple but delicate system. Mechanical components are always subject to a certain amount of wear. They have to be lubricated and protected from environmental influences.
In addition, the positioning accuracy is not exact. The Bowden cable (but also a linkage) can twist and chafe on contact with vehicle components, and the core elongates over time. Hydraulic systems are common in brake systems, for example to build up brake pressure directly via the brake pedal and distribute it via the brake booster. However, solutions with Bowden cables are still used for manual auxiliary braking systems such as the parking brake.
X-by-wire
The automotive industry has already started manufacturing modern cars that do not use a Bowden cable, linkage or hydraulic power transmission when the accelerator, brake or clutch pedal is operated. The magic word is "x-by-wire", which, in our examples, means nothing other than that the driver's preferences are transmitted electronically or electro-hydraulically using the pedals.
Therefore, the pedal set is mechanically decoupled from the rest of the system periphery. A distance sensor (pedal value sensor) determines the exact position of the pedal and passes the information on to the relevant control unit. Special actuators ensure that the pedal information is converted via the relevant system components.
In addition, the x-by-wire approach allows numerous additional pieces of information from other control units to be integrated, for example to reduce engine speed in certain driving situations, to initiate braking, or to disengage or engage the clutch. This means that numerous assistance systems, such as emergency brake assistant or adaptive cruise control, or different load distributions for hybrid vehicles can be implemented.
Classic Bowden cable: The steel core (1) is guided in a sliding tube (2) made of plastic (here in the picture) or flat wire. In addition, a plastic coating (3) protects it from environmental influences. Photograph: Georg Blenk, Krafthand Medien.
The electronic accelerator pedal (gas-by-wire)
The position of the electronic accelerator varies according to the driver's preference. To put it simply, the driver accelerates. The accelerator pedal sensor is the heart of the electronic accelerator pedal. It passes on the load information to the engine control unit. The diesel engine management system uses this as the main control variable for calculating the basic injection quantity and the necessary fuel system pressure, together with the engine speed at the start of injection.
In Otto engine management systems, the accelerator pedal sensor is also used to detect the position of the gas pedal. Depending on the position, the throttle valve actuator is controlled by the engine control unit and adjusted to the throttle valve opening angle, which is dependent on the engine speed and engine load.
The accelerator pedal sensor can be a contactless inductive sensor, a Hall sensor or a potentiometer. Inductive encoders have an internal evaluation circuit and can transmit both analogue and digital signal voltages. HELLA's inductive position sensors (CIPOS®) are highly accurate, robust and extremely compact. The electronic accelerator pedal is a unit consisting of the pedal itself, the electronics, the distance sensor, the haptic unit and the on-board power supply connector.
Depending on the vehicle, the electronic accelerator pedals can be mounted upright or pendant. The accelerator pedal sensor is a safety-relevant component. For this reason, the position data of the accelerator pedal sensor are constantly checked for plausibility, either by switches (idle and full load switches) that are installed as additional components or by one or more additional potentiometers.
The accelerator pedal sensor (APS) transmits the driver's wish to accelerate or brake to the engine electronics by precisely measuring the pedal position.
Feedback function
The interface between vehicle and driver is also becoming increasingly interactive. For example, front cameras use the Lane Assist function to detect whether a car is leaving the centre of the lane. The steering wheel vibrates and the driver receives warnings before the system performs any braking or steering interventions.
The accelerator pedal now also serves as a feedback component and provides feedback depending on the driving situation. Not only can the feedback pedal adjust the haptics (harder and softer accelerator pedal), but the feedback function also enables, for example, the switching point to be detected so that the switching point can be optimally selected using a knocking signal or feedback when the combustion engine is started on hybrid vehicles.
The position measurement technology is the essential part of a feedback pedal sensor, which requires a high degree of accuracy. HELLA is also a leader in this field with its CIPOS® technology.
The electronic brake pedal (brake-by-wire)
In electronic braking systems, the brake pedal is also decoupled from the actual brakes. Whereas in conventional brake systems, hydraulic brake pressure is built up directly via the brake fluid, brake-by-wire systems do this using an electro-hydraulic unit, for example. The pedal travel sensor and/or pressure sensor converts the brake demand into a signal and forwards it either to the relevant actuators or to a separate pressure generator unit with control valves at the respective brake callipers. This means that the brake booster or the ABS/ESP unit, for example, is no longer required. The pedal feel can be regulated dynamically by hydraulic valves or damping units (simulators). The brake control systems include all components required to perform the classic brake functions as well as functions for ABS, ESP, ASR and other assistance functions.
In electric vehicles, depending on the braking situation, the generator function (regeneration) and the classic pedal brake function complement each other. If the central (electro-hydraulic) unit fails, a direct hydraulic connection is established between the brake pedal and the wheel brakes as a safety backup. Ultimately, the driver's active pulse generator is still the actual brake pedal, whether decoupled or direct.
The electronic clutch pedal (clutch-by-wire)
Based on the decoupled accelerator or brake pedal, the electronic clutch pedal also has no mechanical connection to the clutch. An electric motor or actuator opens or closes it. The motor or actuator's position is defined by the position sensor on the clutch pedal. An intelligent control system can be used to optimise the characteristic curve of the coupling process by manually actuating the clutch pedal. This protects the clutch from unnecessary wear. Furthermore, systems, such as a starting assistant, engage assistant, engine stall protection system or a sailing function to reduce fuel consumption, can be implemented.
It all depends on the sensor technology
X-by-wire applications are impossible without sensor technology. HELLA specialises in a wide variety of sensors that are directly or indirectly responsible for making it possible, especially for accelerator pedal sensors and numerous other position sensors. The in-house CIPOS® technology stands for accuracy and reliability.
Further information and test accelerator pedal sensors
By the way: in a separate article, we provide a lot of useful basic knowledge and important tips about accelerator pedal sensors and pedal value sensors.
Figure: Workshop manager talks to customers in a car workshop
