Chapter 4. Human-Machine Interface
2. HMI classifications
The automotive HMIs can be classified in several ways. Our classifications detailed in the next sections are based on the relevance, the direction (I/O) and the applied technology of the HMIs.
The direction parameter can be input or output from the drivers‘ point of view. While through the input interfaces the driver can intervene into the operation of the vehicle, the output interfaces enable the indication of any vehicle parameter, thus notifying the driver about any information.
In the following subsections the relevance groups are detailed. The technical solutions are especially various and diverse, hence the authors considered the detailed explanation important. That is why the applied technologies are described in a new section.
2.1. Primary HMI components
The primary HMI components are used for operating the vehicle basic functions and let the driver control the movement of the vehicle. Furthermore some of these components (and the minimum set of the devices) are regulated by the legal authorities. The following primary components can be considered as a basic primary HMI set in a passenger car with manual gearbox:
• Input devices
• Steering wheel
• Pedals (accelerator, brake, clutch)
• Gear shift lever
• Parking brake
• Turn indicator stalk
• Wiper stalk
• Light switch
• Horn
• Output devices
• Instrument cluster
• Speedometer
• Important warning lights (oil pressure, charging etc.)
• Turn signal indicator light
• Fuel level
2.1.1. Input channels
It is important to explain the evolutionary process of the steering wheel and the accelerator and brake pedals.
(The clutch and gear shift evolved in another way, because of the automatic gear shift systems.) Nowadays the x-by-wire systems are increasingly coming to the fore as mentioned in the Section ―Intelligent Actuators‖. The necessary technical solutions are available, ―only‖ the responsibility and legal questions are not solved yet.
With using solely x-by-wire systems in case of the primary input HMIs, the connections to the actuators are electronic signals or high-level digital messages, which determine the desired motion state of the vehicle. This digital byte stream is often referred to as motion vector. It contains the demanded values for engine torque, brake pressure and steering angle and is extended by further values like gear-shift command. With this indirect connection, the electronic control units can execute the drivers command in a more effective and safer way.
In this technical level the input devices could be any type of human interfaces, the only requirement is to generate the proper electronic output. Beyond the conventional controls, several other possibilities are available, such as a joystick that is (besides the computer games) generally used to control forklifts. Although joysticks are technically perfectly integrated control devices, these solutions are not likely to be widespread in passenger cars, because they require absolute different driving techniques. Undoubtly x-by-wire systems have got a greater importance beyond the already utilized opportunities, namely the capability to control the vehicle by a computer. As the electronic interface (motion vector) can either be generated from the human driver or from an electronic control, the intelligent actuators cannot make a difference if the request was originated from driver or a control ECU. Moreover it is one of the basis of the highly automated or fully autonomous vehicle operation.
In modern vehicles the primary controls are equipped with several sensors which could be the following:
• Steering wheel
• Angle
• Force, torque
• Touching pressure and position (for detecting the driver presence and state)
• Driver pulse detection for medical purposes
The primary HMI output is the instrument cluster. Basically it is responsible for displaying the motion state of the vehicle and the engine (such as velocity and rotational speed), feedback about the execution of the desired task (such as turn signal indicator light), and warning of the problems and errors. Additionally it could inform the driver about the detailed vehicle status, the driving situation and the current level of automation.
If we look through the evolution of the instrument cluster, it can be noticed that the development direction goes from the mechanical to the fully electronic solutions. Until the 1990s the analogue gauge of the speedometer was connected to the gearbox by a Bowden cable. Initial electric gauges used Deprez instruments, later they
were driven by PWM signals using cross-coils technology. With release of the electronic motor control units, the needle of the gauge was driven by a small stepper motor which connected to the ECM. The instrument cluster has been complemented with alphanumeric and later graphical LCD displays. Nowadays the state-of-art solution is the fully colour LCD-based instrument cluster with variable style displaying. These technologies will be detailed in Section 4.3.
2.2. Secondary HMI components
The role of the secondary HMI components is to operate and display the comfort and infotainment functions.
The controls can be found on the dashboard, around the central armrest, on the door armrest, on the steering wheel and sometimes over the central mirror. Naturally nowadays it is a very diverse and large component group. The size and content is highly dependent on the equipment features of the vehicle. Basically it contains the control inputs of the heating, ventilation, and air conditioning (HVAC) system and the radio, and some indicator lights and alphanumeric LCDs.
2.2.1. Input channels
The evolution of the secondary input controls is very diverse, but it has a common property at each manufacturer, namely the multiplication of the comfort and infotainment functions. It has resulted many buttons, switches, sliders and knobs all around the driver. The first solutions for the simplification were the integrated controllers with which the driver can navigate through a menu on an LCD display.
The BMW iDrive Controller was designed to be one single interface for many functions and features of the vehicle through the central console display, as replacing the array of controls of the comfort and infotainment functions with an all-in-one unit. The rotate-and-press mechanism enables one-handed operation: right means
‗continue‘, left ‗back‘, turning the button allows you to scroll through a list and pressing it selects an option.
Frequently used functions like multimedia, radio or navigation have direct access keys. (Source: [51])
Figure 4.4. BMW iDrive controller knob (Source: BMW)
Several other methods exist to simplify the secondary input HMIs, such as steering wheel buttons and touchscreens. Generally most of these solutions are based on a menu, displayed on a relatively large screen.
The design of the secondary input devices is a more and more challenging task, because of the conflicting requirements, i.e. the increasing number of functions (especially infotainment) and the traffic safety.
2.2.2. Output channels
The secondary HMI output channels do not differ that much from the primary output channels, meaning that they are mostly relatively big LCD displays, but they may include special features like touch-screen control or split view of the central console, where on the same display device the driver and the ―navigator‖ views different pictures at the same time.