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Chassis Technologies for Autonomous Robots and Vehicles



1. Introduction

This is part of the PerceptIn technology blog on how to build your own autonomous vehicles and robots. The other technical articles on these topics can be found at https://www.perceptin.io/blog.In this article we briefly introduce the chassis technologies, especially drive-by-wire, required for building autonomous vehicles and robots. Drive-by-wire refers to electronic systems that replace traditional mechanical controls [1]. Instead of using cables, hydraulic pressure, and other ways of providing a driver with direct, physical control over the speed or direction of a vehicle, drive-by-wire technology uses electronic controls to activate the brakes, control the steering, and operate other mechanical systems. There are three main vehicle control systems that are commonly replaced with electronic controls: throttle, brakes, and steering. · Electronic Throttle Control · Brake-By-Wire · Steer-By-Wire​



2. Electronic Throttle Control

Unlike traditional throttle controls that couple the gas pedal to the throttle with a mechanical cable, these systems use a series of electronic sensors and actuators. In vehicles that use true electronic throttle control (ETC), the gas pedal sends a signal that causes an electromechanical actuator to open the throttle.


Figure 1: electronic throttle control

A typical ETC system consists of an accelerator pedal module, a throttle valve that can be opened and closed by an electronic throttle body (ETB), and a powertrain or engine control module (PCM or ECM). The ECM is a type of electronic control unit (ECU), which is an embedded system that employs software to determine the required throttle position by calculations from data measured by other sensors, including the accelerator pedal position sensors, engine speed sensor, vehicle speed sensor, and cruise control switches. The electric motor is then used to open the throttle valve to the desired angle via a closed-loop control algorithm within the ECM. The throttle valve is a part of the ETB. On the vehicles equipped with the throttle controller sensor, the throttle opening is determined based on how far the gas pedal was pressed.



3. Brake-by-Wire

A brake-by-wire system consists of a spectrum of technologies that range from electro-hydraulic to electromechanical, and both can be designed with fail-safes in mind. Traditional hydraulic brakes make use of a master cylinder and several slave cylinders. When the driver pushes down on the brake pedal, it physically applies pressure to the master cylinder. In most cases, that pressure is amplified by a vacuum or hydraulic brake booster. The pressure is then transmitted via brake lines to the brake calipers or wheel cylinders.Anti-lock brake systems were early precursors of modern brake-by-wire technologies, in that they allowed the brakes of a vehicle to be pulled automatically with no driver input. This is accomplished by an electronic actuator that activates the existing hydraulic brakes, and a number of other safety technologies have been built on this foundation. Electronic stability control, traction control, and automatic braking systems all depend on ABS and are peripherally related to brake-by-wire technology. In vehicles that use electro-hydraulic brake-by-wire technology, the calipers located in each wheel are still hydraulically activated. However, they are not directly coupled to a master cylinder that is activated by pushing on the brake pedal. Instead, pushing on the brake pedal activates a sensor or series of sensors. The control unit then determines how much braking force is required at each wheel and activates the hydraulic calipers as needed. In electromechanical brake systems, there is no hydraulic component at all. These true brake-by-wire systems still use sensors to determine how much brake force is required, but that force is not transmitted via hydraulics. Instead, electromechanical actuators are used to activate the brakes located in each wheel.

Figure 2: an example brake-by-wire system

4. Steer-by-Wire

Most vehicles use a rack and pinion unit or worm and sector steering gear that is physically connected to the steering wheel. When the steering wheel is rotated, the rack and pinion unit or steering box also turns. A rack and pinion unit can then apply torque to the ball joints via tie rods, and a steering box will typically move the steering linkage via a pitman's arm. In vehicles that are equipped with steer-by-wire technology, there is no physical connection between the steering wheel and the tires. In fact, steer-by-wire systems don’t technically need to use steering wheels at all. When a steering wheel is used, some type of steering feel emulator is typically used to provide the driver with feedback. The control of the wheels’ direction will be established through electric motor(s) which are actuated by electronic control units monitoring the steering wheel inputs from the driver. Such a system is illegal in most jurisdictions for passenger or commercial vehicles.


Figure 3: an example steer-by-wire system

4. Open Source Car Control

To learn more about drive-by-wire technologies for autonomous robots and vehicles, Open Source Car Control (OSCC) is a good starting point [2]. OSCC is an assemblage of software and hardware designs that enable computer control of modern cars in order to facilitate the development of autonomous vehicle technology. It is a modular and stable way of using software to interface with a vehicle’s communications network and control systems. OSCC enables developers to send control commands to the vehicle, read control messages from the vehicle’s OBD-II CAN network, and forward reports for current vehicle control state. Such as steering angle and wheel speeds. Control commands are issued to the vehicle component ECUs via the steering wheel torque sensor, throttle position sensor, and brake position sensor. This low-level interface means that OSCC offers full-range control of the vehicle without altering the factory safety-case, spoofing CAN messages, or hacking ADAS features. Although OSCC currently supports only the 2014 or later Kia Soul (petrol and EV), the API and firmware have been designed to make it easy to add new vehicle support. A detailed tutorial of converting a Kia soul EV into an autonomous vehicle using OSCC can be found here [3].



5. DragonFly Pod Chassis

Note that at PerceptIn we develop various kinds of autonomous robot and vehicle chassis, tailored for the application needs. Thus far we have developed an autonomous explorer for search and rescue missions, a passenger pod (the DragonFly pod), and an intelligent advertising machine.


Figure 4: PerceptIn autonomous vehicle and robot chassis

Figure 5: PerceptIn intelligent advertising vehicle

Specifically, the DragonFly Pod is a low-speed, affordable, and reliable autonomous vehicle built using PerceptIn’s modular design methodology [4, 5]. The detailed design methodology of DragonFly can be found in [6], and a testing demo can be found in [7]. The vehicle chassis is equipped with electronic throttle control, steer-by-wire, and brake-by-wire technologies. Detailed specifications of DragonFly pod can be found below.


Figure 6: PerceptIn DragonFly Pod




References

1. Nissan Drive by Wire, Accessed 1 November 2018,

https://www.youtube.com/watch?v=MH7e5aUDWYY&feature=youtu.be

2. Open Source Car Control, Accessed 1 October 2018,

https://github.com/PolySync/oscc

3. Drive by Wire Installation, Accessed 1 October 2018, https://github.com/frk2/opencaret/wiki/Drive-by-wire-Installation

4. DragonFly Pod, Accessed 1 November 2018,

https://www.perceptin.io/dragonflypod

5. PerceptIn Launches the DragonFly Pod: The World's First $40,000 AV, Accessed 1 November, 2018, https://www.futurecar.com/2649/PerceptIn-Launches-the-DragonFly-Pod-The-Worlds-First-$40000-AV

6. Build your own autonomous vehicles with DragonFly technologies, Accessed 1 November 2018, https://www.perceptin.io/blog/build-your-own-autonomous-vehicles-with-dragonfly-technologies

7. PerceptIn DragonFly Pod test, Accessed 1 November 2018,

https://www.youtube.com/watch?v=MH7e5aUDWYY&feature=youtu.be

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