Wednesday, October 28

Braking down to the wire


Source: The following article about brake-by-wire systems which operate electronically/electrically rather than mechanically was posted by Deaglán Ó Meachair on BrakeBetter.com.

In this post, we shall spend some time exploring the topic of brake by wire, and look at the various flavors of decoupled brake systems already in the market, as well as the prospects for Brake-by-Wire using electrical energy as the energy transmission system. We will also discuss the advantages of such a system, as well as some of the trade-offs.

Peeling the onion…

When we talk about Brake-by-Wire, what we are really considering is a change in brake actuation architecture, that is, how we take a (human or robot) driver brake request and translate that into deceleration. When you first consider this, most likely you’d imagine a brake request as a pedal being pressed, and deceleration resulting from a disc and pad (or drum and shoe) being pressed together. And for an awful lot of braking, that is correct. 

Related post:
Brake-by-Wire Offers Numerous Benefits

But with the advent of terms like Single Pedal Driving, Remote Parking Assist and Traffic Sign Recognition, the topic of Brake-by-Wire becomes a good bit broader. Brake-by-Wire could also be brake by Wi-Fi, brake by geolocation or brake by tweet. (OK, hopefully not the last one, but you get the idea). So, we need to stratify and refine the topic somewhat.

Firstly, when we talk about by-Wire, we should agree that a primary facet of a by-Wire system is the ability to execute a variety of brake responses to a given brake request. In the main use case today, we’re therefore talking about a system which can vary the deceleration response to a driver’s pedal input. This might mean offering a variety of pedal characteristics, or overlaying additional brake pressure in a panic situation, or modulating applied brake pressure for wheel slip control or indeed generating some deceleration blend in response to a driver lifting off the accelerator (the Single Pedal Driving characteristic that is popular in some EVs). For all these, there is still a direct hydraulic connection between the brake pedal and foundation brake, but these overlays or offsets are generated by a control mechanism and actuated by electronic means. 

But Brake-by-Wire can also mean a hydraulic disconnect between two complimentary circuits – the typical operational mode of a modern 1 Box system. This means that in normal operational conditions, a driver pedal input is fed into a pedal feel simulator, made up of a piston working against a spring in dedicated chamber. The driver input is recreated in the second circuit through electronic interpretation, and a pump or plunger is used to generate the required pressure. In certain safety scenarios the circuits can be brought together, so the driver is directly hydraulically connected to the foundation brakes. 

This system is increasingly common among mass market vehicles as the single unit can incorporate the functions of a brake booster, brake modulation and volume blending for regenerative braking. It sacrifices some brake pedal feel granularity but provides a pretty good solution for the vast majority of drivers in the vast majority of cases while saving weight and cost.

The 1 Box system can also be modified in several ways to bring about a more ambitious Brake-by-Wire layout. A second unit built into the vehicle can provide a back-up brake controller for Autonomous Driving. In this case, the unit will dispense with the physical driver interface, and maintain the pump, reservoir and electrical connections. The unit must also have a discrete pedal travel sensor to interrogate driver requests. This can even be combined with a redundant power source to bring an added layer of back-up to a vehicle. This combination of hydraulic elements connected through electronic means is perhaps the truest Brake-by-Wire in popular use today, but it is still primarily a hydraulic system, albeit augmented with electrical brains and muscle.

The entire post with additional images can be viewed by clicking HERE.

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