Regenerative Braking Background

Hybrid and electric vehicles typically have a system known as regenerative (regen) braking which helps to improve efficiency. Essentially, regen braking turns the motors into generators, thereby converting the cars kinetic energy into electrical energy and later into chemical energy in the battery pack. Since regen braking is only effective at high speeds, all hybrid and electric vehicles also have traditional mechanical brakes. Another important point about regen braking is that if too much kinetic energy is converted to electrical energy in too short a time, damage can incur (thermal runaway, fire, explosions, earthquakes, viral pandemics, alien invasions, etc.)

The problem

While driving, there are scenarios when it is be more desirable to use more regen braking but also scenarios when it is best to use mostly mechanical braking:

The problem we face is designing the interface and control architecture to allow for these two states as well as some middle ground. Some important criteria include:

The system must provide a way to engage regenerative braking without engaging the mechanical brakes
The mechanical brake must be robust and able to provide the necessary maximum braking forces

Possible Solutions

1) Dual stage brake pedal

Design the brake pedal mechanism to have two stages: one where the pedal position is measured with a sensor to control regen braking and one where the pedal engages the mechanical brakes.

The above sketch is a gross abstraction but it illustrates the concept. The mechanical brake would not engage until part way through the pedal travel

2) Two brake pedals

Have two brake pedals, one for regen braking and one for mechanical braking

3) Automatic regenerative braking

When neither the brake pedal or the accelerator pedal is pressed the regen brake would automatically engage. The brake pedal would have a single state (both mechanical and regen braking). To permit coasting, the accelerator pedal would have two positions. Think of the button that controls power windows in a car. There is usually a detent half way down where the logic goes from "stop when button released or window reaches bottom" to "stop when window reaches bottom". Similarly, our accelerator pedal could have a detent part way down where the logic goes from the "coasting" state to "accelerate" state. That way we still have two braking states: regen only when no pedal is pressed, regen and mechanical when the brake pedal is pressed. Overall we would have 4 states as shown below:

Truth Table
Inputs			Outputs
Brake pedal pressed	Accel. pedal pressed	Accel. pedal pressed beyond detent	Regen braking	Mechanical braking	Power Driven to Motors	State Name
0	0	0	1	0	0	Regen braking
0	0	1	Mechanically impossible state
0	1	0	0	0	0	Coasting
0	1	1	0	0	1	Motors driven
1	X	X	1	1	0	Regular Braking

The above diagram is a simplification and obviously the control system would be more complex. For example, the "motors driven" state would be analog and dependant on the accelerator pedal position as well as the car's current velocity. The regen gain would also change relative to the car's current velocity.

This system is similar to the system that Tesla uses:


(excerpt from the the Model S owners manual - section 4.21)

4) Decoupled brake pedal

Have the brake pedal mechanically decoupled from the master cylinders. A hydraulic power unit would actuate the mechanical brakes and be electronically controlled.

Renault explains this system well on their website:

5) Hand paddle for regen brake

Use a hand paddle for regen braking like in MSXI.

Decision Matrix

Proposed Solution	Safety	Control System Complexity	Similarity to Real Car
1) Dual stage brake pedal	Less of the total brake pedal travel would go towards mechanical braking: leading to lower braking power	Simple control system: regen gain can be coupled to pedal position (with interlocks for safety at high speeds)	Driver may forget that they have to press pedal past a certain point to engage mechanical braking
2) Two brake pedals	Driver may step on the regen pedal when mechanical braking is preferred/required Mechanical brake pedal can be made simple and robust for maximum reliability	Simple control system: regen gain can be coupled to pedal position (with interlocks for safety at high speeds)	Far more difficult for the driver to transfer skills from a traditional vehicle Lower practicality score
3) Automatic regenerative braking	Brake pedal can be made simple and robust for maximum reliability Full pedal travel can be used for braking (maximizes power) Risk of damage to battery if the automatic regen braking applies too high a gain for the given speed	More complex control system requiring testing and algorithm vetting	Tesla uses logic similar to this In a car with a combustion engine, when you release the gas pedal completely, the resistance of the transmission slows you down just like regen braking would if it were engaged automatically. Otherwise it would be like shifting to neutral or pushing in the clutch with no pedals depressed
4) Decoupled brake pedal	Would require extensive testing to prove safety Many moving parts increases chance of failure	Very complex control system requiring control over several different braking mechanisms in parallel	Very good control over crossover region between regen braking and mechanical braking Widely implemented in industry with power braking systems
5) Hand paddle for regen brake	Could be difficult to engage while turning (especially if on steering wheel)	Fairly simple	Non intuitive and unlike conversional car (lower practicality score)

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