EHB Self-Pre-pressurization in Autonomous Chassis

Author: Johnny Liu, CEO at Dowway Vehicle
Date: July 13, 2026
Category: Automotive Engineering / Autonomous Chassis / Drive-by-Wire
Reading Time: 7 mins

Author’s Note

“At Dowway Vehicle, we build L4 low-speed autonomous delivery, sanitation, and logistics chassis for real-world deployments. One critical safety item in drive-by-wire vehicles is verifying that the hydraulic braking system is fully functional before the wheels even turn. This technical guide explains how we use EHB Power-on Self-Pre-pressurization to handle safety diagnostics directly on the EHB unit instead of overloading the VCU.” — Johnny Liu, CEO of Dowway Vehicle

Quick Q&A (AEO Snippets)

What is EHB power-on self-pre-pressurization?

EHB power-on self-pre-pressurization is an autonomous safety routine where the brake system builds a low hydraulic pressure of about 1 bar immediately after starting up to close the brake pad gap and test for leaks.

Why is the pre-pressure set to exactly 1 bar?

A 1 bar pressure is high enough to close the 0.1 mm to 0.3 mm gap between the brake pads and the disc, but low enough to avoid causing pad drag, excessive wear, or vehicle range loss.

Does the VCU control the pre-pressurization process?

No, the VCU only sends a basic system enable signal, while the EHB’s own controller autonomously manages the pump motor, runs the leak test, and sends back a simple ready or fault status.

1. What is EHB Power-on Self-Pre-pressurization?

In low-speed autonomous drive-by-wire chassis, EHB Power-on Self-Pre-pressurization (often called autonomous pre-fill or self-pre-charging) is a self-diagnostic routine run by the Electronic Hydraulic Braking (EHB) unit.

[VCU] ─── System_Enable=1 ───► [EHB ECU] ───► Runs Motor ───► Builds ~1 bar ───► Check Sealing ───► Releases Pressure ───► [VCU] ◄─── BrakeReady

Once the EHB completes its internal boot checks and receives an enable signal (via hardwire or CAN bus: EHB_Active = 1 or System_Enable = 1), its integrated controller runs the pump motor. The pump pushes a small amount of brake fluid to the wheel cylinders to build a steady, low pressure of about 1 bar. This action gently pushes the brake pads to touch the brake disc or drum. Afterward, the system releases the pressure back to the reservoir or goes into standby.

This setup decouples the control layers. The VCU does not need to send complex pre-charge commands. It only enables the system and reads the feedback status bits (BrakeReady or BrakeFault).

2. Triggering and Setup Pre-conditions

To keep the system safe and avoid accidental activation, the EHB controller checks several conditions before running the pre-pressurization loop:

Trigger Source: The EHB controller’s internal state machine starts the process automatically after passing its power-on self-test.
Power Check: Both high-voltage and low-voltage power supplies must be within normal limits.
Enable Signal: The VCU must send System_Enable = 1 via CAN or a hardwire wake-up line.
No Active Faults: The system checks for historical errors. It will not run if there are active faults, such as sensor tolerance issues or previous leakage.
When It Runs: Usually, this is a one-time check run right after the vehicle turns on and the EHB is enabled. If the EHB also controls the Electric Parking Brake (EPB), the pre-pressurization starts right after the EPB is released.
Periodic Check: Some setups run a quick re-check every 30 minutes or after the vehicle travels a set distance.

3. Step-by-Step System Workflow

When running the pre-pressurization cycle, the EHB automatically moves through four main steps:

[Step 1: Self-Test] ───► [Step 2: Build Pressure] ───► [Step 3: Hold & Leak Test] ───► [Step 4: Release & Standby]

Step 1: Power-On Self-Test (POST)

The EHB controller checks its own hardware. It calibrates the pressure sensor zero-point, checks the motor stall current, and verifies the motor driver circuits.

Step 2: Low-Pressure Build-up

The EHB motor runs for a brief moment—usually tens of milliseconds to just over 100 milliseconds. The master cylinder opens its valves and pumps fluid to all four wheel cylinders. The motor adjusts its output to hit the target calibration: $$P_{pre} \approx 1\text{ bar}$$

Step 3: Pressure Retention & Leak Test (Optional)

Once the pressure reaches the target, the inlet valves close. The system holds this pressure for 200 to 500 milliseconds with the motor turned off. The controller monitors how fast the pressure drops ($\Delta P / \Delta t$). If the pressure drops too quickly, the system flags a leak in the brake lines or caliper, sets the BrakeFault status, and stops the vehicle from moving.

Step 4: Pressure Release and Standby

The EHB releases the hydraulic pressure back to the brake fluid reservoir. The brake pads slide back slightly, leaving a tiny, near-zero clearance with the discs. Finally, the EHB sets BrakeStatus = Ready (commonly sent as BrakeReady = 1 on the CAN bus) to let the VCU know the chassis is ready for autonomous driving.

4. Why Exactly 1 Bar? The Engineering Behind the Number

For passenger cars, pre-charge pressures are often higher. But for low-speed autonomous vehicles (like L4 delivery pods, street sweepers, or security robots), 1 bar is the standard target. Here is why:

I. It Closes the Mechanical Gap

Disc and drum brakes have a small physical gap of 0.1 mm to 0.3 mm between the pads and the rotors, held in place by return springs and seals. A pressure of 1 bar is just enough to overcome these springs and slide the pads to touch the rotor without actually squeezing it to slow the wheel down.

II. It Stops Brake Drag and Heat

If the pressure is calibrated higher than 1.5 bar, the pads will rub against the rotors while driving. This creates constant drag, which drains the vehicle’s battery, wears down the brake pads prematurely, and causes the brakes to overheat. A 1 bar setting ensures the pads are close but do not drag.

III. It Fits Low-Speed Vehicle Dynamics

Low-speed autonomous vehicles usually weigh between 200 kg and 1000 kg and drive at speeds under 25 km/h. They do not need the ultra-fast, sub-millisecond brake response times required by highway-speed passenger cars. The goal here is checking system readiness and verifying seals, not shaving off millimeters of emergency stopping distance.

IV. It Matches Compact Pump Limits

The small EHB pumps used in low-speed chassis have compact motors and simple control valves. Trying to regulate hydraulic pressure below 0.5 bar causes sensor noise and control errors. A 1 bar target is the lowest limit where these compact pumps can maintain stable control.

5. Four Main Benefits of EHB Self-Pre-pressurization

Why should chassis designers use this autonomous feature instead of relying on active VCU-driven braking?

┌─────────────────────────────────────────────────────────────────────────┐
│                     FOUR BENEFITS OF EHB PRE-PRESSURIZATION             │
├───────────────────┬─────────────────────┬───────────────────────────────┤
│ 1. Zero Gap       │ 2. Early Diagnostic │ 3. Clean VCU Control          │
│    Removes pad    │    Finds leaks and  │    Simple 'Ready' signal      │
│    travel lag     │    sensor issues    │    saves VCU processing power │
└───────────────────┴─────────────────────┴───────────────────────────────┘
                                 │
                        4. Environmental Help
                           Clears air bubbles &
                           restores seal shape

No Mechanical Lag: Pushing the piston close to the disc means that when the autonomous driving system sends a brake command, the hydraulic system builds pressure instantly without a physical delay.
Early Hardware Check: The system checks the pressure sensors, pump, and lines for leaks before the vehicle starts its route. This avoids hidden failures while driving.
Simpler VCU Software: The VCU does not have to run complex hydraulic control loops. It simply checks if the EHB sends a BrakeReady signal before starting a route, keeping the main software clean.
Weather and Storage Help: If a vehicle sits overnight or works in freezing weather, this first pressure cycle helps reshape cold rubber seals, clears small air bubbles from the calipers, and restores normal pedal feel.

6. Fault Diagnostics and Exception Handling

If the pre-pressurization cycle detects a problem, the EHB acts as a safety gate to block the vehicle from driving:

Type A: Pressure Build Failure
- Problem: The pressure does not reach at least 0.6 bar within the allowed timeout window.
- EHB Action: The system registers a PreCharge_Fail diagnostic trouble code (DTC) and sends out EHB_Status = BrakeFault.
- VCU Action: The VCU prevents the vehicle from starting its drive.
Type B: Pressure Leakage
- Problem: During the 200 to 500 ms hold window, the pressure drop rate ($\Delta P / \Delta t$) goes past the allowed safety limit.
- EHB Action: The system flags a leak in the brake lines or wheel cylinder and sets EHB_Status = BrakeFault.
- VCU Action: The VCU stops the vehicle or puts it into a slow limp-home mode.
Type C: Repeated Failures
- Problem: The pre-pressurization test fails several times in a row across different power cycles.
- EHB Action: The system locks out automated operation, stores the recurring fault, and sends the diagnostic data to the cloud fleet management platform so maintenance teams can schedule a repair.

7. Communication and Signal Handshake

The communication between the VCU and EHB is designed to use minimal bandwidth to keep the CAN bus clear.

Signal Handshake Flow

VCU $\rightarrow$ EHB: Sends EHB_Enable = 1 to wake up the system.
EHB ECU Internal Loop: Runs self-test $\rightarrow$ Builds ~1 bar pressure $\rightarrow$ Runs leak test $\rightarrow$ Vents fluid back to reservoir.
EHB $\rightarrow$ VCU: Sends back EHB_Status = Ready (or BrakeReady = 1).
VCU Action: If ready, the VCU allows the autonomous driving system to start the mission.

Message Structure Example

// Downstream VCU to EHB Message
{
  "VCU_to_EHB_Message": {
    "EHB_Enable": 1,           // 1 = Enable System, 0 = Standby
    "EPB_Release_Cmd": 1       // 1 = Release EPB (starts the pre-charge check)
  }
}

// Upstream EHB to VCU Feedback Message
{
  "EHB_to_VCU_Message": {
    "EHB_Status": "Ready",      // Options: Init, PreCharging, Ready, BrakeFault
    "EHB_Live_Pressure": 0.05,  // Real-time pressure in Bar (drops back near 0 after release)
    "PreCharge_DTC": "0x00"     // DTC code (0x01 for build failure, 0x02 for leak)
  }
}

Looking Ahead: Why Decentralized Edge Intelligence Wins

In older drive-by-wire designs, the central VCU had to manage every minor action of the subsystems. But as L4 low-speed autonomous fleets scale up, moving to decentralized control makes more sense.

Handling the power-on self-pre-pressurization directly within the EHB controller brings clear advantages:

Independent Diagnostics: The chassis becomes a self-testing, plug-and-play platform.
Software Safety Isolation: Critical safety code is separated from high-level application software.
Less Downtime: Automatic checks catch hydraulic issues before the vehicle leaves the depot, saving fleet maintenance costs.

For more details on drive-by-wire chassis designs and low-speed autonomous hardware, follow Dowway Vehicle or get in touch with Johnny Liu’s team.

Dowway Vehicle