HDC or Hill Descent Control is an electronic vehicle system that is designed to promptly slow down a vehicle's speed while descending a mountain. Today such a system can be found on many cars, including sedans, however, it was originally intended exclusively for SUVs that have increased cross-country ability.
The hill descent control system also prevents the driver from inertial acceleration of the vehicle while driving on difficult mountain roads. If your car is equipped with an HDC system, you can travel along the serpentine road without worrying about your own safety. Moreover, electronics not only provide safety, but also significantly improve vehicle handling.
As mentioned above, most cars with HDC installed as standard are SUVs. Each car manufacturer is trying to develop its own analogue of the system. Thus, the most common Hill Descent Control was designed specifically for Volkswagen and BMW cars (today it is much more widespread). Downhill Assist Control or DAC is developed by Toyota. Downhill Drive Support or DDS is installed on Nissan vehicles.
DAC system: why does the driver need it?
It is believed that the DAC (Downhill Assist Control) was first introduced by engineers of the famous automobile brand Toyota. The main purpose of the new development was to provide the car with the safest descent from steep slopes, preventing unwanted acceleration and ensuring that a constant safe speed is maintained.
Most often, the English abbreviation DAC is used to denote the function of safe movement on a slope. However, there is no single generally accepted designation. Individual manufacturers may call this system differently. For example, BMW and Volkswagen use the designation HDC (Hill Descent Control) , while Nissan uses DDS (Downhill Drive Support) . The operating principle remains the same regardless of the name.
Downhill Assist System
Most often, a downhill speed control system is installed in off-road passenger cars, which can include crossovers and SUVs, as well as all-wheel drive sedans.
Hill Descent Control (HDC), how does it work and does it help the driver?
Of the entire list of driver assistance systems, none is more understandable by name than the hill descent control system.
But the clear name does not reveal the technical essence of the equipment or its importance for drivers. The system was developed for the SUV class, but over time it began to be equipped with SUVs and even city sedans, hatchbacks, and station wagons. But is equipment needed to control descent from slopes, and is it worth paying money for it?
Purpose and functions
The main task of the system is to provide the car with a stable and safe speed during a steep descent. Based on information received from various sensors, the mechanism controls the speed when driving down a mountain by braking the wheels.
The DAC becomes especially valuable when driving along steep serpentines and mountain slopes. While the system monitors speed, the driver can fully concentrate on the road.
Lift Assist System
The hill assist system is designed to prevent the vehicle from rolling away when starting on an incline (incline). The use of this system ensures that the vehicle starts on an incline, without a mechanical parking brake, and increases safety. The system is installed as an option on some passenger cars.
Depending on the car manufacturer, the system has the following names:
- Hill Hold Control (HHC) from Volkswagen;
- Hill Holder from Subaru, Fiat;
- Hill-Start Assist Control (HAC) from Toyota;
- Uphill Start Support (USS) from Nissan.
The lifting assistance system is built on the basis of the dynamic stabilization system and is a software extension of this system, therefore it is not a system as such.
When stopping on an incline, a rolling force FH acts on the car (see Fig. 1), i.e. The force that causes a vehicle to roll backward when the brake is released. When starting a car after stopping on an incline, its traction force must first balance the rolling force. If the driver presses the accelerator pedal too lightly or releases the brake pedal (or parking brake) too early, the traction force will be less than the rolling force and the car will begin to roll back before moving. To ensure comfortable and safe starting of a vehicle on an incline, some vehicle designs are equipped with a hill assist system that makes starting on an incline easier, allowing it to be done without using the parking brake. To do this, the system slows down the decrease in brake pressure in the hydraulic system when starting off, preventing the vehicle from rolling back while the traction force is still insufficient to compensate for the rolling force.
The system is activated when the following conditions are met:
- the car is stationary,
- rise exceeds 5%,
- the engine is running,
- The driver's door is closed.
The hill start system is based on the ESP system. The ESP sensor unit is complemented by a longitudinal acceleration sensor that detects the vehicle's position.
The operation of the system can be divided into four phases (Fig. 2).
Phase 1 - creating brake pressure. The driver stops or holds the vehicle by pressing the brake pedal. The high pressure valve is closed.
Rice. 2. Phases of operation of the lift assist system (pressure change)
Rice. 3. Phases of operation of the hill assist system (driver actions)
The intake valve is open and the required pressure is created in the brake cylinder. The exhaust valve is closed. The braking torque is sufficient to keep the vehicle from rolling away.
Phase 2 - holding brake pressure. The driver takes his foot off the brake pedal to move it to the accelerator pedal. Hill start control maintains brake pressure at the same level for 2 seconds to prevent the vehicle from rolling backwards. The wheel circuits hold the brake pressure. This prevents premature reduction in pressure.
Phase 3 - dosed reduction in brake pressure. The car is still motionless. The driver presses the accelerator pedal. As the engine increases the torque transmitted to the wheels (traction torque), the starting system reduces the braking torque so that the car does not roll back, but also does not become braked when moving off. The inlet valve is open.
Phase 4 - Brake pressure release. The car starts moving.
The traction torque is sufficient to start and subsequently accelerate the car. Hill Assist reduces brake pressure to zero.
It should be noted that the system always works in an uphill direction, regardless of the direction of movement, which is important for starting on a hill in reverse.
An electromechanical parking brake can be used as a lifting assistance system (Fig. 4).
Rice. 4. General layout of the parking brake with electric drive: 1 — brake disc; 2 — brake pad; 3 — movable bracket; 4 - gearbox; 5 - electric motor; 6 — power supply; 7 — electric motor gear; 8 — drive housing; 9 — drive gear; 10 - swing gear; 11 — driven gear of the electric drive
The parking brake is turned on and off using a special switch. The brake is released by pressing the switch slide while simultaneously pressing the brake or accelerator pedal.
The parking brake can also be activated when the ignition is off by pulling the brake switch towards you. The car can be released from the brake only with the ignition on.
To perform the main function of the parking brake, it is necessary to convert the rotation of the electric motor shaft into a small forward movement of the brake piston. This is achieved by using an oscillating gear reducer or a planetary gearbox in combination with a screw drive.
The rotational motion is converted into translational motion by means of a lead screw 3 connected to the piston 5 of the brake mechanism (Fig. 5). The lead screw is driven directly from a gearbox with an oscillating gear. A cylinder 6 is located in the cavity of the brake piston. A pressure nut 2 is pressed into the thickening of the head of the cylinder. The pressure nut and the associated cylinder can slide freely along the brake piston without rotating relative to it. Rotation of the nut is impossible due to the special shape of the inner surface of the piston, which interacts with the figured surface of the pressure nut.
Rice. 5. Scheme of operation of the parking brake mechanism with an electric drive: a - tightening the brake; b — releasing the brake; 1 — brake disc; 2 - pressure nut; 3 - lead screw; 4 - gearbox; 5 — brake piston; 6 - cylinder; 7 - sealing ring
The speed of the motor shaft is determined using a Hall sensor. Thanks to this, the control unit can calculate the piston stroke.
When tightening the parking brake, the rotation of the lead screw 3 is converted into a translational movement of the pressure nut connected to the cylinder 6, which rests on the piston of the brake mechanism and presses the pads through it to the brake disc (Fig. 5, a). In this case, deformation of the piston sealing ring 7 occurs in the direction of the pads. As the force of pressing the pads against the brake disc increases, the current consumption of the electric motor increases. The electromechanical parking brake control unit monitors the amount of current consumed during the entire process of tightening the brake and, when this current reaches a certain value, turns off the electric motors. The screw thread is self-locking. Thanks to this, after the brake pads are brought together and the voltage supply to the electric motor is stopped, the brake remains tightened.
When removing the brake (Fig. 5, b), the nut moves backward along the lead screw due to the rotation of the lead screw in the opposite direction. The pressure on the cylinder stops. The piston moves away from the brake disc under the action of the elastic forces of the sealing ring 7, which tends to take its original position, and the runout of the brake disc. In this case, the pads also move away from the brake disc.
The car is stationary. The electromechanical parking brake is engaged. The driver decides to get going, engages first gear and presses the accelerator pedal. The dynamic traction system checks all relevant data for determining when to release the parking brake:
- tilt angle (determined by the longitudinal acceleration sensor);
- engine torque;
- accelerator pedal position;
- clutch pedal position (on vehicles with a manual transmission, the signal from the clutch pedal position sensor is used, on vehicles with an automatic transmission, the current value of the engaged gear is requested);
- desired direction of movement (on vehicles with automatic transmission it is set according to the selected direction of movement, on vehicles with manual transmission - according to the signal from the reversing light switch).
Based on these data, the control unit calculates the rolling force acting on the vehicle and the optimal moment to release the electromechanical parking brake, so that the vehicle can move without rolling back. When the vehicle's traction torque becomes greater than the rolling force value calculated by the control unit, the control unit sends a control signal to both actuator electric motors of the rear wheel brakes. The parking brake acting on the rear wheels is released electromechanically. The car starts moving without rolling back. The hill assist system performs its functions without using the hydraulic brakes, it simply uses the information provided by the ESP system sensors.
At temperatures above 10 °C, the function is implemented by the automatic transmission. When the automatic transmission control unit detects (by motion resistance) a rise and at the same time recognizes zero speed, the gearbox engages in second gear, in which the vehicle cannot roll back, since the ring gear of the double planetary gear would then have to rotate in the direction blocked by the freewheel. After the engine traction torque exceeds the rolling moment, the clutch is unlocked and the car starts moving without rolling back.
Let's look at the principle of operation and design of the descent assistance system, also known as DAC. At the end of the article there is a video review of the operating principle of the DAC system.
Car safety today is one of the main details that most often plays an important role. One such example is the hill descent assist system, also known as the DAC system. The mechanism not only makes driving easier, but also significantly improves safety.
Essential elements
In most cases, hill descent assist is available on vehicles with an automatic transmission. This type of system is extremely rare in vehicles with manual transmissions.
In fact, DAC is just an additional function in the vehicle's vehicle stability control system (TCS or ESP). The main elements of the mechanism include:
- a sensor that determines the position of the gas pedal;
- braking force sensor (pedal pressure);
- crankshaft speed sensor;
- vehicle speed sensor;
- ABS wheel speed sensors;
- temperature sensor;
- hydraulic unit, control unit and actuators of the TCS system;
- on/off button.
Each of the sensors helps in the full operation of the system, assessing as fully as possible all related factors that may affect automatic speed control. For example, a temperature sensor can determine what weather conditions a person is driving in.
System activation button
How does the hill descent control system work?
It must be said that automakers and advertisers are disingenuous when they call this assistance system a “system.”
The fact is that the system is only a software role of the exchange rate stability system and does not have any of its own actuators or control units.
Let's figure out how the hill descent assist system works. After pressing the button and fulfilling the above conditions, the electronics begins to monitor the current speed.
If she sees that the car is starting to move faster, the pressure in the brake lines rises to apply the brakes. As a rule, the assistant maintains the speed at 5-7 km/h.
It turns off by pressing a button or touching any of the pedals. It will automatically deactivate if it sees that the slope has become less than 12%.
In principle, that's all that can be said about the descent assistance system. This simple but useful function is present in many modern off-road vehicles.
There is another one, related to it - a lifting assistance system, but read about it in the next publication!
Principle of operation
Regardless of which car model the system is installed in, the principle of its operation remains the same. Downhill speed control is activated by pressing the corresponding button. For the mechanism to start working, several conditions must be met:
- the car engine must be running;
- the gas and brake pedals are not pressed;
- travel speed – no more than 20 km/h;
- slope – up to 20%.
If all conditions are met, after pressing the instrument panel button, the system automatically starts working. Reading information from numerous sensors, it transmits it to the control unit. When a certain speed is exceeded, the pressure in the brake system increases and the wheels begin to brake. Thanks to this, the speed can be maintained at a given level, which depends on the initial speed of the car, as well as on the gear engaged.
Features of the technology
It is not difficult to guess that the descent assist system is designed to facilitate the exit from various slopes. It would seem, what could go wrong here?
In fact, on a descent, the car can accelerate very quickly under the influence of gravity, which will be an unpleasant surprise for the driver. This is where the system comes to the rescue, maintaining a constant speed on the slope.
The difference between the electronic descent assistant and others is that the system is turned on forcibly with a special button and not automatically. In addition, the following conditions must be met:
- the motor must be running;
- feet are removed from the brake pedal and gas pedal;
- speed should not be higher than 20 km/h;
- the slope should be no more than 20%.
Many automakers produce a similar system and, as usual, they all call it differently.
Let's give a few examples: Volkswagen calls its development (Hill Descent Control) HDC, Toyota calls it (Downhill Assist Control) DAC, and Nissan calls it (Downhill Drive Support) DDS. It's easy to get confused.
Advantages and disadvantages
Most motorists agree that DAC has many important advantages, however, it also has its disadvantages. Clear advantages include:
- safe passage of almost any descent;
- automatic speed control, allowing the driver not to be distracted from driving;
- assistance to novice motorists in mastering the features of driving a vehicle.
The downside is that a car with this feature will cost a little more. In addition, DAC operation is not designed for long distances. It is recommended to use automatic acceleration control on a descent on short and most difficult sections of the route.
Hill Descent Control can help the driver navigate difficult sections of the route and ensure a safe speed on the slope. This mechanism is especially useful for novice motorists. But experienced drivers should not neglect the use of DAC, because the safety of the motorist, his passengers and other road users should always remain a priority.
Descent Assist System
Hill descent control is designed to prevent the vehicle from accelerating when driving on mountain roads. The presence of this system on the car increases ease of operation and safety. The hill descent assist system is usually installed on off-road passenger cars. Depending on the car manufacturer, the system has the following names:
- Hill Descent Control (HDC) from Volkswagen, BMW, etc.;
- Downhill Assist Control (DAC) from Toyota;
- Downhill Drive Support (DDS) from Nissan.
If the car is on an inclined plane, the force of gravity acting on it is decomposed according to the parallelogram rule into normal and parallel components (Fig. 1). The latter represents the rolling force FH acting on the car.
Rice. 1. Diagram of the action of forces when a car moves on a descent: FA - traction force; FG - gravity (vehicle weight); FH—rolling force; FN - normal component of gravity
If the vehicle is subject to its own traction force, it is added to the rolling force FH. The rolling force acts on the car constantly, regardless of its speed. As a result, a car rolling down an inclined plane will accelerate all the time, i.e. move faster the longer it rolls. To maintain a constant vehicle speed without hill descent assist, the driver will need to brake and/or downshift by taking his foot off the accelerator pedal. To facilitate the driver's actions in these conditions and ensure safety when descending, a descent assist system is used, which is activated when the following conditions are met:
- vehicle speed is less than 20 km/h;
- slope exceeds 20%;
- the engine is running;
- the accelerator and brake pedals are not pressed.
When these conditions are met, the system receives data on the position of the accelerator pedal, engine speed and wheel speed (which is slightly higher than walking speed) indicating an increase in vehicle speed. The system assumes that the car is rolling downhill and the braking system must be applied. The speed of the vehicle that the brake assistant must maintain by braking all wheels depends on the speed at which the descent was started and the gear engaged. In this case, the descent assist system switches on the return pump. The high pressure valves and ABS intake valves open and the ABS exhaust valves close. Braking pressure is created in the brake cylinders of the wheels, and the car slows down. The system applies wheel braking through the ABS hydraulic modulator, which creates pressure in the brake system.
When the vehicle speed decreases to a safe value, the hill descent assist system stops braking the wheels and again reduces the pressure in the brake system. If the speed then begins to increase without pressing the accelerator pedal, the system assumes that the vehicle is still traveling downhill. In this way, the vehicle's speed is constantly kept within a safe range that can be easily controlled and controlled by the driver.
The downhill assist system is deactivated forcibly (by pressing the button again) or automatically when you press the gas or brake pedal, as well as when the slope decreases to 12%.
Hill descent control is a software extension of the stability control system and uses its design elements, so at its core it is a function, not a system.
