Electronic Stability Program (ESP) - Trouble Shooting

Electronic Stability Program (ESP)

The Electronic Stability Program is now a standard feature in many vehicle models. As the number of vehicles fitted with ESP increases, the fault frequency and garage repair requirements also increase, of course. Here, we would like to briefly outline the function, the individual system components and diagnosis possibilities.

Task of the ESP

The task of the ESP is to avoid the vehicle breaking away to the side when driving through bends or in critical situations such as evasive actions (high-speed swerve test). The system intervenes specifically in the braking system, engine and gear management and keeps the vehicle on track. It is important to remember, however, that physical laws cannot be cancelled. As soon as the limits are exceeded, even the ESP system cannot prevent the vehicle breaking away.

How it works

What happens when the ESP is active?

For the ESP to become active, a critical driving situation has to occur. A critical situation is recognized as follows: The system requires two basic pieces of information to recognize a critical driving situation. Firstly, the driver's wish, and secondly, which direction the vehicle is driving in. If a

comparison of these two pieces of information results in differences, i.e. if the vehicle is driving in a different direction to the one being steered by the driver, this results in a critical driving situation for the ESP. This can be noticed through understeering or oversteering. If the vehicle is understeered, specific intervention in the braking system and the engine management compensate the tendency to understeer. The brake is applied separately to the inner rear wheel. If the vehicle is oversteered and the vehicle tends to skid, specific braking intervention on the outer front wheel will counteract the oversteering.

In the following we would like to explain the system's sensors and actuators. It must be noted here that there are differences in certain functions or structure depending on the vehicle manufacturer. We will focus on a system such as the one installed in a VW Passat, model year 97.

ESP system structure

The control unit

With this system, the ESP control unit is not connected to the hydraulic unit. It is installed in the right-hand front footwell on the bulkhead. The control unit consists of a high-power computer. To guarantee the greatest possible safety, the system is made up of two computers with their own voltage supply and diagnosis interface that use the same software. All the information is processed in parallel and the computers monitor each other. The control unit is also responsible for regulating the ABS/ASR and EDS. All the systems are contained in one control unit.

Steering angle sensor

The steering angle sensor determines the steering angle and forwards the information to the control unit. The steering angle sensor is installed on the steering column. How does the steering angle sensor work? It works in the same way as a light barrier. A coding disc with two rings in the form of a shadow mask, an absolute ring and an incremental ring, is slipped over a light source situated between the two rings. Two optical sensors are arranged opposite the light source.

When the steering wheel is turned and light passes through the openings of the shadow masks onto the optical sensors, a voltage is produced in these. The different shapes of the shadow masks results in different voltage sequences. A regular signal is produced on the incremental ring side, whereas an irregular signal is generated on the absolute ring side. By comparing the two signals, the control unit can calculate how far the steering wheel has been turned. In addition, the steering angle sensor has a counter that counts the full number of steering wheel turns. This is necessary because the angle sensors usually only map angles up to 360° whereas the steering wheel can be turned through a total of +/- 720 (four full turns). The reset ring with slip ring for the airbag are on the underside of the steering wheel sensor.

Transverse acceleration sensor

The transverse acceleration sensor has the task of establishing which lateral forces are acting and trying to bring the vehicle off track. It is always installed as near as possible to the vehicle's centre of gravity. How does the transverse acceleration sensor work? The transverse acceleration sensor is made up of a permanent magnet, a Hall sensor, a damper plate and a spring. Together, the damper, the spring and the permanent magnet form a magnet system. The permanent magnet, which is connected to the spring, can oscillate freely backwards and forwards over the damper plate. If transverse acceleration acts on the vehicle, the damper plate moves away from under the permanent magnet, which follows this movement after a short delay due to its inertia. This movement generates eddy currents in the damper plate and which builds up an opposing field to the magnetic field of the permanent magnet. The weakening of the overall magnetic field resulting from this changes the\ Hall voltage. How much the voltage changes is proportional to the transverse acceleration. In other words, the greater the movement between the permanent magnet and the damper plate, the weaker the overall magnetic field will become and the more the Hall voltage will change. As long as there is no transverse acceleration, the Hall voltage remains constant.

Yaw rate sensor (turning rate sensor)

The yaw rate sensor has the task of establishing whether the vehicle tends to turn around its own vertical axis (spin). It must also always be installed as near as possible to the vehicle's centre of gravity. The yaw rate sensor is made up of a hollow cylinder which has 8 piezo electronic elements attached to it. Four of these elements cause resonant oscillation on the hollow cylinder. The other four elements register whether there is any change to the oscillation nodes where they are located. If a torque acts on the hollow cylinder, the oscillation nodes are displaced. The displacement is recorded by the piezo elements and forwarded to the control unit. This uses the information to calculate the yaw rate.

Combined sensor for transverse acceleration and yaw rate

In newer systems these two sensors are both contained in one housing. They are mounted on a PCB and work according to the micro-mechanical principle. This has a number of advantages such as reduced design space and a more accurate alignment of the two sensors to one another. This combined sensor also has a different structure than the individual sensors. The transverse acceleration sensor is structured as follows: A capacitor plate with a moving mass is suspended in such a way that it can oscillate backwards and forwards. This moving plate is framed by two capacity plates installed in fixed positions. This results in two capacitors (K1 and K2) switched one behind the other. The charge quantity (capacity C1 and C2) that the two capacitors can absorb can now be measured through electrodes. In the quiescent state the measured charge quantities are identical for the two capacitors. If a transverse acceleration acts on the sensor, the movable plate is displaced against the direction of acceleration by inertia. This displacement changes the distance between the plates and thus the charge quantity of the capacitors. This change in capacity quantity is the measured variable for the control unit.

The yaw rate sensor is located on the same board as the transverse acceleration sensor but in a different spot. It is structured as follows:

An oscillating mass to which conductive tracks are attached is fixed in a carrier in a constant magnetic field between a north pole and a south pole. If alternating current is applied, the oscillating mass with the tracks begins to oscillate in a straight line to the applied alternating current. If a rotary movement now occurs, the inertia of the oscillating mass changes the regular backwards and forwards movement. The change in movement of the mass in the magnetic field also causes a change in the electrical behaviour of the tracks.

This electrical change is the variable to measure the amplitude of the rotary movement. This structure is installed in duplicate to guarantee maximum safety.

Sensor for brake pressure

The sensor for the brake pressure is installed in the hydraulic pump for the ESP. It has the task of recording the current braking pressure in the braking circuit for the control unit. The control unit uses the values of the braking pressure sensor to calculate the wheel brake forces that are integrated in the calculations when the brakes are used. The braking pressure sensor is made up of a piezo electrical element, on which the pressure of the brake fluid acts, plus an electronic evaluation unit. A change in pressure leads to a change in the charge distribution in the piezo electrical element. If the element is without pressure, the charges are distributed evenly. As pressure increases, the charges are displaced

and voltage is generated. The more the pressure increases the more the charges are separated. The voltage continues to increase. The evaluation electronics amplify this voltage and pass it on to the control unit.

On/off switch for the ESP switch

In certain situations it makes sense to switch off the ESP system, e.g. on a capacity test stand or when driving with snow chains on the vehicle. To enable the driver to do this, an on/off switch is installed. If the system is switched off via the switch and not switched on again, it will switch on again automatically after the engine has been restarted. If the ESP system is active it cannot be switched off. Nor can it be switched off if a certain speed has been exceeded.

The hydraulic pump