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ABS sensors – types and fault diagnosis

01.03.2022

Wheel speed sensors are an important component necessary for systems such as ABS and ESP. Sensor data is also important for chassis control systems, hill start assistants and other similar systems. The task of the sensors is to provide information about the current speed of each wheel separately.

In its simplest variant, the ABS controller counteracts wheel locking by reducing the braking force when a wheel stops. In more complex ESP systems, the controller additionally detects loss of grip when starting or accelerating and uses data from other sensors (steering wheel turn, acceleration, vehicle rotation) to actively counteract skidding and maintain the desired trajectory.

How do ABS sensors work?

As we can see, the performance of these inconspicuous elements has an impact on driving safety. There are two basic variants of ABS sensors: active sensors and inductive sensors. They differ both in construction and operation, as well as in diagnostic methods. Therefore, it is useful to be able to initially identify the type of sensor.

Inductive sensors

Inductive (reluctance, or passive or passive) sensors were the first to appear. This element has a simple structure – it consists of a magnet, which is the core of the sensor, and a coil. The sensor and the rotating gear wheel (pulse ring) form a magnetic circuit. The rotational movement of the pulse ring causes a change in the magnetic field in the coil. The alternating magnetic field induces an alternating voltage in the winding, close to a sine wave, which is the output signal from the sensor.

inductive sensor

The advantage of inductive sensors is their simple design and the fact that they do not need power. The disadvantage is that the output voltage depends on the wheel speed, so they are less accurate at lower speeds. Because the ABS controller responds to voltage above a certain threshold, the system has a certain minimum speed below which it cannot detect wheel movement. These sensors tend to be larger and heavier than active sensors. They work with an impulse ring connected to a wheel hub or half-shaft, for example. The ring can also be part of brake discs.

Active sensors

Active sensors (also referred to as Hall sensors) are electronic circuits integrated with a magnetic field sensor (which can be a Hall or magnetoresistive sensor). The sensors themselves are not suitable for use with a controller, so they are connected to an integrated electronic circuit (usually a small integrated circuit in the sensor housing). If the sensor is to work with a steel pulse ring, then a magnet that generates a magnetic field is placed next to the Hall effect sensor. Magnetoresistive (magnetoresistive) sensors, on the other hand, work in cooperation with a multipole magnetic ring. The main advantage of Hall and magnetoresistive sensors is that they do not require a minimum wheel speed to operate. They measure the signal practically from the start of the vehicle. This is because the ABS sensor system generates a rectangular signal with fixed levels depending on the detection of a magnetic field.

The active sensor itself does not generate a signal – it requires power from the controller. The way the information is transmitted is familiar from industrial solutions – the speed sensor generates a current signal. This is a convenient solution – one pair of wires is enough to power the sensor and receive data. A typical sensor in an inactive state draws a base current (about 7mA) which is doubled (14mA) when the sensor output is active. The current output increases noise immunity, and facilitates diagnostics on the controller side. A defective ABS sensor can be detected by the controller if the current drawn is incorrect. This will also happen when the electrical circuit of the sensor is interrupted.

ABS sensor faults

The customer usually arrives at the service center after the ABS or ESP light on the dashboard comes on. But ABS sensor failure manifests itself not only by activating the ABS light, but also by locking the wheels under hard braking. ABS sensors at the same time are among the components that are quite susceptible to damage due to their location. Proximity to the road increases the risk of dirt, moisture or mechanical damage affecting both the sensor itself and the mating impulse wheel.

Initially, check the fuses responsible for the operation of the system. This simple step avoids many misunderstandings. It is worth assessing the condition of the brake system – brake discs and pads. It is best to test the functioning of the brakes on a roller stand. We then run a scan with a diagnostic tester to look for ABS system errors.

Examination of the ABS system starts well with a visual inspection of the mechanical components. Especially pay attention to the wiring of the sensors, damage to which is a common problem. Malfunction of the ABS sensor may be due to excessive wheel bearing clearance, for example. If the pulse ring moves too far away from the sensor, it will cause an incorrect signal.

How to check the ABS sensor?

The method of proceeding depends on the type of sensor. If we do not know which sensor we are dealing with, we can measure its supply voltage. To do this, we disconnect the sensor from the controller and measure the voltage on the plug side of the controller. If the sensor is not powered – we are dealing with an inductive sensor (of course, remember that the cause can also be a broken circuit or a defective controller).

If we use the QST-5 sensor tester, we do not need to determine the type of sensor before the measurement, the tester will do it for us. It is enough to connect the sensor itself to the tester after unplugging (the way of connection is arbitrary).

We test the inductive sensor

We can check the inductive sensor by measuring its resistance with a multimeter. The result should be similar for a set of sensors. We can theoretically measure its inductance (the sensor is a coil), but not every universal instrument offers such a measurement, and the value itself can be large (on the order of 1H or more).

If we have the QST-5 we can do better – not only will we know the resistance and inductance of the sensor coil, but at the same time we can read the parameters of the output signal. If you turn the wheel during the test, you will see the current and maximum voltage amplitude and frequency indications. In this way, we can compare whether the sensors perform similarly.

Users of the oscilloscope additionally have the opportunity to visually check the uniformity of the generated pulses. With the help of an oscilloscope, we will see, for example, how a damaged tooth of the wheel cooperating with the sensor interferes with the signal.

We test the active sensor

In the case of an active sensor, without additional tools we will not do much. At most, we can verify the supply voltage – see above. We won’t check such sensors with a multimeter – first of all, we won’t know if the sensor works anyway, and if the meter in question takes measurements at higher voltages, we may even damage the sensor.

We can check the active sensor with the QST-5 device. We can not only observe the sensor signal, but also count the pulses per full revolution. To do this, simply mark the position of the wheel and turn it exactly a full revolution during the test. The number of pulses is read from the device’s screen.

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As always, oscilloscope users can count on the most data. It is important to remember that the ABS sensor measurement will not provide a high-amplitude signal. Connect the oscilloscope between one of the sensor outputs and the ground of the controller. In this way, we measure the voltage on the internal resistor in the controller. This is a way to indirectly measure the signal, which is a current signal.

For active sensors, the condition of the magnetic ring is particularly important. The ring is usually located on the side of the wheel bearing. Keep in mind that visual inspection for damage will not do much. We can use a test card for magnetic rings, but the surest way is to verify the number and regularity of pulses with an oscilloscope. This is the only way to check whether the magnetic field level is suitable for the cooperating sensor. Remember that the ring can be magnetically damaged, for example, by storing it near a strong magnet.

Summary

It is worthwhile to gain a basic knowledge of ABS sensors, because it has a concrete impact on the work in the workshop. Often these components are not cheap, especially when integrated into the wheel hub. Better knowledge of the subject means more accurate diagnoses and fewer unnecessarily listed items. Remember that ABS and ESP systems have a significant impact on driving safety. Replacement of the ABS sensor and other system components should be done in accordance with the manufacturer’s service manual.

 

Author: Product Engineer Piotr Libuszowski

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