Oscilloscope Patterns and Waveforms

Fig. 6 Saw-Tooth Wave-Form:

MAGNETIC INDUCTIVE PICKUPS
The wheel speed sensors are of a magnetic inductive type sensor design.

Magnetic inductive pickup sensors operate using the following characteristics of magnetic fields:

- A magnetic flux field is always present around and through a magnet.

- When a material with low reluctance (low resistance to magnetic lines of force) and high permeability (high ability to provide a path for a magnetic field) moves into a magnetic field, the magnetic lines of force are concentrated and the strength of the field is increased.

When a low-reluctance, high-permeability material moves out of a magnetic field, the strength of the field decreases. In other words, the field collapses.

Magnetic inductive pickup sensors use permanent magnets to produce the magnetic field and a stator tip of permeable, low- reluctance material to concentrate the field near a rotating armature. The armature usually has evenly spaced teeth - also made from a permeable, low reluctance material.

- When the stator tip is close to an armature tooth, the armature tooth strengthens the magnetic field.

- When the stator tip and an armature tooth are exactly adjacent, the magnetic field is at its strongest.

- When the stator tip is between two armature teeth, an air gap exists between the stator tip and armature. Because air is a high- reluctance medium, the magnetic field is weakest with the armature in this position.

As the field strengthens and collapses, the change in the field induces a voltage in a pickup coil. The voltage varies from positive to negative in a sawtooth shaped wave-form similar to sine wave (fig. 6).

NOTE:
- Positive pulses occur as the field strength is increasing.

- Negative pulses occur as the field strength is decreasing (collapsing).

- The pulses DO NOT occur when the stator tooth and the armature tooth are exactly adjacent. The pulses occur when the field strength is increasing and decreasing at its most rapid rate.

- For example, when the armature tooth is approaching and when the armature tooth is leaving the stator tip.