Variable hysteresis circuit

Details Variable hysteresis circuit Variable hysteresis circuit

2000-10-31

2002-06-18

Tran, Toan (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Particular stable state circuit

Reexamination Certificate

active

06407605

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to circuits which provide a hysteresis function, and more specifically relates to circuits used in anti-lock braking systems(ABS).
ABS used in conjunction with air-braked truck trailers are in common use. An objective of an ABS is to allow the wheels of a vehicle to continue to rotate during braking, including heavy braking. Keeping the wheels moving during braking generally provides more efficient braking. As shown in
FIG. 1
, an ABS generally consists of wheel speed sensors
10
, an Interface circuit
12
, an Electronic Control Module(ECM) circuit
12
, an Electronic Control Module(ECM)
14
, a Pneumatic Control Module (PCM)
16
, and brake mechanisms
18
associated with the wheels of the vehicle. The ECM
14
is sometimes referred to as the Electronic Control Unit (ECU). The PCM
16
is sometimes referred to as the modulator and, in some anti-lock braking systems, the PCM is integrated into another component such as a relay valve.
An ABS for air brakes generally works as follows. Each wheel speed sensor
10
measures the speed of a wheel and forwards this information, in the format of an electrical signal, through the interface circuit
12
. The interface circuit
12
transforms the electrical signal produced by the wheel speed sensor
10
into a square wave. The square wave is then used by the ECM
14
to calculate wheel speed information. The wheel speed information is then transmitted to the PCM
16
which sends instructions to the braking mechanisms
18
which modify the air pressure to control the braking level. This braking process is well known and is described in numerous patents and in pending, U.S. patent application Ser. No. 09/306,921, which is hereby incorporated herein in its entirety by reference.
One type of wheel speed sensor which is commonly used in ABS systems is a variable reluctance (VR) sensor. A typical VR sensor
20
is,shown in
FIG. 2
, and consists of a permanent magnet
22
, a magnetically soft pole piece
24
and a coil of wire
26
wound around the pole piece
24
. A magnetic field extends from the permanent magnet
22
, through the pole piece
24
and coil
26
out into the air space proximate the face
31
of the VR sensor
20
. The return path of the magnetic field is from the air space proximate the face
31
to the opposite end of the permanent magnet
22
.
Each end of the coil
26
is attached to a cable
28
which extends through the sensor housing
30
opposite the face
31
of the VR sensor
20
. The electrical signal produced by the VR sensor
20
flows through the cable
28
. Another type of sensor commonly used in ABS's is a Hall sensor, which construction is generally known in the art.
As shown in
FIG. 3
, when a VR sensor
20
is employed in an ABS, the VR sensor
20
is typically associated with a toothed wheel
32
(sometimes referred to as an exciter ring or a tone wheel). Usually in the truck and trailer industry, the toothed wheel is attached directly to the back of the wheel hub
34
and rotates with the road wheel (not shown). The VR
20
is mounted so that the face
31
of the VR sensor is proximate the toothed wheel
32
and perpendicular to the axle
38
.
The toothed wheel
32
includes a row of teeth
40
around the perimeter of the toothed wheel
32
. A gap
42
is located on either side of each tooth
40
along the perimeter of the toothed wheel
32
. As the road wheel rotates, the teeth
40
of the toothed wheel
32
pass the face
31
of the VR sensor
20
. Because the toothed wheel
32
is made of a ferrous material, as each tooth
40
approaches the face
31
of the VR sensor
20
, the magnetic field increases. As each tooth.
40
becomes further away from the face
31
of the VR sensor
20
, the magnetic field decreases. The magnetic field, or flux, is greatest when the tooth
40
is in front of the VR sensor
20
. Conversely, when a gap
42
is in front of the VR sensor
20
, the flux is least. Thus, as the teeth
40
pass the face
31
of the VR sensor
20
, the flux increases and decreases.
Through basic electromagnetic theory, this changing flux induces an AC voltage signal across the coil
26
. The induced voltage is ideally a sinusoidal signal. The frequency of the signal is directly proportional to the number of teeth
40
which pass the face
31
of the VR sensor
20
in a given period of time. The amplitude of this voltage signal is proportional to the speed of the teeth
40
passing the face
31
of the VR sensor
20
. When the road wheel is turning at high speeds, the AC signal has a high amplitude. When the road wheel is turning at low speeds, the AC signal has a low amplitude. As the wheel speed becomes very slow, the signal becomes generally unreliable. Typically, designers of ABS's assume that when the wheel speed is less than 2 mph, the signal, received from each wheel sensor is unreliable. The speed at which the signal becomes unreliable, however, is dependent upon many factors of the overall system design, including, for example, the sensitivity of the circuit, the VR sensor, the toothed wheel, and the gap maintained between the face of the VR sensor and the toothed wheel.
Because the signal received by the ECM will be used to generate wheel speed information, it is critical that this signal be as accurate as possible. However, certain factors create imperfections in the signal. These imperfections can result in the ECM
14
incorrectly calculating the wheel speed.
One factor which adversely affects the sinusoidal signal produced by the VR sensor is mechanically induced noise. At times, even though the vehicle is moving, the road wheel is not rotating. The road wheel therefore rubs on the road surface producing a noise sometimes referred to as tire scrub. This noise may additionally be amplified by suspension resonances. Because sensors, such as VR sensors, are generally prone to mechanically induced noise, the sensor will produce significant AC voltage even though the wheel is stationary. Therefore the sensor will send an output signal which indicates that the road wheel is rotating when, in fact, it is stationary. This situation is further complicated by the fact that the mechanical noise tends to be at a relatively high frequency. Thus the high frequency noise causes a great rate of change of flux. Because a VR sensor, for example, responds to the rate of change of flux, the resulting signal will have significant frequency imperfections.
Other factors such as electrical interference effects from onboard or off board radios, radars and other radio frequency interference affect the overall sensing scheme. Depending on the severity, these effects may combine and prevent the ECM
14
from operating correctly and imperfections in control performance may result. When greater degradation of the signal occurs, the ECM
14
determines that the signal is unworkable and the ABS system shuts down.
Another factor which leads to imperfections in the signal produced by the VR sensor is a varying gap between the face
31
of the VR sensor
20
and the toothed wheel
32
. As shown in
FIG. 3
, in the heavy truck and trailer industry, a VR sensor
20
is oriented along the axle of the vehicle resulting in the face of the VR sensor
20
being perpendicular to the wheel hub
34
on which the toothed wheel
32
is mounted. Axial slack causes the gap between the VR sensor
20
and the toothed wheel
32
to vary. When the gap between the VR sensor
20
and the toothed wheel
32
is large, the amplitude of the AC voltage signal is low. When the gap is small, the amplitude of the AC voltage signal is larger. This results in amplitude modulation of the sinusoidal waveform due to variation of the sensor gap.
An additional effect of the varying sensor gap is best understood by considering the concept of a “toothless” tone wheel. Initially, one might conclude that in the case of a toothless wheel, as the wheel rotates no signal would be generated, as there is no variation in the magnetic flux. However, it is clear that as the tone wheel mov

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