Specifications

Anti-Skid Circuit Functions
Motor Vehicle Standards 121 place certain stopping require-
ments on heavy vehicles which require the use of electronic
anti-skid control devices.
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These devices generally use vari-
able reluctance magnetic pickup sensors on the wheels to
provide inputs to a control module. One of the questions
which the systems designer must answer is whether to use
the average from each of the two wheels on a given axle or
to use the lower of the two speeds or to use the higher of
the two speeds. Each of the three functions can be generat-
ed by a single pair of LM2907-8 as illustrated in
Figures 26–
28.
In Figure 26 the input frequency from each wheel sensor
is converted to a voltage in the normal manner. The op
amp/comparator is connected with negative feedback with
a diode in the loop so that the amplifier can only pull down
on the load and not pull up. In this way, the outputs from the
two devices can be joined together and the output will be
the lower of the two input speeds. In
Figure 27 the output
emitter of the onboard op amp provides the pullup required
to provide a select-high situation where the output is equal
to the higher of two speeds. The select average circuit in
Figure 28 saves components by allowing the two charge
pumps to operate into a single RC network. One of the am-
plifiers is needed then to buffer the output and provide a low
impedance output which is the average of the two input fre-
quencies. The second amplifier is available for other appli-
cations.
TL/H/7451–47
V
OUT
is proportional to the lower of the
two input wheel speeds
TL/H/7451–48
FIGURE 26. ‘‘Select-Low’’ Circuit
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