JPS6152018B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a wheel rotation speed calculation circuit, and more particularly to a wheel rotation speed calculation circuit that is most suitable for application to anti-skid control of a vehicle.

Recently, various anti-skid control devices have been developed as braking devices to provide more effective and safe braking to automobiles on any road surface. In each of these anti-skid control devices, the rotational state or skid state of the wheel is calculated, and the brake fluid pressure transmitted to the wheel cylinder of the wheel brake device is controlled according to this result. Alternatively, in calculating the skid state, the wheel rotation speed, that is, the wheel speed is required. For example, in calculating the rotational state, deceleration is calculated from the wheel speed, and in calculating the skid state, an approximate vehicle speed is calculated from the wheel speed, and the slip rate is calculated from these results. The wheel deceleration is compared to a predetermined deceleration reference value, the wheel slip rate is compared to a predetermined slip rate, and
The brake fluid pressure is controlled based on these results, but the basis of control is the wheel speed in any case.

Recent anti-skid control tends to be digitally controlled, but in order to obtain the wheel speed mentioned above, the output from the wheel rotation speed sensor installed on the wheel is converted into a pulse train, and the wheel speed is calculated based on this pulse train. . In other words, the frequency of this pulse train is proportional to the wheel rotation speed, but conventionally, the pulses of this pulse train during a certain period of time are counted, or high-frequency pulses are calculated between the pulses of the pulse train, that is, the wheel rotation at that time. The wheel speed was calculated by counting at a period inversely proportional to the speed.

However, in the former method, in order to increase the accuracy of the wheel speed, the above-mentioned fixed time must be made considerably longer, and the calculation requires a long time. Furthermore, in the latter method, the calculation accuracy differs greatly depending on whether the wheel speed is high or low.

Furthermore, both of the above methods required complex circuitry to obtain wheel speed signatures, ie, wheel deceleration or acceleration.

The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a wheel rotation speed calculation circuit that calculates wheel speed almost instantaneously, and furthermore, allows a very simple classification operation. This object, according to the invention, comprises: a counter controlled by a wheel speed signal with a frequency proportional to the rotational speed of the wheels; a comparator for comparing a predetermined digital value with the digital output of said counter; A computing unit that computes and outputs a digital value corresponding to the wheel rotation speed based on the output of this comparator, and a computing unit that receives the digital output of this computing unit and a signal of a constant frequency and is proportional to the digital output of the computing unit. a pulse generator that generates a pulse train of a certain frequency; the counter receives the pulse train of the pulse generator and counts the pulse train that occurs within a cycle of the wheel speed signal or a period related to the cycle; This is achieved by a wheel rotation speed calculation circuit characterized in that the digital output of the counter obtained thereby is compared with the predetermined digital value by the comparator.

Hereinafter, details of the present invention will be explained with reference to the drawings.

FIG. 1 shows a wheel rotation speed calculation circuit for anti-skid control of an automobile according to an embodiment of the present invention. This circuit mainly consists of a counter 1 composed of a binary counter, a correction circuit 2 composed of a presettable up-down binary counter, a comparator 3, and a presettable up-down binary counter. It consists of a wheel speed calculator 4 comprised of a counter and a pulse generator 5 comprised of a binary rate multiblier.

A clear signal CLR as shown in FIG. 2D is supplied to the clear signal input terminal 6.
is connected to the input terminal CLR of the counter 1. In addition, the input terminal 7 has a wheel speed pulse RI shown in Fig. 2B.
is supplied, and this terminal 7 is connected to the input terminal CP of the counter 1 via the NAND element 12. The output terminals Oo to Om of the counter 1 are connected to the preset input terminals Po to Pm of the correction circuit 2, respectively, and the input terminals
The CPU and CPD are connected to an UP terminal 8 and a DWN terminal 9, respectively, which are supplied with pulses as described later. Note that the output terminal Om of the most significant bit (MSB) of the counter 1 is connected to the input terminal of the above-mentioned NAND element 12 via an inverter 13.

An input terminal 10 is supplied with a latch signal LT as shown in FIG.
It is connected to the latch signal input terminal L of. Output terminals Oo to Om of the correction circuit 2 are connected to comparison input terminals Ao to Am of the comparator 3, respectively. A predetermined digital value "M" is applied to the reference input terminals Bo to Bm of the comparator 3.

The first comparison output terminal A<B and the second comparison output terminal A>B of the comparator 3 are NAND elements 14 and 1, respectively.
5, NOR elements 19, 20 and inverter 23,
24 to the input terminals CPU and CPD of the wheel speed calculator 4. Input terminal Po of computing unit 4
A predetermined digital value "No" that sets the lowest temperature of the computing unit 4 is applied to Pn. This value "No" corresponds to a set minimum speed Vmm of 5 Km/H, for example. Further, a negative direction pulse is applied to the input terminal L of the arithmetic unit 4 via the input terminal 30 when the power of the entire circuit shown in FIG. 1 is turned on. Therefore, when the power is turned on, the digital value "No" is preset in the arithmetic unit 4.

The output terminals Oo to On of the wheel speed calculator 4 are connected to the input terminals A, B, C, etc. of the pulse generator 5, respectively, and are terminals that become "1" when the output of the output terminals Oo to On is "No". All the terminals except for are further connected to the input terminals of a NOR element 25 for detecting the lowest speed. Further, although not shown, the output terminals Oo to On are each connected to an input terminal of a NAND element 28 for maximum speed detection. The NOR element 25 is
The above-mentioned
Connected to the other input terminals of the NOR element 20, NAND
The element 28 is connected to the other input terminals of the above-mentioned NOR element 19 via the inverter 27, the NOR element 22, and the inverter 18.

The input terminal CP of the pulse generator 5 has a frequency o
The clock pulse of the above-mentioned NAND element 12 is supplied through the input terminal 29, and the output terminal OUT is the clock pulse of the above-mentioned NAND element 12.
Connected to the input terminal. The frequency o of the clock pulse is given by rimax, which is the highest frequency of the wheel speed pulse RI supplied to the input terminal 7.
It is set to rimax×M. Here, “M”
is a predetermined digital value applied to comparator 3.

Note that the above-mentioned input terminal 30 is further connected to the inverter 2.
6 to the other input terminals of the NOR elements 21 and 22, and the output terminal of the wheel speed calculator 4.
It is assumed that Oo to On are connected to an approximate vehicle speed calculation circuit and a slip rate calculation circuit, as indicated by C in the figure, even though they are not shown. In addition, input terminal 1
A timing pulse of, for example, 2 KHz is supplied to the input terminal 1, and is supplied to the other input terminals of the above-mentioned NAND elements 14 and 15 via an inverter 16. According to the output of the comparator 3, the pulses obtained from the NAND elements 14 and 15 are applied to the above-mentioned UP terminals 8 and 15, respectively.
Supplied to DWN terminal 9.

The wheel speed calculation circuit according to the embodiment of the present invention is constructed as described above, and its operation will be explained next.

A wheel speed sensor (not shown) is provided on the wheel, and a sinusoidal signal is obtained from it as the wheel rotates. By waveform shaping this signal, a wheel speed signal as shown in FIG. 2A is obtained. The wheel speed signal is a rectangular signal whose frequency is proportional to the rotational speed of the wheel, and its rise forms the above-mentioned wheel speed pulse RI in the negative direction. Further, the falling and rising edges of the wheel speed pulse RI form the latch signal LT and clear signal CLR as shown in FIG. 2C and D, respectively. The clear signal CLR, wheel speed pulse RI, and latch signal LT are supplied to input terminals 6, 7, and 10, respectively, while the car is running, but for the sake of clarity, we will explain this when the car is stopped. Suppose that the vehicle accelerates from this state and reaches the maximum speed set for this circuit.

First, at the start of running, a negative direction pulse is applied to the input terminal 30 by turning on the power supply for the entire circuit.
As a result, the wheel speed calculator 4 is preset to a digital value of "No". Note that the output value of the calculator 4 is such that when the wheel speed reaches the set maximum speed Vmax, the outputs of the output terminals Oo to On all become "1". In other words, the digital value 2 ⁰ +2 ¹ +2 ² +...2 ⁿ = 2 ⁿ⁺¹ -1 is the maximum speed
It is a digital value corresponding to Vmax.

The pulse generator 5 is composed of a rate multiblier, and from its output terminal OUT, a pulse train of a frequency proportional to the digital output of the arithmetic unit 4/2 ⁿ⁺¹ = ox digital output of the arithmetic unit 4 is obtained. It will be done. Here, o is the frequency of the clock pulse applied to the input terminal CP, and "n+1" of 2 ⁿ⁺¹ is the number of bits of the input terminals A, B, . . . of the rate multiblier plus 1. Therefore, when the entire circuit is powered on, a pulse train with a frequency of =oxNo/2n ⁺¹ is generated. A pulse train of this frequency is applied to one input terminal of the NAND element 12, and the NAND element 12
Since the signals shown in FIG. 2B and the output of the inverter 13 are applied to the other two input terminals of the counter 1, the output pulse train of the pulse generator 5 starts to be counted by the counter 1. That is, with the generation of the wheel speed pulse RI, the latch signal LT and clear signal CLR shown in FIG.
The output of the counter 1 at this time (set to O when the power is turned on) is latched in the correction circuit 2, and then the contents of the counter 1 are cleared. Inverter 13 is connected to terminal Om, which is the MSB of counter 1, and since the output of this terminal Om is "0", inverter 1
3, an output of "1" is applied to the NAND element 12. Therefore, the wheel speed pulse RI is “0”
becomes "1" and the pulse train from the pulse generator 5 starts counting.

By the way, when the wheels start running, the wheel speed V
is lower than the set minimum speed Vmm, so the output of the calculator 4 is "No", and the interval between wheel speed pulses RI at this time, that is, the period 1/ri, is 0×No/2 ⁿ⁺¹ ×f/ri. Pulse is counter 1
It is counted by When the next wheel speed pulse RI occurs, the output of the counter 1 is latched into the correction circuit 2 as described above. This digital value is applied to the input terminals Ao to Am of the comparator 3 and compared with the above-mentioned predetermined digital value "M", but since the wheel speed is lower than the set minimum speed, the digital value is applied to the input terminals Ao to Am. Since the value is larger, a signal of "1" is generated from the output terminal A>B of the comparator 3. This is applied to one input terminal of the NAND element 15, and in this embodiment, a 2KHz timing pulse is applied from the inverter 16 to the other input terminal, so this 2KHz frequency pulse is applied to the NAND element 15.
It is applied to the -input terminal of the NOR element 20 via the element 15. At this time, the output of the arithmetic unit 4 is "No", and therefore the output of the NOR element 25 for detecting the lowest speed is "1", so the other input terminals of the NOR element 20 receive "1" from the inverter 17. An output is being applied, and the NOR element 20 is in a pulse non-conducting state. Therefore, the pulse from the inverter 16 is not transmitted to the CPD terminal of the calculator 4, and the output of the calculator 4 remains "No".

As mentioned above, the wheel speed calculator 4 is preset to a digital value of "No" when the power is turned on, and its output is kept at "No" as long as the wheel speed is below the set minimum speed Vmm.

Note that a negative direction pulse is applied to the inverter 26 from the input terminal 30 when the power is turned on, but this prevents noise that occurs when the power is turned on, and its output is "O" while the circuit is operating. .

Further, the value of the predetermined digital value "M" applied to the reference terminal of the comparator 3 is the pulse train of the pulse generator 5 counted in one period of the wheel speed pulse RI when the wheel speed is the set minimum speed Vmm. Therefore, if the frequency of the wheel speed pulse RI corresponding to the set minimum speed Vmm is rimin, then M=No/2 ⁿ⁺¹ ·o×I/rimin.

Here, "No" is the digital value of the arithmetic unit 4 corresponding to the set minimum speed Vmin, and o is the frequency of the clock pulse supplied to the input terminal 28.

Even if the wheel speed increases and reaches Vmin and exceeds this, as long as the number of pulse trains counted within the period of the wheel speed pulse RI is "M", the output of the wheel speed calculator 4 does not change. Wheel speed is Vmin
The pulse train counted within the period between the first wheel speed pulse RI and the next pulse RI after exceeding +△V is shortened by an amount corresponding to the period of the wheel speed pulse RI corresponding to △V. Nevertheless, since the frequency of the pulse train emitted from the pulse generator 5 is the same as that for the previous period of the wheel speed pulse RI, it is one less than the previous period, and becomes "M-1".
As a result, the output signal of the output terminal A<B of the comparator 3 becomes "1" and is supplied from the inverter 16.
One 2KHz negative direction pulse is supplied to the CPU terminal of the wheel speed calculator 4 via the NOR element 19 and the inverter 23. That is, a 2KHz pulse is
It is also added to the CPU terminal of the modification circuit 2 from the NAND element 14, and the contents of the modification circuit 2 are counted up by one, and its output becomes "M" and the output signal of the output terminal A<B of the comparator 3 becomes "0" again. This is because the NAND element 14 will no longer pass any more pulse trains.

Here, if ΔV is the speed difference in which the number of counts counted between wheel speed pulses RI is reduced by "1" from the predetermined digital value "M", ΔV is given by the following relationship.

Now, if the frequency of the wheel speed pulse when the wheel speed is Vmin+△V is ri, and the number of counts in that period is M', then from M-M'=1, No/2 ⁿ⁺¹ ×o×1/rimin −No/2 ⁿ⁺¹ ×o×1/f′ri = No/2 ⁿ⁺¹ ×o(1/rimin-1/f
′ri)=1 ′ri−rimin=2 ⁿ⁺¹ /N×o×rimin×f′ri=1/M×′ri Therefore, if ′ri−rimin=△ri, then △ri=1/M×f′ ri In the above equation, the frequency ri of the wheel speed pulse RI is proportional to the wheel speed V, so △V=1/M×(Vmin+△V) ∴△V=Vmin/M-1 (however, M>1) . That is, the value of ΔV can be arbitrarily set by appropriately selecting the value of "M".

In addition, although the above was obtained when the wheel speed increased by ΔV from Vmin, it is clear that the above relationship holds true even when the wheel speed increases by ΔV from any Vi.

As described above, the output value of the wheel speed calculator 4 changes from "No" to "No+1", and the pulse generator 5 generates a pulse train with a frequency of oxNo+1/2 ⁿ⁺¹ . i.e. front wheel speed pulse
From the frequency of the pulse train in the period of RI, No+1/N
A pulse train of o times higher frequency is supplied to the counter 1.

The above will be explained in more detail as follows.
In other words, as shown in Figure 3A, the wheel speed is
The period of wheel speed pulse at Vmin RI ₁ ~ RI ₂ is T ₁
Then, the period of the subsequent wheel speed pulse is RI ₂ ~ RI ₃
(T ₂ ), ..., RIn~RI _o+1 (Tn), RI _o+1 ~ _{RI o+2}
(T _o+1 )... becomes shorter depending on the acceleration of the wheel (T ₁ > T ₂ >...>Tn>Tn+ ₁ ...). Then, after the wheel speed exceeds Vmin+△V=Vm, if the first wheel speed pulse is RIn, the period Tn at this time is No/2 ⁿ⁺¹ ×o from the period T ₁ when Vmin.
It is shorter than one pulse of frequency. In other words, the period
The pulse train count number at Tn is “M-1”
becomes. As a result, the output of the wheel speed calculator 4 increases by "1", and the frequency of the pulse train generated by the pulse generator 5 is shorter by one pulse having a frequency of No/2 ⁿ⁺¹ xo. In other words, the count number of the pulse train in the period Tn is "M-1". As a result, the output of the wheel speed calculator 4 increases by "1", and the frequency of the pulse train generated by the pulse generator 5 becomes No+1/2 ^n+1.
xo, and this pulse train is counted in the next period Tn+1. Therefore, the wheel speed pulse
The number of pulse counts within the period of RI becomes "M" again.

Until the wheel speed further increases and exceeds Vm+ΔV=Vr, each time the wheel speed pulse RI is applied to the NAND element 12, clearing, latching, and comparison are repeated as described above. Output terminal A<
Since the output signal of B, A>B remains at "O", the contents of the wheel speed calculator 4 do not change. However, the first wheel speed pulse after the wheel speed exceeds Vr
The period from when RI is applied to the NAND element 12 until the next wheel speed pulse RI is applied, that is, the period of the pulse RI at this time, after exceeding the above-mentioned Vm, becomes further △V compared to the period Tn of the first pulse RI. The frequency of the pulse train generated by the pulse generator 5 is reduced by a corresponding amount, and the frequency of the pulse train generated by the pulse generator 5 is reduced by a corresponding amount.
Since the magnitude is before reaching Vr, the pulse train counted by counter 1 within this period is “M-
1", the signal at the output terminal A<B of the comparator 3 becomes "1", and the output of the wheel speed calculator 4 increases by 1.

As the wheel speed increases, the digital output of the wheel speed calculator 4 increases as described above, and when the wheel speed reaches the set maximum speed Vmax, the output signals of the output terminals Oo to On of the calculator 4 all become "1". becomes. As mentioned above, all the output terminals Oo to On of the arithmetic unit 4 are connected to the NAND element 28, so the output of the NAND element 28 becomes "O", and therefore the output of the inverter 18 becomes "1". ,
The NOR element 19 becomes non-conductive with respect to the pulse train. That is, even if a pulse train is supplied from the NAND element 14, the NOR element 19 is in a non-conductive state, so the contents of the wheel speed calculator 4 are not counted up. This causes the next pulse to
All signals applied to the output terminals Oo to On are prevented from becoming "O".

In reality, the wheel rotational speed rarely reaches the set maximum speed Vmax, but the above method ensures that a reliable and reliable wheel speed is always obtained. If the wheel speed exceeds Vmax, a constant speed value corresponding to 2 ⁿ⁺¹ −1
Vmax can be obtained.

As explained above, when the wheels are accelerated from a stopped state to the set maximum wheel speed, the digital output of the wheel speed calculator 4 changes, but the relationship between this digital output and the wheel speed is arbitrary. If the wheel speed is Vi, the frequency of the wheel speed pulse RI at that time is ri i, and the output of the calculator 4 is Ni, then the number of counts M' counted by the counter 1 in one cycle of the wheel speed pulse RI is M′=Ni/2 ⁿ⁺¹ ×o×1/ri i =o/2 ⁿ⁺¹ ×Ni/ri i , and this M′ is always the predetermined digital value “M”.
M=o/2 ⁿ⁺¹ ×Ni/ri i ∴Ni=2 ⁿ⁺¹ /o×M×ri i Also, the wheel speed Vi is the frequency ri of the wheel speed pulse RI.
Since it is proportional to i, if its proportionality constant is k, then ri i=kVi Therefore, Ni=M×2 ⁿ⁺¹ /o・k・Vi (however, Ni>No) In the above equation, M, o, 2 ⁿ⁺¹ , are constants, it is clear that the output Ni of the calculator 4 changes in proportion to the wheel speed Vi. In addition, the computing unit 4
The output 1-bit speed value of can be arbitrarily selected by appropriately setting the above constants.

Next, when the car is traveling at a constant speed,
In other words, a case will be explained in which the brake is applied while the wheels are rotating at a constant speed.

For example, if the wheel speed is now in a constant speed state of Vm, the number of pulses of the pulse train counted within the period of the wheel speed pulse RI at this time is "M" as described above, and The output of is constant. However, when the brakes are applied to reduce the wheel speed, the period of the wheel speed pulse RI becomes longer as shown in FIG. 3B: Tm<T _n+1 <...<Tp<T _p+1 . Now, in Figure 3B, the wheel speed pulse
Between RIm and RI _n+1, the velocity is constant, and from RI _n+1 to
If the brake is applied between RI _n+2 , the period T _n+1
becomes longer than Tm, but does not become longer as the number of counts in counter 1 increases. Wheel speed is Vm―
In the period Tp between the wheel speed pulses RIp and RI _p+1 where ΔV=Vp or less, the period Tp is longer than the period Tm by one pulse, and the output of the counter 1 changes from "M" to "M+".
1". As a result, the output terminal A of comparator 3
>B signal becomes "1" and the contents of the arithmetic unit 4 are counted down by 1. At the same time, the frequency of the pulse train generated by the pulse generator 5 decreases in proportion to this, and the pulse count number of the counter 1 in the next period T _p+1 becomes "M" again. Then the wheel speed pulse
After several to tens of cycles of RI (depending on how the brakes are applied), the wheel speed changes to Vp - △V
When the value becomes even smaller, the pulse count number of the counter 1 becomes "M+1" again, and the content of the arithmetic unit 4 becomes 1 again.
will be counted down. As described above, the contents of the calculator 4 are counted down until the desired wheel speed is obtained, and when the brake is released, a digital output corresponding to the current wheel speed is obtained from the calculator 4 as a constant value. When the wheels are stopped, the digital output of the computing unit 4 remains as "No", but by turning off the power to the entire circuit, all the circuits return to their initial states.

In the embodiment of the present invention, as described above, the wheel speed is immediately calculated, and the speed change is detected by the number of pulses as a count-up signal or a count-down signal from the NAND elements 14 and 15. Calculation of acceleration or wheel deceleration is performed very easily. That is, in the above embodiment, one pulse is generated as a count-up signal or a count-down signal every time the wheel speed changes by a predetermined ΔV, so if this number of pulses is counted within a predetermined time, the deceleration can be immediately detected. can be obtained.

In addition, in this embodiment, the above-mentioned pulses are obtained in synchronization with the 2KHz clock pulse applied to the terminal 11, so even if the speed suddenly changes due to temporary impact on the wheels, for example, when driving on the road, Even if the wheel temporarily gets stuck in the dent, it is 1/2000
Changes within seconds are not detected and stable anti-skid control is guaranteed.

Although the digital output Ni of the wheel speed calculator 4 is not shown, it is supplied to the approximate vehicle speed calculation circuit and the slip rate calculation circuit, and provides control signals for the valves that control the brake fluid pressure of the wheels as well as the acceleration and deceleration mentioned above. However, since various configurations are known, a description thereof will be omitted.

Although the embodiment of the present invention is configured as described above,
Of course, the present invention is not limited to this, and various modifications can be made based on the technical idea of the present invention.

Although the reference digital value M to be added to the comparator 3 can be freely selected, as is clear from the above explanation, in the present invention, the number of counts within the wheel speed pulse period of the counter 1 is always constant. The count-up signal or count-down signal is, so to speak, an error signal, and from the viewpoint of control accuracy, it is not preferable to make the above-mentioned count number too small. Therefore, it is not preferable to make the reference digital value to the comparator 3 too small.

Furthermore, in the above embodiment, in order to prevent overflow of the wheel speed calculator 4, a
Although a NAND element 28 is provided, it can of course be omitted if a counter with a sufficiently large capacity is used. Also, NOR element 25 for minimum speed detection
is provided to prevent the output of the calculator 4 from becoming less than "No", but this means that when the wheel speed is close to 0, the output of the wheel speed sensor is almost non-existent or very unstable. This is because stable calculation results cannot be obtained. Therefore, the maximum output of the arithmetic unit 2 ⁿ⁺
It is desirable to set it according to the value of ^1-1 .

Further, in the above embodiment, the content of the arithmetic unit 4 is preset to "No" when the power of the entire circuit is turned on, but it is not necessarily limited to this value.

Further, in the above embodiment, the pulse train from the pulse generator is counted for each period of the wheel speed pulse RI, but the pulse train is not limited to this, and for example, 4 cycles are counted for each plurality of periods of the wheel speed pulse RI. The pulse train within one cycle may be counted every cycle or every multiple cycles, for example every four cycles. In this way, it becomes possible to calculate the wheel speeds of a plurality of wheels using the single circuit shown in FIG.

In addition, in the above embodiment, the counter 1 outputs a pulse.
Although the case where the count changes by ±1 per period of RI has been explained, it is obvious that the above explanation can be applied as is even if the count changes by ±2 or more.

The wheel speed calculation circuit of the present invention has the above-described configuration, and has the extremely remarkable effect of being able to instantaneously calculate the wheel speed and performing the differential operation extremely easily.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a wheel speed calculation circuit according to an embodiment of the present invention, and FIGS. 2 and 3 are waveform charts and time charts for explaining the operation of the circuit. In the figure, 1...counter, 2...correction circuit, 3...comparator, 4...wheel speed calculator, 5...
...Pulse generator.

Claims (1)

[Claims] 1. A counter controlled by a wheel speed signal with a frequency proportional to the rotational speed of the wheel, a comparator that compares a predetermined digital value with the digital output of the counter, and a A computing unit that calculates and outputs a digital value corresponding to the wheel rotation speed, and a pulse generator that receives the digital output of this computing unit and a signal of a constant frequency and generates a pulse train of a frequency proportional to the digital output of the computing unit. The counter receives the pulse train of the pulse generator and counts the pulse train generated within the period of the wheel speed signal or the period starting from this period, and the counter receives the pulse train generated by the pulse generator. A wheel rotation speed calculation circuit characterized in that a digital output is compared with the predetermined digital value by the comparator.