GB1601643A

GB1601643A - Vehicle brake system having a safety circuit

Info

Publication number: GB1601643A
Application number: GB2053378A
Authority: GB
Original assignee: Wabco Fahrzeugbremsen GmbH
Current assignee: Wabco Fahrzeugbremsen GmbH
Priority date: 1977-05-18
Filing date: 1978-05-18
Publication date: 1981-11-04
Also published as: DE2722435B2; FR2391096B1; JPS6229267B2; FR2391096A1; DE2722435C3; JPS53142712A; DE2722435A1

Description

(54) A VEHICLE BRAKE SYSTEM HAVING A SAFETY CIRCUIT (71) We, WABCO FAHRZEUGBREMSEN GmbH, formerly WABCO WESTING HOUSE GmbH, a company organised according to the laws of the Federal Republic of Germany, of 3000 Hannover 91, Postfach 9112 80, Federal Republic of Germany; do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a vehicle brake system having a safety circuit.

An essential feature of a safety circuit for anti-lock vehicle brake systems is the use of monitoring circuits to detect inadmissibly long operation of the valve magnets of inlet and outlet valves controlling the pressure in the brake cylinders of the vehicle brake system.

These monitoring circuits signal incorrect operation of the system, as when a valve magnet has been operating for longer than, for example, one second, after a delay period, by means of appropriate error warning devices. If an ALB control circuit, controlling the brake pressure in dependence on the load of the vehicle, having an ALB safety circuit (ALB-SIS) is used together with an ABV control circuit, controlling the brake pressure so as to prevent wheel locking under braking, the danger of unstable braked driving conditions may arise as a result of the adverse mutual influence of the control circuits and the associated safety circuits. The condition may arise in which the ALB control circuit comes into operation, for example, during braking processes that do not result in the ABV control circuit being actuated.This means that at least the magnet of the inlet valve is excited for the whole braking period so as, for example, to reduce the brake pressure at the rear axle to a certain level or to maintain it at a certain value. As such processes generally last longer than the maximum period during which the valve magnets are operated by the ABV control circuit, there is a danger that the monitoring circuit will respond and produce an error signal, which can cause an incorrect reaction of the system.

If, in the hypothetical case above, the ALB control circuit fails after the desired pressure has been reached, the inlet valve continues to be excited even if the pressure Pe supplied from the pedal operated brake valve is reduced again. As a result, an inadmissible condition occurs because the limited brake pressure set by the ALB control circuit is not reduced again.

Another error condition occurs when the ALB control circuit should become operative, but no limitation of the rear axle pressure takes place and hence the behaviour of the brakes is affected.

Furthermore. the ALB-SIS also may fail and activate the warning device by a false error signal. This too can cause incorrect reactions of the control system.

An object of the present invention is therefore to provide a safety circuit in which the incorrect reactions in anti-lock and load-dependent vehicle brake systems as described above can be avoided with certainty.

According to the present invention there is provided a vehicle brake system having an anti-lock brake pressure control circuit and a circuit for the load-dependent regulation of the brake pressure, which, in use, control the pressure in brake cylinders by means of controllable inlet and outlet valves, and including a first safety circuit for the anti-lock control circuit, and a second safety circuit for the load-dependent regulation circuit, wherein the anti-lock control circuit and the load-dependent regulation circuit and their safety circuits are so linked to one another via logic elements and to the inlet and outlet valves that during an anti-lock controlled braking process error signals from the safety circuit for the load dependent regulation circuit are blocked, and that during a load-dependent brake force regulation when there is no simultaneous anti-lock regulation of the brake pressure error signals from the safety circuit of the anti-lock control circuit are blocked.

The invention ensures that in braking processes that do not result in the response of the ABV control circuit but rather in the response of the ALB control circuit, the relatively long operational period at least of the magnets of the inlet valves is not recognised as an error by the monitoring circuit for the magnet operational period, and that, further, when anti-lock brake pressure regulation is being carried out, error signals of the ALB-SIS cannot adversely affect the control of the braking process.

As already mentioned, in the case of ALB regulation by itself, the ALB control circuit can fail. When this happens, as soon as the inlet valve has closed after reaching the reference pressure and after the pressure Pc introduced from the pedal operated brake valve has been reduced again, the valve continues to be excited and an inadmissible condition occurs. On the one hand, the brake pressure that was limited by the ALB control circuit is not reduced and, on the other hand, the ABV-SIS remains blocked for monitoring the operated period of the valve. In one example of the invention measures are taken to prevent this occurring. By means of these measures it is possible to reduce the cylinder pressure again.

The ABV-SIS usually monitors the energised periods of the valve magnets downstream of amplifier stages driving the magnets. In order that the ABV control circuit can continue to be monitored during ALB brake pressure regulation, a further development of the invention provides that in the case of ALB regulation the energising signals produced by the ABV control circuit for the inlet and outlet valves can be monitored before control by the ALB control circuit. In practice, this means that the valve-energising signals are monitored directly at the output of the ABV control circuit. If the ALB control circuit itself is faulty, it may result in the disconnection of the ABV-SIS by the ALB-SIS so that too long an operated period of the valve magnets for other reasons cannot be detected.To prevent this it may be arranged that in the unbraked state, i.e. at Pc = 0, the disconnection of the ABV-SIS by the ALB control circuit is prevented. In the braked state, a signal tv4 is produced by the ABV control circuit, which signal represents a criterion for the response of the ABV control circuit. This signal likewise blocks the disconnection of the ABV-SIS, i.e.

the ABV-SIS is switched in again. Under these conditions, however, only the signal tv4 of the energising signals for the outlet valve from the ABV control circuit logic may be used.

To prevent incorrect regulation of the brake pressure in the event of a faulty ALB control circuit, according to a further development of the invention, in the event of an error signal of the ALB-SIS the energising line of the ALB control circuit to the inlet and outlet valves and to the ABV-SIS is blocked.

So as to be able to signal an error which results in no limitation of the rear axle brake pressure although the ALB control circuit should in fact be effective, the ALB-SIS may include a comparator for comparing the brake pressure with a reference value depending on the load and producing an error signal if the difference is too large.

According to a further development of the invention, mutual monitoring of the ALB control circuit and the ALB safety circuit is effected by providing a device for monitoring the coincidence of comparator outputs of the ALB control circuit and the ALB safety circuit, which device produces an output when coincidence does not occur.

In a simple design of the safety circuit for the ALB regulator it is feasible that in the unbraked state (Pe = 0) monitoring of the operated period of the magnets by the ABV-SIS can detect whether a possible faulty energisation of the valve magnets originates from the ABV control circuit or the ALB control circuit, since the energisation of the magnets is effected both upstream and downstream of the connection of the ALB. As soon as the error has been detected it would be arranged to disconnect the ALB control circuit.

In order that the invention may be fully understood and readily carried into effect it will now be described in more detail with reference to the accompanying drawings of which: Figure 1 shows a block diagram of the safety circuit according to an embodiment of the invention; Figure 2 shows the reference curve of a load-dependent brake force regulator, this being a simple break point regulator; Figure 3 shows schematically an ALB safety circuit using analogue signal circuit design for a break point regulator having the performance indicated in Figure 2; Figure 4 shows a block diagram for simulating the ALB control curves in digital form, e.g. for the comparison of the actual and reference values in the ALB safety circuit; Figure 5 shows a block diagram for an ALB control circuit in digital form; and Figure 6 shows a block diagram for improving quantisation of signal values.

In Figure 1, the reference numeral 2 denotes a wheel speed sensor, 4 denotes an anti-lock (ABV) control circuit, 6 denotes an ABV safety circuit (ABV-SIS), 8 denotes a load-dependent brake pressure (ALB) control circuit and 10 denotes an ALB safety circuit (ALB-SIS).

The variables employed in Figure 1 are an introduced brake pressure Pc, a brake cylinder pressure pz and a load L, which variables are supplied to the ALB control circuit and to the ALB-SIS via connections 12, 14 and 16.

Comparators are provided in the ALB-SIS for comparing the introduced pressure Pc with the brake cylinder pressure Pz( ePz) and for comparing the actual curve with the reference curve of the ALB regulator (pz = f(UL,pe), UL being a signal value corresponding to the load, for example in the form of voltage.

The ALB-SIS has an error signal output line 18 which leads via an AND-gate 20 to an ALB-error-indicating device 22.

The ABV control circuit and the ABV-SIS are linked via a logic circuit 24 to the ALB control circuit and the ALB-SIS. The logic circuit includes two AND-gates 26 and 28 to which the output signals of the ALB control circuit and those of the ALB-SIS are applied.

The error signals of the ALB-SIS are fed to inverting inputs of the AND -gates 26 and 28 and the output signals of the ALB control circuit are applied to non-inverting inputs of the AND-gates 26 and 28.

The logic circuit also includes a further AND-gate 30 with three inputs. A non-inverting input of the gate 30 receives an output signal of the ALB control circuit, and two inverting inputs of the gate 30 receive respectively the error signal of the ALB-SIS and the output signal of a comparator 32 are supplied. The comparator 32 detects whether the introduced pressure Pc is zero (Pe = 0).

The logic circuit also has two OR-gates 34 and 36 in which the output signals of the AND-gates 26 and 28 and output signals of the ABV control circuit are combined. The output of the OR-gate 34 energises an inlet valve 40 (EV via an amplifier 38 and the output of the OR-gate 36 energises an outlet valve 44 (AV via an amplifier 42.

The output of the AND-gate 30 is connected to a non-inverting input of a further AND-gate 46 which has an inverting input to which is supplied a signal (tv4) from the ABV control circuit that indicates the presence of a control process. The output of the AND-gate 46 is connected to energise the ABV-SIS and is blocked by the ALB control circuit.

The ABV-SIS 6 monitors the operated periods of the inlet valve and outlet valve magnets 40 and 44 via lines 48 and 50 and monitors the output signals of the ABV control circuit via lines 52 and 54 before these output signals are combined with the signals from the ALB circuit in gates 34 and 36. The output signals of the ABV-SIS are applied to an ABV-error-indicating device 56.

The circuit according to Figure 1 operates as follows: 1. If the ALB control circuit 8 is operating correctly, no ABV control takes place and if Pc = 0, then firstly, by means of the AND-gate 26 and the OR-gate 34, as soon as the reference pressure point Ki (Figure 2) is reached EV is blocked and the further supply of pressure medium in the rear axle brake cylinder is cut off; this being the principle of operation of a break point regulator. On reducing the introduced brake pressure Pc, by means of an AND-gate 28 and OR-gate 36 the outlet valve 44 is also energised, the inlet valve still being energised, and thus the pressure in the brake cylinder is reduced. The ABV-SIS is blocked via the AND-gates 30 and 46.This prevents the ABV-SIS from responding to the generally relatively long operated periods of the valve magnets 40 and 44 as a result of ALB regulation, and thus causing the incorrect generation of an error signal.

2. If the ALB-SIS detects an inadmissible condition, for example, in the case of failure of the ALB control circuit, which consists in the reference pressure in the brake cylinder lying outside the permissible tolerance range monitored by the ALB-SIS, then the ALB-SIS produces an error signal which, if no ABV regulation is taking place and therefore the signal tv4 is not produced, is indicated by the device 22. This error signal also blocks the AND-gates 26, 28 and 30, thereby preventing the blocking of the ABV-SIS by the output of gate 46 and cutting off the energisation of the valve magnets 40 and 44. As a result of this, the brake cylinder pressure can be reduced again and monitoring of the magnet operated period can be allowed to take place again.

3. Both in the unbraked and braked state the disconnection of the ABV-SIS by the ALB can be incorrect if, for example, the ALB control circuit itself is faulty, so that too long an operated period of the valve magnets cannot be detected for one reason or another.

In the unbraked state the introduced brake pressure Pc is equal to 0. The comparator 32 then emits a "1" signal which blocks the AND-gate 30 thus terminating the blocking of the ABV-SIS. In the braked regulated state the ABV control circuit produces the signal (tv4) indicating that a regulation process is taking place which signal blocks the AND-gate 46 and thus ends the blocking of the ABV-SIS. In this case, however, only the signal tv4 from the ABV logic may be used.

4. If, by comparing the values of the actual curve and the reference curve of the ALB regulator, the ALB-SIS detects that the actual value lies outside a given tolerance range of the reference values (Upz > U(L)), then the ALB-SIS produces an error signal which indicates the error by energising the device 22 and also blocks the AND-gates 26, 28 and 30, in a similar manner to the case of error discussed in paragraph 2 above. This prevents an error such that when the ALB control is meant to be active, there is no limitation of the rear axle brake pressure and hence the behaviour of the brakes is totally different from their customary behaviour. This kind of error will be described in more detail with the aid of Figures 2 and 3.

In Figure 3 the reference numerals 60, 62 and 64 denote transducers for the introduced brake pressure (pre) the brake cylinder pressure (pz) and the load (FL). The output variables of these transducers, for example in the form of voltages, indicated by U with appropriate suffix, are supplied both to an ALB control circuit 66 and to an ALB-SIS 68. In the ALB-SIS a safety circuit is represented by way of example for the simple case of a break point regulator, having a characteristic like that shown in Figure 2. The ALB-SIS has two comparators 70 and 72 which compare the measured values for the introduced brake pressure Pc and the brake cylinder pressure pz and also compare the measured values for the brake cylinder pressure pz and the load L.The comparator 70 emits a signal if the brake cylinder pressure p is greater than the introduced pressure Pc and the comparator 72 emits a signal if the measurable variable of the brake cylinder pressure pz deviates from that of the load L by a predetermined amount or more, i.e. when the actual curve of the ALB regulator lies outside a certain tolerance range of the reference curve of the ALB regulator.

After passing timing elements 74 and 76, both delayed response circuits, the output signals of the comparators 70 and 72, which respresent error signals, are combined in an OR-gate 78 and indicate the particular error by means of a warning device 80, which device can also be activated by the ABV-SIS (not shown). The output signals of the ALB control circuit 66 energise the inlet and outlet valve magnets (EV and AV), as in Figure 1.

Example: If the pressure in the rear axle brake cylinder is controlled according to the curve Pzi (see Figure 2), then the inlet valve EV is excited from the point K1 onwards and the further increase in pressure is carried out in steps. The ALB-SIS then carries out a comparison of the actual value of the pressure and the reference value to ascertain whether the difference lies within the tolerance range marked by the dotted lines. If the actual pressure value lies outside this range relative to the reference value, there is an error and the ALB-SIS signals this fact to the warning device 80.

The ABV-SIS usually checks the magnet value operated periods downstream of the amplifier stages 38 and 42 via the lines 48 and 50 (Figure 1). So that the ABV control circuit is operating, a circuit is provided in which the magnet valve-energising signals from the ABV control circuit are monitored directly at the output of the ABV control circuit, that is by means of lines 52 and 54.

For anti-lock vehicle brake systems several kinds of control systems can be used, for example, individual wheel regulation, diagonal regulation, etc. In the case of individual wheel regulation by the ABV control circuit and axle regulation by the ALB control circuit, the ALB-SIS must be switched off when a wheel is being regulated by the ABV control circuit. As two pressure transducers are necessary for each wheel in this case, the associated measuring channels for the transducers feeding the ALB-SIS must also be switched off.

Above, an analogue signal circuit design of the circuit arrangement has been described with reference to Figures 1 - 3. Below, an example of a digital signal circuit design will be described with reference to Figures 4 - 6, which may be constructed using large-scale integration.

Figure 4 shows the essentials of a circuit for the digital comparison of the actual value and the reference value in the ALB-SIS, e.g. for preventing incorrect behaviour of the vehicle brake system in the event of failure of the ALB control circuit.

After, for example, the analogue measurement of the brake cylinder pressure pz, the introduced brake pressure Pc and the axle load or vehicle load FL, for example in the form of voltage values, and after an A/D conversion of the voltages in suitable converters, not shown, the analogue measured values take the form of digital words as output variables which are supplied to the circuit of Figure 4 by way of lines 90, 92 and 94 to stores 96, 98 and 100.

The digital words for Pc and FL are used to address a read only memory 102, which in dependence on the input address, provides a specific output word on output lines 104 representing a reference value (Bo ... BK) for the pressure to be set in the rear brake cylinder in accordance with a predetermined reference curve for the ALB regulator. This reference value in the form of a digital word is supplied to a digital variable comparator 106 and is there compared with a digital value (Ao ... Ak), representing the actual value, of the brake cylinder pressure pz If the actual value is smaller than the reference value (A < B) then no valve magnet is excited.In the case A = B the inlet valve magnet (EV) is excited, and in the case A > B the inlet valve magnet and the outlet valve magnet (EV and AV) are both excited.

To produce another curve, in the present case it is necessary only to use a read-only memory 102 programmed in another manner.

The digital control unit shown in Figure 5 comprises inter alia the stores and the digital variable comparator of Figure 4.

As the ALB control concept permits relatively coarse quantisation, 6 bits, for example, are sufficient for describing the total brake pressure range of approximately 0 - 7 bar. The same applies to the pressure in the rear axle brake cylinder and to the axle load-dependent voltage.

The programmable read-only memories are commercially available in units of 128 addresses, so that two units provide 256 or 28 addresses. The introduced pressure Pc can thus be subdivided into 5 bit values and the axle load into 3 bit values. The pressure Pc can thus be subdivided into 32 value steps and the axle load L into 8 value steps. Both variables together represent a data word of 8 bits. Depending on the range for Pc and for UL a bit combination can'then be produced which addresses the associated desired rear axle pressure pz and is available as the reference value at the output of the read-only memory.

Example: Pc = 6 bar UL = 3000 kg Pz = f(pe,UL) = 3.5 bar.

The maximum axle load is, for example, 10,000 kg; then,^ for example, axle load quantisation stages of about 100 kg are obtained; they need not be uniformly distributed.

The pressure quantisation Po is Po = (P,),,, ~ 7 bar = 0.22 bar 32 6 bar then corresponds to approximately 27 x 0.22 = 11011 and 3000 kg then corresponds, expressed in binary form, to the digit sequence 011.

The resulting data word for the reference value B is then B = 11011/011. The digital value for 3.5 bar is then located at this address in the read-only memory. If the read-only memory produces an output using the same quantisation as the introduced pressure then the reference value of 3.5 bar would take the form:- 3.5 ~ 15.9 = 16 = 10000 (in 5-bit binary code) 0.22 If the actual value of the brake cylinder pressure Apz is equal to 10001, then A > B or Pz > Preference. In this case the magnets of both the inlet valve and the outlet valve should be excited.

If higher resolutions for the pressure range and load range are required, a read-only memory that has more than 2 addresses must be used. Such stores may, for example have 2" addresses, which corresponds to a storage capacity of 2,048 bytes (8 bit words). With these, the pressure range of 7 bar for example, could be subdivided into quantisation stages of 0.22 bar and the load range of 10,000 kg into 312 kg (say 300 kg) quantisation stages.

One possible way of improving quantisation which is shown schematically in Figure 6, is to use two 256 x 4 read-only memories. Each measurable variable (pressure, load) is used as an input address of a respective one of the memories and the output bit combination, 4 bits from each memory, is then used as an address for a further memory (256 x 8), the output word of which represents the pressure reference value.

WHAT WE CLAIM IS: 1. A vehicle brake system having an anti-lock brake pressure control circuit and a circuit for the load-dependent regulation of the brake pressure, which, in use, control the pressure in brake cylinders by means of controllable inlet and outlet valves, and including a first safety circuit for the anti-lock control circuit, and a second safety circuit for the load-dependent regulation circuit, wherein the anti-lock control circuit and the load

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. reference value in the form of a digital word is supplied to a digital variable comparator 106 and is there compared with a digital value (Ao ... Ak), representing the actual value, of the brake cylinder pressure pz If the actual value is smaller than the reference value (A < B) then no valve magnet is excited. In the case A = B the inlet valve magnet (EV) is excited, and in the case A > B the inlet valve magnet and the outlet valve magnet (EV and AV) are both excited. To produce another curve, in the present case it is necessary only to use a read-only memory 102 programmed in another manner. The digital control unit shown in Figure 5 comprises inter alia the stores and the digital variable comparator of Figure 4. As the ALB control concept permits relatively coarse quantisation, 6 bits, for example, are sufficient for describing the total brake pressure range of approximately 0 - 7 bar. The same applies to the pressure in the rear axle brake cylinder and to the axle load-dependent voltage. The programmable read-only memories are commercially available in units of 128 addresses, so that two units provide 256 or 28 addresses. The introduced pressure Pc can thus be subdivided into 5 bit values and the axle load into 3 bit values. The pressure Pc can thus be subdivided into 32 value steps and the axle load L into 8 value steps. Both variables together represent a data word of 8 bits. Depending on the range for Pc and for UL a bit combination can'then be produced which addresses the associated desired rear axle pressure pz and is available as the reference value at the output of the read-only memory. Example: Pc = 6 bar UL = 3000 kg Pz = f(pe,UL) = 3.5 bar. The maximum axle load is, for example, 10,000 kg; then,^ for example, axle load quantisation stages of about 100 kg are obtained; they need not be uniformly distributed. The pressure quantisation Po is Po = (P,),,, ~ 7 bar = 0.22 bar 32 6 bar then corresponds to approximately 27 x 0.22 = 11011 and 3000 kg then corresponds, expressed in binary form, to the digit sequence 011. The resulting data word for the reference value B is then B = 11011/011. The digital value for 3.5 bar is then located at this address in the read-only memory. If the read-only memory produces an output using the same quantisation as the introduced pressure then the reference value of 3.5 bar would take the form:- 3.5 ~ 15.9 = 16 = 10000 (in 5-bit binary code) 0.22 If the actual value of the brake cylinder pressure Apz is equal to 10001, then A > B or Pz > Preference. In this case the magnets of both the inlet valve and the outlet valve should be excited. If higher resolutions for the pressure range and load range are required, a read-only memory that has more than 2 addresses must be used. Such stores may, for example have 2" addresses, which corresponds to a storage capacity of 2,048 bytes (8 bit words). With these, the pressure range of 7 bar for example, could be subdivided into quantisation stages of 0.22 bar and the load range of 10,000 kg into 312 kg (say 300 kg) quantisation stages. One possible way of improving quantisation which is shown schematically in Figure 6, is to use two 256 x 4 read-only memories. Each measurable variable (pressure, load) is used as an input address of a respective one of the memories and the output bit combination, 4 bits from each memory, is then used as an address for a further memory (256 x 8), the output word of which represents the pressure reference value. WHAT WE CLAIM IS:

1. A vehicle brake system having an anti-lock brake pressure control circuit and a circuit for the load-dependent regulation of the brake pressure, which, in use, control the pressure in brake cylinders by means of controllable inlet and outlet valves, and including a first safety circuit for the anti-lock control circuit, and a second safety circuit for the load-dependent regulation circuit, wherein the anti-lock control circuit and the load

dependent regulation circuit and their safety circuits are so linked to one another via logic elements and to the inlet and outlet valves that during an anti-lock controlled braking process error signals from the safety circuit for the load-dependent regulation circuit are locked, and that during a load-dependent brake force regulation when there is no simultaneous anti-lock regulation of the brake pressure error signals from the safety circuit of the anti-lock control circuit are blocked.

2. A system according to claim 1, further including transducers for deriving signals representing an introduced brake pressure and a brake cylinder pressure, and wherein the safety circuit for the load-dependent regulation circuit has a comparator for comparing the introduced pressure and the brake cylinder pressure and which produces an output signal when the introduced pressure is less than the cylinder pressure which signal interrupts the energisation of the inlet and outlet valve magnets.

3. A system according to claim 1 or 2, wherein at least during the regulation of the brake pressure in dependence on the load the energising signals for the inlet and outlet valves, produced by the anti-lock control circuit, are monitored by the safety circuit for the anti-lock control circuit upstream of the connection to the valves of the load-dependent regulation circuit.

4. A system according to claim 1, 2 or 3, wherein the logic elements include means for blocking the operation of the safety circuit for the anti-lock control circuit in response to a signal from the load-dependent regulation circuit indicating that such regulation is taking place, except when the anti-lock control circuit is controlling braking.

5. A system according to any preceding claim, wherein the logic elements include means responsive to an error signal from the safety circuit for the load-dependent regulating circuit to block connections from the load-dependent regulating circuit to the inlet and outlet valves and to the safety circuit for the anti-lock control circuit.

6. A system according to any preceding claim including the features of claim 2, wherein the safety circuit for the load-dependent regulating circuit has a second comparator for comparing a variable corresponding to the brake cylinder pressure with reference variable corresponding to the load and producing an error signal when the reference variable is exceeded or the brake cylinder pressure variable exceeds the reference variable by a predetermined tolerance depending on the value of the reference variable.

7. A system according to claim 1, wherein the logic element include a device for monitoring the coincidence of the outputs of the load-dependent regulating circuit and its safety circuit, which device signals an anti-coincidence.

8. A system according to any of claims 1 to 6, wherein the logic elements further include a gate connected to receive error signals from the safety circuit for the load-dependent regulating circuit, and signals blocking the gate from the anti-lock control circuit, which latter signals indicate that an anti-lock control process is in progress.

9. A system according to claim 4, wherein the means for blocking the operation of the safety circuit for the anti-lock control circuit includes a first AND-gate which has a non-inverting input which is connected to the load-dependent regulating circuit and also has an inverting input which is connected to the output of the anti-lock control circuit for the signal indicating an anti-lock control process is in progress.

10. A system according to claim 9, further including means responsive to an introduced brake pressure for producing an output signal when that pressure is substantially zero, and wherein the logic element include a further AND-gate, to an inverting input of which the output signal of the responsive means is applied, and to a non-inverting input of which is connected the blocking output from the load-dependent regulating circuit, the output of the further AND-gate being connected to a non-inverting input of the first AND-gate.

11. A system according to claim 5, wherein the means responsive to an error signal from the safety circuit for the load-dependent regulating circuit includes two AND-gates, each having an inverting input to which the error signal is applied and a non-inverting input, each of which is connected to an output of the load-dependent regulating circuit, and a further AND-gate having a first inverting input to which the error signal is applied, a second inverting input to which a signal indicating that an introduced brake pressure is substantially zero is applied, and a non-inverting input to which an output of the further AND-gate being connected to block the operation of the safety circuit for the anti-lock control circuit.

12. A system according to claim 1, wherein the two safety circuits are linked together in such a manner that a function of the safety circuit for the load-dependent regulating circuit is taken over by the safety circuit for the anti-lock control circuit so that in the unbraked driving state a failure of the load-dependent regulating circuit can be detected as a result of the monitoring of the operated period of the magnets by the safety circuit for the anti-lock control circuit and, if necessary the load-dependent regulating circuit can be switched off.

13. A brake system having an anti-lock brake pressure control circuit, a circuit for the load-dependent regulation of the brake pressure and a safety circuit substantially as described herein with reference to Figures 1, 2 and 3 of the accompanying drawings, or modified as described with reference to Figure 4 or 5 with or without Figure 6 of the drawings.