GB2381052A

GB2381052A - Evaluating coil current in an electromagnetically releasable brake

Info

Publication number: GB2381052A
Application number: GB0124988A
Authority: GB
Inventors: Timothy John Martin
Original assignee: Lansing Linde Ltd
Current assignee: Linde Material Handling UK Ltd
Priority date: 2001-10-17
Filing date: 2001-10-17
Publication date: 2003-04-23
Anticipated expiration: 2021-10-17
Also published as: GB2381052B; DE10248362A1; GB0124988D0

Abstract

An electromagnetically releasable brake eg. a parking brake, has at least one electrical coil and in which a brake body can be moved relative to the coil using a magnetic force which can be produced with the coil. During an operating state in which there is a clearance between the coil and the brake body, and the brake body is intended to be moved in the direction of the coil, the coil has voltage continuously applied to it and the characteristic of the coil current I is evaluated in an electronic control apparatus. Preferably the value of the coil current I 1 flowing through the coil shortly before the instant c at which the brake body comes fully into contact with the coil is recorded in the control apparatus (partial contact d is not recorded). In one embodiment the width of the clearance is ascertained using a linear relationship between the recorded coil current I 1 and the width of the clearance. In this context, the voltage across the coil and/or the temperature of the coil are not taken into account. An electronic controller is provided, and a pulsed current holds the brake in the released position.

Description

Method for operating a brake which has an electromagnet and electronic controller therefor The invention relates to a method for operating an electromagnetically releasable brake which has at least one electrical coil and in which a brake body can be moved relative to the coil using a magnetic force which can be produced with the coil. The invention also relates to an electronic controller for carrying out such a method.

Vehicles, particularly commercial vehicles such as industrial trucks, have known brakes which can be used equally as a parking brake and as a service brake. Such brakes generally have a non-rotatable brake body which has a brake pad attached directly or indirectly to it. To actuate the brake, the brake body is moved in the direction of a brake rotor, so that the brake pad is pressed against the brake rotor.

For operation as a parking brake, such brakes generally have an energy storage mechanism in the form of a spring, which exerts a force on the brake body and thereby presses the brake pad onto the brake rotor when the vehicle is not in operation. For operation as a service brake, an actuator in the form of an hydraulic cylinder, for example, is provided, which is used to actuate the brake in accordance with the position of a brake actuation element in the form of a brake pedal, for example. In this instance, the actuator directly or indirectly exerts a force on the brake body, so that the brake pad is pushed onto the brake rotor.

To release the brake, the brake body needs to be released from the brake rotor counter to the force of the energy storage mechanism. To this end, a brake solenoid which is frequently formed by an electromagnet is provided. In this context, the maximum movement path of the brake body corresponds to the width of a clearance which exists between the brake body and the electromagnet when the brake is actuated, and exists between the brake body and the brake rotor when the brake is

released. The width of the clearance increases continuously during the lifetime of the brake as the brake pads wear. The thickness of the brake pads can thus be ascertained by measuring the width of the clearance. The width of the clearance can then be used to obtain knowledge about the state of wear and about the servicing measures which are necessary on the brake To ascertain the width of the clearance, it is known practice to use optical or electrical sensors, but these tend to malfunction frequently on account of the build-up of dust and the evolution of heat in the region of the brake, or need to be protected from these influences with a corresponding level of complexity.

An object of the invention is to provide a method for operating a brake which allows the width of the brake's clearance to be ascertained easily and with little susceptibility to fault. It is likewise an object of the invention to provide an electronic controller for carrying out such a method.

According to the invention there is provided a method for operating an electromagnetically releasable brake which has at least one electrical coil and in which a brake body can be moved relative to the coil using a magnetic force which can be produced with the coil, wherein during an operating state in which there is a clearance between the coil and the brake body, and the brake body is intended to be moved in the direction of the coil, the coil has voltage continuously applied to it and the characteristic of the coil current is evaluated in an electronic control apparatus. In this operating state, the parking brake has been actuated and needs to be released using the magnetic force produced by the coil. To this end, a constant voltage is applied to the coil, and this results in a continuous rise in the current flowing through the coil, on account of the inductance of the coil. In this case, the magnetic force exerted on the brake body by the coil likewise rises continuously. Since the clearance influences the inductance of the coil, the current's time characteristic can be used to draw conclusions about the width of the clearance. The characteristic of the current is recorded and evaluated in an electronic control apparatus for the brake.

Particularly advantageous results are achieved if the value of the coil current flowing through the coil shortly before the instant at which the brake body comes fully into contact with the coil is recorded in the control apparatus. In this context, the term 'coil'includes the coil winding, a coil core and any housing components and insulating layers there may be, forming a unit with the coil winding. If the electronic control apparatus detects that the brake body has come into contact with the coil, the control apparatus records and stores the coil current present shortly before this event. Thus, the coil current present at an instant before the brake body has started its movement in the direction of the coil is recorded.

Preferably the control apparatus uses the recorded value of the coil current to ascertain the width of the clearance between the coil and the brake body. The coil current at the instant shortly before the brake body started its movement in the direction of the coil permits clear conclusions about the width of the clearance.

In this context, the width of the clearance can be ascertained using a linear relationship between the recorded coil current and the width of the clearance. This relationship can be stored in the control apparatus in the form of an algorithm or family of characteristics, for example.

It is particularly advantageous if the voltage across the coil and/or the temperature of the coil are not taken into account for ascertaining the width of the clearance. The force required to move the brake body is constant in each case; similarly, the magnetic force of the coil is directly dependent on the coil current, which means that the only unknown variable, the width of the clearance, can be derived from the recorded value of the current. Correction on the basis of the voltage or the temperature is therefore not necessary.

The control apparatus preferably detects that the brake body has come into contact with the coil on the basis of a sudden drop in the current flowing through the coil. When the brake body moves in the direction of the coil, the inductance of the coil rises sharply, so that the coil current decreases abruptly. Such an abrupt decrease in

the coil current is detected by the control apparatus, whereupon, as described above, the current flowing through the coil shortly before this event is recorded.

During automatic detection of movement of the brake body using the coil as described above, a sudden drop in the current flowing through the coil is assessed as meaning that the brake body has come into contact with the coil only if the drop reaches a prescribed level. This means that other events influencing the inductance, such as rattling of the brake body, are not assessed as meaning full contact being made. Rattling is denoted as meaning when not the entire brake body but only an edge of the brake body comes into contact with the coil.

The invention also provides an electronic controller for carrying out the method defined above.

Further advantages and particulars of the invention are explained in more detail with the aid of the exemplary embodiment shown in the schematic figures, in which :- Figure 1 shows the characteristic of the current during release of the brake, and Figure 2 shows the characteristic of the current when the brake has been released.

Figure 1 shows the characteristic of the current I during release of the brake.

From that instant onward, a constant voltage is applied to the coil. The current I flowing through the coil then rises continuously. From this instant onward, the characteristic of the current is measured continually in an electronic control apparatus using a sampling technique.

At the instant b, the coil current decreases in one step. The control apparatus detects from the size of the step that the brake body has not come fully into contact with the coil. Instead, only one edge of the brake body has touched the coil. The current value at the instant b is therefore not recorded in the control apparatus.

The current I then rises further until the brake body comes fully into contact with the coil at the instant c. The control apparatus detects this event from the pronounced abrupt decrease in the coil current. The control apparatus records the value 11 of the current before the step and uses this current value 1, to calculate the width of the brake's clearance.

The coil then continues to have voltage applied to it for a prescribed period of time in order to ensure that the full current level is reached. If the control apparatus has not reliably detected that the brake body has come fully into contact with the coil, the current is turned off with an error message.

In the normal case, after successful release of the brake, that is to say when the brake body is already on the coil, the coil current is reduced. This minimizes the evolution of heat in the coil and the current consumption of the brake.

The characteristic of the current when the brake has been released is shown in Figure 2. In this case, from the instant d onwards, the current flowing through the coil is controlled by means of pulse-width modulation and is continuously reduced up to the instant e. The coil current is then kept constant, and a sampling technique is used to measure the current, and possibly to correct the pulse width, during each pulse.

If, as at the instant f, a rise in the coil current exceeds a prescribed tolerance value, the control apparatus detects from this that the brake body has been unintentionally released from the coil. The control apparatus then attempts to reverse this unintentional release by re-invoking the coil energisation from the instant a. If this fails, the coil is automatically turned off and an error signal is produced.

Claims

1. Method for operating an electromagnetically releasable brake which has at least one electrical coil and in which a brake body can be moved relative to the coil using a magnetic force which can be produced with the coil, wherein during an operating state in which there IS a clearance between the coil and the brake body, and the brake body is intended to be moved in the direction of the coil, the coil has voltage continuously applied to it and the characteristic of the coil current is evaluated in an electronic control apparatus.

2. Method according to Claim 1, wherein the value of the coil current flowing through the coil shortly before the instant at which the brake body comes fully into contact with the coil is recorded in the control apparatus.

3. Method according to Claim 2, wherein the control apparatus uses the recorded value of the coil current to ascertain the width of the clearance between the coil and the brake body

4. Method according to Claim 3, wherein the width of the clearance is ascertained using a linear relationship between the recorded coil current and the width of the clearance.

5. Method according to Claim 3 or Claim 4, wherein the voltage across the coil and/or the temperature of the coil are not taken into account for ascertaining the width of the clearance.

6. Method according to any one of Claims 2 to 5, wherein the control apparatus detects that the brake body has come into contact with the coil on the basis of a sudden drop in the current flowing through the coil.

7. Method according to Claim 6, wherein a sudden drop in the current flowing through the coil is assessed as meaning that the brake body has come into contact with the coil only if the drop reaches a prescribed level.

8. Electronic controller for carrying out a method according to any one of Claims 1 to 7.

9. Method for operating an electromagnetically releasable brake substantially as hereinbefore described with reference to the accompanying drawings.

10. Electronic controller for operating an electromagnetically releasable brake substantially as hereinbefore described with reference to and as shown in the accompanying drawings.