GB2360335A

GB2360335A - A method for operating a brake which has an electromagnetic release function

Info

Publication number: GB2360335A
Application number: GB0006569A
Authority: GB
Inventors: Timothy John Martin
Original assignee: Lansing Linde Ltd
Current assignee: Linde Material Handling UK Ltd
Priority date: 2000-03-17
Filing date: 2000-03-17
Publication date: 2001-09-19
Anticipated expiration: 2020-03-17
Also published as: GB2396672B; DE10112902B4; GB0006569D0; GB0405894D0; GB2360335B; GB2396672A; DE10112902A1

Abstract

A parking brake is held in a release position by an electromagnet having at least one coil, against the force of mechanical actuation means. During an operating state in which there is a clearance between the coil 1 and the brake body (ie. brake 'on') the coil is supplied with voltage pulses (so as not to energise the coil sufficiently to overcome the mechanical actuating force) and the characteristic of the coil current (ie. inductance) is evaluated in an electronic controller 4, so as to signal movement of the body towards the coil. Alternatively or additionally, during an operating state in which there is no clearance between the coil and the brake body (ie. brake off), the coil is continuously supplied with voltage and the characteristic of the coil current evaluated, to thereby signal movement of the body away from the coil (ie. brake actuation or malfunction of the electromagnetic brake release function).

Description

2360335 Method for operating a brake which has an electromaQnet and

electronic controller therefor This invention relates to a method for operating a brake which has an electromagnet and in which a brake body can move relative to at least one coil of an electromagnet.

The invention also relates to an electronic controller for carrying out the method for operating the brake.

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 geneially 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 a 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.

2 - During normal operation of the industrial truck, in which the brake is not actuated, the brake body needs to be held against the force of the energy storage mechanism in a position in which the brake pad is released from the brake rotor. For this, a brake lifter is provided, which is frequently in the form of an electromagnet. The force which can be exerted on the brake body using the electromagnet varies greatly depending on the clearance between the electromagnet and the brake body. In this respect, the electromagnet can be proportioned to be so small that, although the force which can be exerted on the brake body using the electromagnet is sufficient to hold the brake body when the brake pad is already released from is the brake rotor, the force of the electromagnet is not sufficient to move the brake pad, when it is against the brake rotor, against the force of the energy storage mechanism. The parking brake must then be released by an external force which can be produced using the actuator for the service brake, for example. Such a brake is described in GB 2 333 335 A, for example.

In order to be able to recognize the operating state or malfunctions of the brake at the present time, and subsequently to be able to actuate the brake itself and other units of the vehicle correctly, it is necessary to detect the position of the brake body using a suitable device. Customarily used sensors or microswitches are found to be unreliable in practice because they are not sufficiently shielded against the environmental influences arising in the region of the brake, such as vibrations, electromagnetic radiation or abrasive dust.

An object of the invention is to provide a method which allows the operating state of the brake to be 3 recognized reliably.

According to the invention, during an operating state in which there is a clearance between the coil and the brake body, the coil is supplied with voltage pulses and the characteristic of the coil current is evaluated in an electronic controller, and/or, during an operating state in which there is no clearance between the coil and the brake body, the coil is continuously supplied with voltage and the characteristic of the coil current is evaluated in an electronic controller.

In the first operating state mentioned, in which there is a clearance between the coil and the brake body, the brake is actuated so that its function is that of a parking brake. In this operating state, no effective force is intended to be exerted on the brake body by the electromagnet. However, the electronic controller, which is a component part of the brake, is intended to recognize immediately when the brake body is moved toward the coil of the electromagnet by an external force and the clearance between the coil and the brake body is reduced in the process. The width of the clearance affects the inductance of the coil. The current characteristic through the coil during the voltage pulses applied in accordance with the invention is affected by the inductance. This current characteristic is measured and evaluated using a suitable circuit in the controller. The pulse duration and the time between the voltage pulses are chosen to be such that the magnetic force exerted on the brake body is negligibly small.

In the second operating state mentioned, there is no clearance between the coil and the brake body, and the coil is continuously supplied with voltage. In this case, the brake body is held by the magnetic force produced by means of the coil, and the parking brake function is not actuated. For this, the coil is continuously supplied with voltage, so that the magnetic force exerted on the brake body holds the brake body. During this operating state, the controller is intended to recognize whether the brake body is unintentionally released from the electromagnet. In this case, too, the characteristic of the current flowing through the coil is measured and evaluated in the electronic controller.

During the first operating state mentioned, the electronic controller preferably determines the gradient of the coil current over time. In contrast to the absolute current level, the gradient of the coil current, that is to say the slope of the current curve, is barely dependent on external conditions such as the temperature of the coil.

It is advantageous if, in this case, the electronic controller compares the gradient of the coil current during the present voltage pulse with the gradient of the coil current during at least one of the preceding voltage pulses. By comparing the gradients, it is an easy matter to be able to recognize a change in the inductance of the coil. To rule out assessment errors during the movement of the brake body, the present gradient can be compared with an average value or with a maximum value for the gradients of a particular number of past voltage pulses.

It is particularly advantageous if the electronic controller divides the pulse duration (t2) of each voltage pulse into a plurality of measurement ranges (sampling points) (tl) and determines the average gradient of the coil current over a predetermined number of measurement ranges. By dividing into measurement ranges, a plurality of measurements can be determined for the voltage over the duration of a pulse, and the gradient of the coil current can be calculated from this.

The electronic controller compares the average gradient of the present voltage pulse with the maximum average gradient of at least two preceding voltage pulses. Since the comparison is carried out using a plurality of preceding voltage pulses, measurements taken during a movement of the brake body are not taken into account.

After the end of the present voltage pulse, the electronic controller continues to supply the coil with voltage if the comparison between the gradient of the coil current during the present voltage pulse and the gradient of the coil current during at least one of the preceding voltage pulses produces a particular result. When the controller detects the particular comparison result, which is stored in the controller, the flow of current through the coil is maintained, so that the coil produces a holding force which acts on the brake body. The particular result signals to the controller that the brake body has been moved toward the electromagnet by an external force.

The particular result is defined in that the average gradient of the present voltage pulse is lower by at least a predetermined percentage than the maximum average gradient of at least two preceding voltage pulses.

The voltage pulses each have the same pulse duration. Similarly, the measurement ranges each have 6 the same duration. This produces reciprocally comparable measurements.

In the second operating state mentioned, in which there is no clearance between the coil and the brake body, the electronic controller determines average values of the coil current during predetermined time periods. In this operating state, in which the brake body is held by the magnetic force of the coil, the coil current is normally constant. Movement of the brake body causes a change in the inductance of the coil, which intermittently affects the coil current. Such a change can be detected by forming average values for the current level during predetermined time periods.

The electronic controller compares the average value of the coil current during the present time period with the average value of the coil current during at least one of the preceding time periods. The comparative measurement is carried out because it is not possible to evaluate absolute values for the coil current owing to the temperature dependence of the inductance of the coil.

After the end of the present time period, the electronic controller interrupts the voltage supply to the coil if the comparison of the characteristic of the coil current during the present time period with the characteristic of the coil current during at least one of the preceding time periods produces a particular result. The particular result, defined in the controller, signals an operating state in which the brake is intended to be actuated by the coil current being turned off.

The particular result is defined in that the average of the coil current during the present time period is higher by at least a predetermined percentage than the average of the coil current during at least one of the preceding time periods. Such a rise in the coil current signals that the brake body has become released from the electromagnet, despite the coil being supplied with voltage. The electronic controller then actuates the brake by way of a safety function, by turning off the coil current.

To obtain comparable measurement results easily, the time periods preferably each have the same duration.

Further advantages and details of the invention are explained in more detail using the illustrative embodiment shown in the schematic figures, which show, by way of example, the following:

Figure 1 is a schematic view of the brake control system; Figure 2 shows the coil reactance varying with the temperature; Figure 3 shows a current pulse shown in detail; Figure 4 shows current pulses to coil in the plate detection phase; Figure 5 shows current rise to maximum once the plate comes into contact with the coil; Figure 6 shows drops in current illustrating that the plate is brought harder to the coil; and Figure 7 shows current surge indicating that the plate has come off the coil.

Figure 1 shows the switching and sensing electronics which is a standard current sensing circuit. The present invention is a technique that can detect whether a brake body, e.g. an armature plate, is covering the face of an electromagnet. The invention describes a method of detecting when the plates are on the coils by using their reactive characteristics, since the inductance increases significantly when the plates are on the coils. This change in inductance can be measured by sensing the current through the coils.

The current through the coils 1 is represented by a voltage across the current sensing resistor 2. This appears at the A-D converter input 3 on the controller 4. A controller output 6 is connected to the gate of a field effect transistor (FET) 5, which controls the current supplying the coils 1. The current can only be monitored whilst the FET 5 is switched on since the A-D converter input 3 is shorted to OV through the current sensing resistor when it is switched off. This means that the current decay does not appear at the A-D converter input.

Another input 6 of the controller 4 is connected to a park brake switch, whereas another output 7 is connected to a LED indicating the operating status of the parking brake. A further output 8 of the controller is used for error reporting.

Figure 2 shows how an arbitrary value of coil reactance changes with temperature of a period of about 1 hour for plate on and off the coil. Since the reactance of a coil varies with temperature, as well as there being a tolerance between coils, the technique according to the invention measures the change in characteristics of the coil rather than comparing it with a fixed value. Since the inductance of the coil increases when the plate is in full contact, the coil's response to a step increase in voltage can be measured.

9 - The brake mechanism can be mounted directly on top of an electric motor and therefore is in an extremely electrically noisy environment. This noise will be picked up and will appear on the A-D converter input 3. A sampling technique must calculate an average value over a range of samples in order to smooth out the noise, rather than look at single point samples.

The technique consists of two phases - the plate detection phase and the plate holding phase. The plate detection phase detects when the plate has come firmly in contact with the coil 1 and allows the current to rise to maximum when it does so. The plate holding phase detects when the plate becomes dislodged from the coil 1.

Figure 3 shows in detail one of the short pulses of current which are applied to the coil during the plate detection phase. During each of these pulses with the pulse duration t2 a number of measuring ranges t, defined. The gradient of current is calculated over each pulse duration t2 by summing the current differences between the measuring ranges tl.

Figure 4 shows the pulses of current applied to the coil phase before the plate comes into contact with the coil. The time interval between the beginning two pulses is indicated as T.

As the plate nears the coil 1, the gradient decreases. When the plate comes into full contact with the coil 1, there is a 50-70% reduction in gradient. In this particular arrangement the reduction in gradient is about 60%. The gradient of each new pulse is compared to a reference gradient (the maximum gradient of the previous pulses) and the percentage difference is calculated. Since the technique always takes the highest gradient as a comparison value ignoring gradual changes until the 60% decrease indicating that the plate is on the coil - means that plate 'clatter' on the coil does not falsely indicate that the plate is on.

When the pulse gradient becomes less than 40% of the reference gradient, the software recognises that the plate is on the coil and the FET5 is left switched on, allowing the current to rise to maximum. This is shown in Figure 5. When the current reaches its maximum value, the gradient becomes zero and the software moves to the plate holding phase.

While the FET5 is on, the current is sampled constantly and gradient and average are calculated at the end of every time period of the duration t2. An increase in gradient of greater than 5% while the current is rising to maximum indicates that the plate has come off the coil 1. The software now calculates the average of the current by integration of the current over each t2 period (measuring range). Each new average calculation is compared to a reference average (which is updated only for ≤2% decreases in average to allow for gradual heating effects) and the percentage difference is calculated.

Figure 6 shows how the current can change if the plate is brought harder on to the coil 1. This is an allowable event and may occur when particles between the plate and the coil 1 becomes dislodged. This current change is due to the sudden step increase in inductance of the coil 1.

When the plate comes off the coil 1. the inductance suddenly drops. Since the coil is now storing more energy than this new inductance will allow, the energy is released in the form of a current surge. An increase in average current of greater than 10% indicates that the plate has come off the coil.

This is shown in Figure 7.

When the plate comes off the coil 1 unexpectedly, the software indicates this and switches off the current to the coil 1 in order to apply the park brake. When going back to the plate detection phase, there must be a time interval of T between switching off the current and the beginning of the first current pulse to allow the current in the coil 1 to decay completely.

12 -

Claims

CLAIMS:

1. Method for operating a brake which has an electromagnet and in which a brake body can move relative to at least one coil of an electromagnet, wherein, during an operating state in which there is a clearance between the coil and the brake body, the coil is supplied with voltage pulses and the characteristic of the coil current is evaluated in an electronic controller, and/or, during an operating state in which there is no clearance between the coil and the brake body, the coil is continuously supplied with voltage and the characteristic of the coil current is evaluated in an electronic controller.

2. Method according to Claim 1, wherein, during the operating state in which there is a clearance between the coil and the brake body, the gradient of the coil current over time is determined in the electronic controller.

3. Method according to Claim 1 or Claim 2, wherein, during the operating state in which there is a clearance between the coil and the brake body, the electronic controller compares the gradient of the coil current during the present voltage pulse with the gradient of the coil current during at least one of the preceding voltage pulses.

4. Method according to any one of the preceding claims, wherein, during the operating state in which there is a clearance between the coil and the brake body, the electronic controller divides the pulse duration of each voltage pulse into a plurality of measurement ranges (sampling points) and determines j the average gradient of the coil current over a predetermined number of measurement ranges.

5. Method according to any one of the preceding claims, wherein, during the operating state in which there is a clearance between the coil and the brake body, the electronic controller compares the average gradient of the present voltage pulse with the maximum average gradient of at least two preceding voltage pulses.

6. Method according to any one of the preceding claims, wherein, during the operating state in which there is a clearance between the coil and the brake body, the electronic controller continues to supply the coil, after the end of the present voltage pulse, with voltage if the comparison between the gradient of the coil current during the present voltage pulse and the gradient of the coil current during at least one of the preceding voltage pulses produces a particular result.

7. Method according to Claim 6, wherein, during an operating state in which there is a clearance between the coil and the brake body, the particular result is defined in that the average gradient of the present voltage pulse is lower by at least a predetermined percentage than the maximum average gradient of at least two preceding voltage pulses.

8. Method according to any one of the preceding claims, wherein, during the operating state in which there is a clearance between the coil and the brake body, the voltage pulses each have the same pulse duration.

9. Method according to any one of the preceding claims, wherein, during the operating state in which there is a clearance between the coil and the brake body, the measurement ranges each have the same duration. 5

10. Method according to any one of the preceding claims, wherein, during the operating state in which there is no clearance between the coil and the brake body, average values of the coil current during predetermined time periods are determined in the electronic controller.

11. Method according to any one of the preceding claims, wherein, during the operating state in which there is no clearance between the coil and the brake body, the electronic controller compares the average value of the coil current during the present time period with the average value of the coil current during at least one of the preceding time periods.

12. Method according to any one of the preceding claims, wherein, during the operating state in which there is no clearance between the coil and the brake body, the electronic controller interrupts the voltage supply to the coil, after the end of the present time period, if the comparison of the characteristic of the coil current during the present time period with the characteristic of the coil current during at least one of the preceding time periods produces a particular result.

13. Method according to Claim 12, wherein, during the operating state in which there is no clearance between the coil and the brake body, the particular result is defined in that the average of the coil current during the present time period is higher by at least a predetermined percentage than the average of the coil current during at least one of the preceding time periods.

14. Method according to any one of the preceding claims, wherein, during the operating state in which there is no clearance between the coil and the brake body, the time periods each have the same duration.

15. Electronic controller for operating a brake using a method according to any one of the preceding claims.

16. Method for operating a brake which has an electromagnet substantially as hereinbefore described with reference to the accompanying drawings. 15

17. Electronic controller for operating a brake substantially as hereinbefore described with reference to and as shown in the accompanying drawings.