BACKGROUND OF THE INVENTION
The invention relates to a method for controlling the operation
of a crane brake, the crane comprising an electric motor controlled by means of
a frequency converter for hoisting and lowering movements of the crane; an electromechanical
brake having an impact on said movements, the brake being opened when the motor
is started for a hoisting or lowering operation; a load sensor weighing the load
to be handled; and an overload protection connected to the sensor. Document
EP-A-0 841 298
discloses such a method.
Electromechanical brakes are currently delivered either
with no controls or provided with a micro switch arranged to measure the movement
of the brake anchor plate. When the brake is in operation this movement is extremely
small, typically 0.2-0.4 mm. The operating range of the switch is therefore very
small and its mounting and tuning is difficult. A control or monitoring system is
provided with a measuring circuit to ensure that when a load hoisting or lowering
movement begins, the brake opens, i.e. the micro switch is closed. If the switch
does not close within a predetermined time, the operation of the crane is stopped.
Systems delivered without any monitoring arrangements are
problematic if the brake control system malfunctions or if the brake for some reason
lags and does not open properly, in which case the brake heats up in only a few
seconds to the extent that the friction properties of the friction material collapse
and the brake is unable to hold the load.
FR 2 675 790
disclose brake control methods based on the above micro switch solutions
to indicate brake position. Such unreliable solutions have been used in shoe brakes
already for decades.
teaches a method for detecting a brake malfunction, in which method motor
current is gradually increased and measured with a speed sensor to indicate when
the motor starts to rotate. The differences in currents between brakes with and
without voltage are then compared. If the difference between the currents is too
small, the brake is detected to be faulty.
US Patent 4,733,148
discloses a method in which the motor is driven at a nominal torque, and
a speed feedback sensor reading will show whether the motor is running. This method
cannot be applied as such to cranes. If the hoisting member is provided with a nominal
load and the load is being brought downward at a nominal torque against the brake,
the brake is in fact required to slip.
US Patent 5,343,134
teaches a similar system in which the brake is monitored by checking the
rotating speed of the motor. Although the system works for cranes, it also requires
the speed of rotation of the motor to be known.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a
method that allows the above problems to be solved. This is achieved with a method
of the invention which is primarily characterized in that a frequency converter
is used for calculating the torque of a motor, which information is compared with
load information, or weight, obtained from a load sensor.
The invention is based on simply comparing the load weighing
information with the motor torque information whenever the crane is operated. If
the torque does not correspond to the information provided by the load sensor, there
are additional losses in the system, either in the brake or some other mechanical
structure. The crane can thus be halted before the brake warms up excessively.
The comparison according to the invention must be made
taking the drive or hoisting direction into account: when the load is taken upward,
the torque is scaled such that a nominal torque (100% torque) is required to hoist
a nominal load (100%), whereby mutually corresponding load information and torque
information are obtained within the entire load range. When the load is being lowered,
the motor functions as a generator, the torque with nominal load (100%) being (machinery
efficiency)2 x 100% torque (of a minus sign), which is -80 ... -90%,
depending on the efficiency.
If the brake drags, or if there is some other mechanical
friction, the torque needed by the motor increases when the load is being hoisted
and, correspondingly, the torque on the generator side decreases when the load is
The control unit is provided with settable limits in which
the torque information must be proportional to the load information, and with necessary
filtering elements for filtering the torque needed for accelerating flier masses,
although the torque can also be removed computationally on the basis of known acceleration
and deceleration times and flier masses of the machinery.
The control can be implemented for example as follows:
- 1. Hoisting or lowering is initiated at a low speed and the brake is opened.
- 2. The load information to be obtained from the load sensor is compared with
the torque information of the frequency converter, taking the driving direction
- 3. If the information correspond to each other, a frequency converter ramp is
released and a higher speed drive is allowed. If during the drive it is detected
that the information differ from one another, the operation is halted.
This control can be programmed into the frequency converter
or another programmable device. The most advantageous solution is naturally to program
the function into the frequency converter itself because then the control can be
implemented without any additional equipment.
A significant advantage of the invention is that systems
already existing in the crane can be used for making reliable conclusions about
the performance of the brake and another mechanical system, whereby additional sensors,
which are expensive and difficult to provide, can be avoided. In addition, increased
crane reliability is obtained compared with solutions based on micro switches because
the system also detects other than brake faults in the hoisting apparatus (such
as bearing damages, rope jamming, etc.)
LIST OF DRAWINGS
In the following, the invention will be described with
reference to an example of a preferred embodiment and the accompanying drawing,
which is a flow diagram of the method of the invention.
DETAILED DESCRIPTION OF THE INVENTION
With reference to the Figure, a crane hoisting apparatus
denoted with reference numeral 1 comprises an electric motor 2 serving as an operating
power source, the motor operating a lifting drum 4 through gearing 3, the drum being
provided with lifting ropes 5 spooled on the drum, and the ends of the ropes being
in turn provided with a lifting hook 6 for seizing a load 7. The hoisting apparatus
1 further comprises a load sensor 8 measuring the weight of the load 7 and an electromechanical
brake 9 for braking the motor 2 and, thereby, the lifting drum 4.
The motor 2, and thereby the hoisting and lowering movements
of the crane, are controlled by means of a frequency converter 10 to which the crane
operator issues commands on a direction information line 13.
For controlling the braking operation, there is provided
a control unit 12 which is placed in this case into the frequency converter 10 itself.
The control unit 12 receives hoisting direction information over the line 13. Reference
numeral 11 denotes a supply voltage line of the hoisting machinery.
Between the control unit 12 and the brake 9 there is connected
a brake controller 14 which either keeps the brake 9 entirely open or closed, depending
on the information received from the control unit 12.
When the crane is in operation, i.e. when the motor 2 is
running and the load 7 moves either upward or downward, the frequency converter
10 calculates the torque of the motor 2 and transmits the information to the control
unit 12. During the crane operation, the unit 12 continuously compares this computational
torque information with the weighing information produced by the load sensor 8.
If the load information and the torque information do not correspond to each other
within the set tolerances (with the above described hoisting direction taken into
account), the brake 9 is applied to stop the motor 2 which remains stopped for as
long as there is the difference between said information, i.e. a disturbance or
malfunction in the brakes.
The above specification is only meant to illustrate the
basic idea of the invention. A person skilled in the art may, however, implement
its details in various ways within the scope of the accompanying claims.