A processing unit coverts the output vector to an amount of voltage and current at a given frequency, and can direct the variable frequency drive to send a frequency to the motor at a frequency substantially equal to the frequency at which the motor is presently rotating. This creates a speed match that reduces ‘spikes’ in the motor during its operation, and substantially eliminates open-circuit decay. There is a hydraulic brake that operates in connection with the processing unit and the variable frequency drive to slow the crane without driving the motor into the brake. Thus, the inventor claims, the crane movement control is improved compared to previous systems.
Background
Fig 3 shows a conventional overhead crane including a horizontal ‘bridge’ (204) that moves the suspended load across the span (202) of the work area. A reversible, variable-speed, electric motor, coupled to conventional electromechanical transmission components, moves the bridge. The speed and direction of the motor is powered and controlled by a variable frequency motor drive (VFD).
Conventional manual controls are possible with the operator using a master switch to command motion, direction, torque/speed and acceleration. A foot-operated hydraulic brake, working similarly to a car brake, slows or stops movement of the crane independent of the motor control. The hydraulic pressure and braking torque is proportional to the amount of the force applied to the foot pedal. Since the operator controls braking in this case, the crane is essentially coasting when the operator moves the master switch to ‘neutral’.
Conventional braking is also available by electrical means including a ramped deceleration time associated with the variable frequency drive. According to the inventor, most traditional crane controls use techniques and systems that are not efficient, and also cause abnormally large amounts of destructive forces on the motor and drive systems.
Fig 4 is a chart of an extreme example of this effect with conventional current and voltage spikes associated with supplying a reversed polarity voltage and current to a motor traveling in a given direction (‘reverse plugging’).
It is also claimed that conventional controls do not facilitate adjustments to the crane speed and can be detrimental even when the user wants a speed adjustment in the same direction as travel.
As a changed voltage and frequency is applied to the coasting motor with the control in ‘neutral’, the motor is re-energized but is not synchronous with the line voltage and frequency being applied, thus the residual voltage and frequency is not equal to the desired values. This can cause sizeable current transients, vibrations and significant wear to the motor, controls and machinery with time, the effects of which can be more than tripled when the controls are reversed plugged (Fig 4).
Thus, says the inventors, there is a need for an overhead crane bridge control system that effectively and efficiently controls the velocity and direction of the crane without undue wear to the motor, controls and machinery.
System assembly
The system for overhead crane control shown in Fig 1 (10) controls a velocity vector (speed and direction) (Fig 3 – 12). The system includes a motor (14), a variable frequency drive (VFD) (16) and a processing unit (20). The motor is coupled (by conventional transmission components if required) and positioned to move the crane and generates an output vector (22) of rotational directional and rotation speed. It also provides speed and direction feedback to the sensor (26). The VFD, and thus the motor, is connected to a power source that can also supply power for other components. The VFD provides operating power and voltage to the motor.
Preferably the VFD and processing unit are contained within the motor drive (AC) to facilitate control over the motor. The motor drive collects data from the control inputs (31) and from the sensor (26) for control purposes, and can receive power from the power source.
The processing unit is operatively connected to the motor and VFD, and converts the output vector to amounts of voltage, current and a frequency. It can also instruct the VFD to transfer a level of voltage, current and a frequency from the VFD to the motor. This maintains the frequency, and in some cases, the amounts of voltage and current in the motor as read by the processing unit. The system needs to know the frequency to apply the voltage to the motor, and provides the active frequency in the motor from the VFD substantially equal to the frequency within the motor.
The traverse direction of the velocity vector can be in either direction along the bridge span, with the speed limited to predetermined limit values within the processing unit.
The sensor is preferably a shaft encoder connected to the motor and VFD. It provides an electronic signal (vector signal 48) to the processing unit with information about the rotational speed and direction of the motor shaft.
The master switch (28) is connected to the processing unit to regulate the crane velocity vector. The positioning of the switch determines the levels of voltage and current generated in the VFD for transfer to the motor. The degree of rotation of the handle (30) can directly correlate the amount of power sent to the motor, and the torque generated, to determine the acceleration and speed of the crane. Other embodiments of the control switch (28) are possible.
A brake (32) is operatively connected to the crane, motor, processing unit and VFD to regulate the velocity vector of the crane. It is preferably a manual, hydraulic, foot brake with a pedal (34) and brake switch sensor (36). The latter determines when the brake has been activated and then sends an electronic signal to the processing unit.
Once the crane is in motion the control switch can be moved to ‘neutral’ to allow the crane to coast until one or all of the following occur: mechanical friction and wind resistance will stop motion, the operator applies the brake, or the operator activates the control switch to apply torque in the opposite direction of motion (reverse plugging).
Procedure
Fig 2 is a flow diagram of the basic logic software of the processing unit. Input variables are preferably made every five milliseconds and come from the motor shaft the control switch and the brake sensor. The processing unit first checks for any velocity vector on the motor and if there are any commands from the input devices. If not, the internal run command of the processing unit is set at the lowest level, and that the sequence of the process is over.
The processing unit then analyses the control inputs separately. If the user is trying to change the crane speed using the control switch, the input is converted to an amount of torque and a run command timer is initiated. The processing unit’s internal parameters are set at a high command indicating imminent manipulation of operating speed. The timer holds the ‘run’ command in an ‘on’ state to keep the motor magnetised whilst the crane may be coasting.
The processing unit then checks the brake for engagement. If it is not, it sets the VFD voltage level sent to the motor to match the existing direction and speed of the crane, for example, the frequency of the rotating motor. On activation of the brake, the processing unit receives input from the control switch to determine the desired direction of movement. If,on braking, the crane is moving in the same direction as the input from the control switch on braking, the processing unit reads this combined input as a desire by the operator to retard the crane. Therefore the processing unit reduces the input from the VFD to the motor so that the torque within the motor is zero. However, if the wish is to move the crane in the opposite direction to actual movement, the processing unit reads this as an attempt to quickly decelerate, or reverse plug, the crane.
In either case the processing unit sends the levels of current, voltage and the corresponding torque, at a given coasting frequency, to the motor. These levels are sent from the VFD to the motor to change the velocity vector of the crane, facilitating brake stand prevention.
ï„·This article is an edited version of the original patent and may omit legally or technically important text.
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The patent details various methods using the above equipment to achieve a smooth change of crane motor speed, and direction if required, to minimise damage to the components. In the preferred embodiment, such as when the crane user decides to change the speed of the crane, the voltage frequency sent to the motor corresponds continuously to the frequency within the motor. This procedure also matches the two torque levels, and facilitates the smooth transfer of power and controls for the direction of movement and velocity of the crane. The procedure reduces the wear and tear on the motor, drive train and crane.
Other methods within the patent include:
– Control of power levels from the VFD
– Brake application to vary the velocity of the crane
– Keeping the rotor of the motor magnetised during crane operation to prevent open circuit voltage decay. The VFD will only transfer enough voltage to keep the motor magnetised when the crane is coasting or the operator provides additional input
– Prevention of a motor driving into a brake when the brake is applied by setting the torque input at zero.
About the patent
This article is an edited version of US Patent 7,190,146, published March 13 2007 from an application filed August 18 2003. The inventor is Aaron S Kureck of Nashotah, Wisconsin. The patent is assigned to Magnetek, Inc., Milwaukee, WisconsinWisconsin, which employs Mr Kureck as Controls Product Manager. He has had three other patents issued including US 06598859 – Multiple hoist synchronisation apparatus and method and US 06653804 – Method and apparatus for controlling a bucket hoist using a flux vector AC drive.
Disclaimer
This article is an edited version of the patent and may omit legally or technically important text. To see the full patent go to www.hoistmagazine.com/patents
Marketing
Fig 4 – A graph of conventional current and voltage spikes associated with supplying a reversed polarity voltage and current to a motor traveling in a given direction Fig 4 Fig 3 – Perspective of an overhead bridge crane Fig 3 Fig 1 – Schematic representation of one embodiment of a system for controlling an overhead crane Fig 1 Fig 2 – A logic flow cart diagram characterizing the functionality of one embodiment of the software associated with the processing unit of the invention Fig 2
