Electricity: motive power systems – Braking
Reexamination Certificate
2000-07-11
2002-04-16
Donels, Jeffrey (Department: 2837)
Electricity: motive power systems
Braking
C318S756000, C388S932000
Reexamination Certificate
active
06373207
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a braking system for braking an electric DC motor.
BACKGROUND OF THE INVENTION
Industrial machines that use moving parts are often provided with DC motors to impart movement to those parts. The DC motors are well suited for these applications because their torque and speed of rotation can be easily controlled. In turn, this enables to precisely control the rate of movement and the positioning of the moving part.
Once a moving part has been set in motion by a DC motor, that is usually effected through any suitable transmission system, the part must at some point be immobilised. This is usually the case of actuators designed to pick-up a component and deposit the component in a precise location. In fact, in many applications the ability to terminate the movement of the part repeatably is an important design consideration that determines the overall performance of the machine.
Several possible approaches to terminate the movement of a part or component driven by a DC motor exist. One approach relies on mechanical braking systems that dissipate the kinetic energy through friction between two contact surfaces. This strategy enables to very quickly decelerate the moving part, however, the braking system that can accomplish this task is complex and often not practical. Another approach is to control the DC motor so the latter decelerates the moving part through dynamic braking. In essence, when the movement of the part is to be terminated and power to the motor armature has been removed, a load is placed across the terminals of the DC motor. The former then ceases to act as a driving source and becomes a generator driven by the inertia of the part in motion. The electric current generated by the motor is dissipated in the load in the form of heat. This approach is widely used because the control mechanism for switching the motor from the driving mode to the braking mode is easy to implement. Indeed, it suffices to provide a suitable mechanism that will switch across the terminals of the motor, when braking is desired, a load to dissipate the energy.
To obtain a high rate of energy dissipation that is necessary to quickly stop the motor, the load across the terminals of the motor should be as high as possible. A high load (low impedance) is likely to generate high current though the winding of the DC motor that in turn can damage the permanent magnets of the motor because such high current can exceed demagnetisation levels. In practice, manufacturers of DC motors specify a peak current value that the motor can sustain without causing any damage to the magnets of the motor. Any braking system using this motor is thus designed not to exceed the peak current value, otherwise the motor can be damaged.
When a DC motor is operating in the dynamic braking mode, the current generated by the motor that is passing through the load reaches its peak value as soon as the braking mode is entered because at this point the rotary speed of the shaft is highest. As soon as the braking takes effect, however, the current progressively diminishes. The same relationship holds true with the rate of energy dissipation. The bulk of the energy is dissipated at the front end of the dynamic braking cycle, while less energy is dissipated near the end of the cycle. This observation highlights a fundamental deficiency in existing dynamic braking systems, where the average rate of energy dissipation during the braking cycle is relatively low, and cannot be increased to avoid exceeding the peak current limitation that occurs only during a small fraction of the cycle.
SUMMARY OF THE INVENTION
Under a first broad aspect, the present invention provides a braking system for a DC motor that is characterized by a maximum braking current value and that features first and second terminals. The braking system comprises a power supply that is capable of drawing electrical energy from the two terminals when the braking system is connected across these same terminals. A current control element receives electrical power for operation from the power supply. This current control element is capable of regulating the magnitude of a current passing through the windings of the DC motor when the braking system is connected across the first and second terminals, such that an average current passing through the windings of the DC motor during a braking cycle of the DC motor tracks the maximum braking current value over a major portion of the braking cycle.
The present inventor has made the unexpected discovery that the repeatability of movement of a DC motor can be significantly improved by dynamically braking the motor aggressively in order to reduce the unpowered coasting stroke. The unpowered coasting stroke has been found to be a source of repeatability error because of the duration of the coasting motion. During a long coasting motion, the impact of uncontrollable system variances such as friction shaft/bearings, among others, reduces repeatability. By reducing the time the motor spends in the coasting mode, the impact of those system variances is reduced. Consequently, the repeatability of movement is improved.
In a preferred embodiment, the braking system in accordance with the invention includes a current control element connected between the terminals of the DC motor to control the magnitude of the current passing in the motor windings during the braking mode of operation of the motor. The control strategy that the current control element implements is such as to allow the rate of energy dissipation during the dynamic braking cycle to be significantly increased by comparison to prior art devices, while preventing the system from exceeding the peak current value established for the motor.
In a most preferred embodiment, the system includes a current control element that can selectively acquire different operative states, namely a first operative state and a second operative state, in the first operative state the current control element manifesting a substantially lower impedance to the passage of current through the windings of the DC motor than in the second operative state. In other words, the operative states correspond to different levels of conduction; the current control element when in the second operative state allowing less current (or no current at all) to pass through the windings than in the first operative state. In a specific example, the current control element includes a semiconductor switch connected across the terminals of the DC motor. The semiconductor switch can be an N-channel MOSFET transistor which can acquire either one of the open, non-conducting condition (no current passing through it) and the closed, conducting condition (acting as a short circuit). The transistor can switch between the closed condition and the open condition, in the closed condition current being allowed to pass through it and also through the windings of the DC motor that act as energy dissipation elements by virtue of their inherent resistance.
In order to initiate the braking event, the DC motor is switched from the running mode, during which power is supplied to the motor windings, to the braking mode, during which the current control element is placed in series with the motor windings. The power supply module of the current control element quickly absorbs, filters and regulates some of the power generated by the motor, for supplying the braking circuit with sufficient power to energize the transistor with pulse width modulation (PWM) based oscillations. These oscillations remain in effect until almost the very end of the braking cycle. More specifically, throughout the braking cycle, the transistor will pulse on and off, according to a variable duty cycle. As the motor is braked, it is this duty cycle that will vary accordingly such that the average current flowing through the motor windings is maintained for as long as possible at a maximum allowable braking current value.
A control signal is provided to actuate the transistor. This control signal can be obtained from any
Clark & Elbing LLP
Donels Jeffrey
Duda Rina I.
Kalish Inc.
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