Electricity: motive power systems – Open-loop stepping motor control systems
Reexamination Certificate
2001-10-31
2003-12-30
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Open-loop stepping motor control systems
C318S700000, C318S727000
Reexamination Certificate
active
06670783
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the efficient operation of variable speed stepper-type electric motors, and more particularly to a method for providing improved control of an electric stepper motor by varying the amount of power supplied to the motor at different speeds in relation to the amount of power required by the motor according to a pre-configured table, without relying on feedback from the motor itself.
2. Description of the Prior Art
Variable speed electric motors, particularly stepper motors, are employed in a wide variety of applications where precise movements are desired. A typical application of a variable speed motor is in a closed circuit television (CCTV) system where a surveillance camera unit is mounted on a movable base. Movement is imparted to the base (and hence to the camera) using one or more variable speed electric motors that cause the camera to scan, pan and/or tilt. It is common to use stepper motors in these applications because such motors are capable of providing the precise movements required by surveillance cameras.
A stepper motor's shaft has permanent magnets attached to it, together these are called the rotor. Around the body of the motor is a series of coils (windings) that create a magnetic field that interacts with the permanent magnets. When these coils are turned on and off the magnetic field causes the rotor to move. As the coils are turned on and off in a certain sequence the motor will rotate forward or reverse. This is called the phase pattern and there are several types that will cause the motor to turn.
To make a stepper motor rotate, the coils must be constantly turned on. If one coil of the motor is energized, the rotor will jump to that position and stay there resisting change. This energized coil pulls full current even though the motor is not turning. This ability to stay put at one position rigidly is an advantage of stepper motors. The torque at stand still is called the holding torque. Because stepper motors can be controlled by turning on and off coils, they are easy to control using digital computers. The computer simply energizes the coils in a certain pattern and the motor moves accordingly. At any given time the computer will know the position of the motor since the number of steps given can be stored.
When other than full power is applied to the coils, the rotor will move to intermediate positions. This is referred to as “micro-stepping” the motor. A common type of signal used for micro-stepping is a pair of orthogonally related pseudo-sine waves. Different actual pseudo-sine waves may be generated. Stepper motors are limited in the amount of torque they can produce in comparison with DC brush motor (this is the other common type of DC motor). Stepper motors are typically rated with a holding torque which is the amount of torque they can hold without slipping with the coils energized at the rated voltage. The holding torque is not the amount of torque they can actually turn. Stepper turning torque is a fraction of the rated holding torque.
Unlike brush motors whose torque increases with speed, steppers have more torque at lower speeds. Stepper motors also have a much lower maximum speed than a brush motor.
The stepper motor coils are typically rated for a particular voltage. The coils act as inductors when voltage is supplied to them. As such, the coils do not instantly draw their full current, and in fact may never reach full current at high stepping frequencies. The electromagnetic field produced by the coils is directly related to the amount of current they draw. The larger the electromagnetic field the more torque the motors have the potential of producing. The solution to increasing the torque is to ensure that the coils reach full current during each step. This is accomplished by increasing the voltage the coil is excited with while never exceeding the manufacturer's current rating. To accomplish this, some kind of current limiting scheme is necessary. A series resistor between the higher than rated power supply and the coil is common. (As current increases so will the voltage drop across the resistor and therefore limit the voltage across the coils and protect them from damage.)
Some level of power must be supplied to a stepper motor at all times, even when the rotor is in a stationary position. The direction of rotation of the motor rotor is determined by variably switching the amount and polarity of the power supplied across the motor windings. The speed (frequency) of rotation of rotor is determined in part by the amount of power delivered, but primarily by how quickly the supply of power to the motor coils is changed. The amount of power supplied may be varied by varying the amount of the voltage or by varying the amount of time that a fixed voltage is provided. The latter procedure is known as pulse width modulation (PWM).
A typical stepper motor has a given inductance based on the number of motor windings, with more/longer windings providing additional inductance. Additional windings are preferred because they allow for smoother operation and more strength, or torque, in the motor. Thus, a typical strong stepper motor will have a higher inductance than a weaker motor for equal amounts of current.
As an electric motor is operated, its speed (RPM or frequency) may be increased according to the needs of the application. The usable speed of a stepper motor is determined by available torque which is directly proportional to current driven through the motor. The current in the motor is limited by the resistance of the windings, the inductance of the windings, and the back EMF (electromotive force) of the motor. As the frequency of the applied power to the motor increases, so also does the reactance in the motor circuit, the reactance being a function of the inductance (i.e. size and number of windings) and the resistance of the motor itself. Thus, at higher speeds additional power must be supplied for proper operation (i.e. sufficient torque) in order to compensate for the increased reactance and back EMF. At lower speeds, there is less reactance, and less back EMF such that less power is required.
If too much power is supplied when back EMF is low (i.e. when the motor is moving at low speeds or is stationary), the motor will draw excessive current which may damage or shorten the life of the motor components. If the applied power is not increased when back EMF increases (i.e., as speed increases), there will be insufficient current flow, and the motor will not have enough power available to operate correctly (i.e., it will run under-power with diminished strength or torque).
Even when not moving, a typical stepper motor will continue to require a small amount of power in order to maintain its current position In surveillance camera applications, the motors operating the camera may be brought to a stationary position and left there for long periods of time (days, weeks or even months). Providing full power over such long periods of time to a stationary motor is wasteful and is likely to burn out motor components. It is estimated that on average, the amount of power required by a stepper motor in a CCTV application is between ¼ and ⅓ of the maximum power available. It is therefore desirable to limit the amount of power supplied to the motors operating a surveillance camera in relation to the amount of power required by the motors whether moving or not.
Many stepper motor manufacturers recommend that a particular current be provided to the motor for proper performance. It is typical for the current supplied to a stepper motor to be changed in accordance with the demands of the motor. However, in many surveillance camera applications as well as other applications, power is only available at one voltage level making it impossible to vary the level of current actually supplied to the motor. In some situations, the power supply may not be consistent resulting in variations in the available voltage.
In surveillance camera system
Hamilton William Eric
Waehner Glenn
Miller Mark D.
Miller Patrick
Nappi Robert E.
Pelco
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