Electricity: motive power systems – Open-loop stepping motor control systems
Reexamination Certificate
1999-01-29
2001-09-04
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Open-loop stepping motor control systems
C318S685000
Reexamination Certificate
active
06285156
ABSTRACT:
The invention relates to a process for the sensorless step recognition in connection with stepping motors by analyzing the current flowing through the motor.
Various possibilities are known according to the state of the art for determining the position of systems driven by stepping motors. For example, the position of the driven structural element can be directly detected by means of absolute-value position sensors. Even though this version is not limited to stepping motors it is eliminated in most cases because of high expenditure in terms of materials and costs.
Owing to the property of stepping motors of performing discrete steps at the time, i.e., angular increments, the position can be determined in a simple manner by monitoring, i.e., counting by means of an electronic counter the number of steps performed from a spatially defined starting point. For such position recognition it is necessary to substantially realize the following preconditions: On the one hand, the actual starting point, which, as a rule, is a final stop, has to be recognized as the starting point of the movement; and it is necessary, on the other hand, to continually monitor whether a step has in fact been performed with each current flowing through the winding of the stepping motor.
Said two preconditions can be controlled with external sensors, for example limit switches and coding disks. This, however, requires expenditure, and it is cost-intensive. Sensorless processes for step recognition are known as alternative methods, by which the feed current or the feed voltage is analyzed as current is flowing through the motor windings.
Such a method and a device for recognizing the movement of a multi-phase stepping motor are known, for example from DE 40 35 970 A1, where the voltages of two phases, i.e., the voltages applied to the motor windings are evaluated by means of a micro-computer. In detail, voltage drops occur in case of blocking in connection with the phase voltages, which have to be registered. This method is said to be insensitive to high-frequency interferences; however, it has a number of other drawbacks because recognition of the movement is possible only on multi-phase, unipolar stepping motors, but not on bipolar stepping motors. Furthermore, the voltage breaks are measurable only if the current source is a high ohmic source. Therefore, shunt resistors have to be employed, as a rule. This, of course, conditions a substantial expenditure in terms of structural elements and, moreover, an unfavorable energy balance due to the loss heat reacted on the resistor.
A process and a circuit for detecting the failure of a stepping motor to keep in step are described also in document EP 0 462 050 A1, where the current flowing through the motor windings is tapped on a shunt resistor and evaluated. As in connection with the first-mentioned published document, the problems arising to the use of the shunt resistor exist here as well. Another problem is that current measurements are relatively susceptible to errors under certain operating conditions, for example in the presence of voltage variations as they frequently occur in on-board networks of motor vehicles. This can then be compensated only by means of costly interference suppression and stabilizing measures.
Furthermore, EP 0 402 220 A1 and EP 0 574 339 A2 describe methods of step recognition in connection with stepping motors. Said methods, however, are afflicted with the aforementioned shortcomings as well.
The problem of the invention of avoiding said drawbacks arises from the shortcomings stated above. In particular, the process of the invention is to be as insensitive as possible to interference, and assure safe operation under any operating conditions. Furthermore, both unipolar and bipolar stepping motors are to be employable.
For solving said problems the invention proposes a process with the following process steps:
Transmission of a defined current to a second motor winding after reversing the direction of the flow of current in a first motor winding;
Switching of a motor winding connection of the second motor winding with a high ohmic value;
Detection of the response signal on the motor winding connection switched with a high ohmic value; and
Evaluation of the duration of the response signal.
A modulated flow of current through the individual motor windings takes place in the course of execution of the process as defined by the invention. Advantages arise from the fact that no current measurements are carried out, which means no shunt resistors and no analog components needed for the evaluation are required, so that as opposed to methods known according to the state of the art, the entire circuit can be integrated in a more advantageous manner in an integrated circuit, for example on an ASIC (application specific integrated circuit).
As opposed to methods known according to the state of the art, which, for recognizing a step loss require the state of oscillation, i.e., the so-called toggling on a fixed or elastic stop means, and which are consequently not capable of recognizing full blocking of the shaft in a fixed state, the process as defined by the invention permits recognition of such operating conditions as well. In the event of toggling on the stop means, the process as defined by the invention is substantially less sensitive to line-bound interferences.
In detail, for carrying out the process as defined by the invention, a defined current is transmitted to a second motor winding after the direction of the current flowing in a first motor winding has been reversed, i.e., after a step pulse has been completed. As a rule, the amount of the current should be smaller than the typical operating current. According to a preferred implementation of the process, provision is made that the winding ends are short-circuited with each other only for a defined time. The coil current is reduced in this way to a defined value by the ohmic resistance component.
In the next step, one end of the winding of the second motor winding is switched with a high ohmic value, i.e., the current circuit is opened and the one end of the winding is connected to the potential-free input of an evaluation electronics.
In the motor winding switched to a high ohmic value and connected to the evaluation electronics, a pulse referred to as the response signal is induced due to the inductivity of said motor winding. The length of said pulse is correlated with the type of motor employed and the type of operation. The process as defined by the invention makes use of said circumstance in a way such that differences in the duration of the response pulses are already evaluated as a clear indication of a deviating rotor position, i.e., as a step loss.
Measuring the length of time of the response signal requires only relatively minor expenditure in terms of circuit technology. As opposed to absolute current measurements, measurement of the duration is at the same time largely insensitive to voltage variations in the system and other interferences.
After the length of time of the response signal has been evaluated, the second motor winding is again switched to the supply lines, so that the feed of current to the motor winding can be continued depending on the result of the preceding evaluation. The current feed cycle is usefully interrupted in this connection if any blocking is detected. A limit switch can be saved in this way in electromechanical systems; at the same time, trouble in the form of noise caused by oscillation on the mechanical stop means as well as mechanical overload are avoided.
In the process as defined by the invention, the motor winding is preferably short-circuited against ground (GND) and the length of time of the high pulse is subsequently measured after a preset time interval. Owing to the short-circuiting against ground, the motor winding current is fixed free of interference. In this wiring scheme, the response signal has a high pulse whose length of time permits supplying information about the operating condition.
One beneficial feature offered by th
Collard & Roe P.C.
Duda Rina I.
Elmos Semiconductor AG
Nappi Robert E.
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