FR2693326A1 - Process for detecting rotation of stepper motor for car air conditioning system - using simulation circuit to calculate winding current with comparison with actual winding current providing precise rotation - Google Patents

Abstract

The process is for detecting the rotation of a stepper motor (M) which is rotated by a pulse generator (3) and amplifier (2). A simulation circuit (6) calculates the current flowing in the motor winding (Is) from an input from the pulse generator. A current detector (4) in the stepper motor and amplifier (5) relay the actual current in the windings (Ir) to a subtractor (7) where a comparison is made with the simulation current. The difference current is fed to an amplifier (8) and comparator (9) to produce corrected positional information (S). USE/ADVANTAGE - For controlling flow rate of air in vehicle air conditioning system. Exact rotation of stepper motor can be determined.

Description

DETECTION METHOD AND CIRCUIT
OF THE POSITION OF A STEPPER MOTOR
The invention relates to stepper motor controls and more particularly the detection of the position of these motors.

Stepper motors are often used to perform position controls. Compared to position controls using other types of motors, the use of stepper motors has the advantage of not requiring position sensors to provide feedback signals from the control. The position of the motor is in principle defined by the number of current pulses applied to the motor windings.

However, the total absence of a position sensor does not make it possible to ascertain whether the motor has actually rotated by the number of steps imposed by the motor control. This becomes a drawback in cases where the motor load has a resistive torque which can vary in relatively large proportions.

For example, a stepping motor can be used to actuate an air flow adjustment shutter in an air conditioning system for a vehicle. However, the mechanical part of the actuation has variations in torque depending first on the position and on the other hand, because of parameters that are difficult to control, such as the manufacturing dispersions of the mechanical parts or the influence of the temperature. and the aging of the materials used.

The above problem can of course be resolved by adding a position sensor connected to the control.

However, this solution would have the effect of reducing the advantage of using a stepping motor.

According to another possibility, the use of a sensor can be avoided by analyzing the supply currents of the motor windings.

This solution exploits the fact that the current flowing in the windings is different depending on whether the motor is rotating or blocked. In accordance with this approach, European patent application EP-A 305 876 published on March 8, 1989 and entitled "Stepper motor shaft position sensor" proposes a circuit for detecting the rotation of a stepping motor based on the measurement of the circulating current in the windings. The current measurement signal is derived over time and compared to a predetermined threshold value. The result of the comparison makes it possible to determine whether the motor has rotated by one step or not.

Although the previous solution theoretically makes it possible to detect the effective rotation of the motor, its implementation proves to be difficult. Indeed, the currents measured in the motor windings do not have very well defined characteristics. In particular, the variations of the current as a function of time in the case where the motor is running generally presents significant fluctuations compared to the theoretical current. As a result, the derivative of the measured current could be interpreted as representative of a blocked engine when it has operated normally.

The invention aims to remedy the above drawbacks. It is based on the following observations 1) the rise time of the current flowing in the
windings of a blocked motor is less than that
for a rotating motor; 2) variations as a function of time of the current measured
in the windings when the motor is blocked
are less prone to fluctuations than those
for a rotating motor.

More specifically, the subject of the invention is a method for detecting the rotation of a stepping motor by measuring the currents flowing in the windings of said motor, characterized in that it consists in producing a reference signal whose variations with respect to time are representative of the variations of the real current flowing in said windings when the motor is blocked and to compare said reference signal with a measurement signal representative of the real current flowing in said windings.

The invention also relates to a detection circuit for implementing the method. The detection circuit includes a current sensor providing a signal for measuring the current flowing in the motor windings and it is characterized in that it includes a simulation circuit for producing a reference signal whose variations with respect to time are representative of the variations in the actual current flowing in said windings when the motor is blocked and comparison means connected to said detector and to said simulation circuit, said comparison means providing a signal indicative of the state of rotation of the motor when the difference between the reference signal and the measurement signal in absolute value exceeds a predetermined value.

Usually, the motor is supplied by a power circuit controlled by rotation control pulses. According to an alternative embodiment of the circuit according to the invention, the simulation circuit includes a time constant circuit synchronized by the rotation control pulses.

The invention also relates to the application of the detection circuit for controlling the opening angle of the air flow adjustment flap of an air conditioning system for a motor vehicle.

Other aspects and advantages of the method and of the circuit according to the invention will appear in the following description with reference to the figures - FIG. 1 is an overall diagram of a control for
stepper motor associated with the detector according to
the invention; - Figure 2 is another diagram using a
microcontroller; - Figure 3 is a diagram of a detailed embodiment
the detection circuit according to the invention; - Figures 4 to 7 show timing diagrams
to explain the operation of the circuit
FIG. 3, and - FIGS. 8 to 10 represent a control block for
air conditioning using a stepper motor and the
detection circuit according to the invention.

The diagram in FIG. 1 shows a stepping motor M powered by a power circuit 2, controlled by a pulse generator 3. According to the invention, a current sensor 4 is connected to an amplifier 5 which provides a current measurement signal Ir flowing in the windings of the motor M to a subtractor 7.

The subtractor 7 receives, on the other hand, an output signal Is supplied by a current simulation circuit 6 synchronized by the pulse generator 3.

The output of the subtractor 7 is connected to an amplifier 8 which supplies a comparator 9 with a signal Id representing the difference between Is and Ir. The elements 4 to 9 make up the constituent elements of the rotation detector according to the invention. The constitution and detailed operation of these elements will be given with reference to FIG. 3.

FIG. 2 represents a particular embodiment of a stepping motor control associated with the detection circuit according to the invention. The stepping motor M comprises two windings La, Lb supplied by the power circuit 2 controlled from a microcontroller 3 provided for example for controlling the air conditioning of a motor vehicle. The microcontroller 3 supplies pulses c of rotation control to the circuit 2. The circuit 2 is supplied by a voltage source 1 which is for example the vehicle battery. As a function of the pulses c received, circuit 2 produces pulses of current Ip in the windings La, Lb which can be measured by the voltage across a resistor 4. This voltage is applied to the input of the synchronized rotation detector D by the pulses c and supplying the rotation detection signal s applied to the microcontroller 3.

The power circuit 2 can consist of a step-by-step control circuit available on the market such as the UDN 2916 circuit from the company SPRAGUE. This integrated circuit also makes it possible to limit the current flowing in the windings La, Lb by a pulse width modulation whose frequency is adjustable by a PWM input provided for this purpose. The chopping of the current is carried out in the following way: the current is cut as soon as a value is reached, the restoration in conduction is carried out after a time fixed by external components. Similarly, the microcontroller can be an integrated circuit of the conventional type offered by various manufacturers.

This arrangement allows the microcontroller 3 to control the direction of rotation and the number of steps to be taken by the motor as a function of various regulation parameters. Thanks to the rotation detection signal s supplied by the detector D according to the invention, the microcontroller 3 can diagnose operating anomalies and send a fault detection signal.

FIG. 3 represents a detailed embodiment of the rotation detector D. It includes a simulation circuit 6 comprising an NPN bipolar transistor 24 whose base receives the rotation control pulses c via a resistor 20.

The emitter of transistor 24 is connected to ground via a resistor 21 while its collector is connected to the positive terminal of the battery 1 via variable resistance elements 18, 19. The collector of transistor 24 is, on the other hand, connected to a terminal of a capacitor 23, the other terminal of which is connected to ground. The collector of transistor 24 is also connected to a first terminal of a potentiometer 22 whose cursor is connected to ground via a zener diode 25. The second terminal of potentiometer 22 provides the simulation signal Is. simulator 6 therefore constitutes a time constant circuit characterized by the values of resistors 18, 19, 21 and of capacitor 23. Resistor 19 is adjustable so as to be able to adjust this time constant. On the other hand, the associated potentiometer 22 at the zener diode 25 limits the voltage of the simulation signal Is.

The voltage across the resistor 4 is applied to the input of an amplification circuit 5 essentially consisting of a differential amplifier 35 whose direct input is connected to ground via a resistor 14 and the inverting input of which is connected to a terminal of the resistor 4 via a resistor 12. The output of the amplifier 35, providing the measurement signal Ir representative of the actual current flowing in the windings of the motor, is connected to its inverting input via a feedback resistor 13.

The subtractor 7 is formed by a differential amplifier 36 whose direct input receives the reference signal Is via a resistor 17 and whose inverting input receives the measurement signal
Ir via a resistor 15. The output of amplifier 36 is connected to its direct and reverse inputs respectively through resistors 26 and 16.

The output of amplifier 36 is also connected to an inverting amplifier 8 which includes a differential amplifier 37 whose inverting input is connected to the output of amplifier 36 via a resistor 27 and whose input direct is connected to ground via a resistor 29. The output of amplifier 37, which delivers the signal Id representative of the difference between Ir and Is, is connected to its inverting input via a feedback resistance 28.

The comparator 9 includes a differential amplifier 38 whose direct input is connected to the output of the amplifier 37 via a resistor 33. The inverting input of the amplifier 38 receives a reference voltage It supplied by a voltage divider 30 supplied by the battery voltage 1. The output of the amplifier 38 provides the rotation detection signal s.

The operation of the circuit of FIG. 3 will now be described with reference to the timing diagrams of FIGS. 4 to 7. FIG. 4 represents the variations as a function of time of the reference signal Is which constitutes a simulation of the winding current of the motor when this last one is blocked. FIG. 5 represents the variations as a function of time of a signal for measuring the current Ir flowing in a winding of the motor when the latter is in rotation. The difference Id between the simulation signal
Is and the current measurement signal Ir is shown in FIG. 6. The signals Is, Ir and Id being synchronized by the rotation control pulse, the difference signal Id varies as a function of time according to the shape of the curve shown in FIG. 6. At the appearance of the control pulse, the signal Id passes from the value 0 to a maximum value then decreases again towards 0. The comparator 9 performs the comparison between the difference signal Id and a predetermined value It and provides the rotation detection pulse s shown in FIG. 7.

Of course, the curves of FIGS. 4 to 6 are simplified representations of the real or simulated currents that can be observed in reality. In particular, the current Ir generally presents disturbances which are difficult to remove.

It should be noted that the current Ir has a transition time between the value 0 and a maximum value generally imposed by a current limiting device already mentioned during the description of FIG. 4. In order to facilitate the comparison between Is and
Ir, it is therefore preferable to adjust the maximum value of the reference signal Is to the same value Im as the maximum value of the current measurement signal. This can be obtained by appropriately dimensioning the components of the time constant circuit and by the fact that the same voltage source supplies the motor and the simulation circuit. This last measure also has the effect of making the comparison between
Is and Ir practically insensitive to variations in the supply voltage.

Finally, it can be seen that the synchronization of the reference signal by the rotation control pulses makes it very easy to obtain a rotation detection signal of the pulse type itself synchronized by the control pulses. The width of the pulses of the detection signal will depend weakly on the supply voltage by planning to supply the voltage divider 30 from the same source as the motor.

FIG. 8 represents an air conditioning unit B actuated by a stepping motor using the detection circuit according to the invention. The air conditioning unit B is provided with an air inlet flap V actuated by the motor M via a reduction gear R and a linkage T. The opening angle of the flap V is determined by the position of the motor M. Figures 9 and 10 respectively show the maximum and minimum opening positions of the flap V.

Claims (10)

1. Method for detecting the rotation of a stepping motor (M) by measuring the currents (Ip) flowing in the windings (La, Lb) of said motor M characterized in that it consists in producing a reference signal (Is) whose variations with respect to time are representative of the variations of the actual current (Ip) flowing in said windings (La, Lb) when the motor (M) is blocked and to compare said reference signal (Is) with a signal Ir measurement representative of the actual current (Ip) flowing in said windings (La, Lb). 2. Method according to claim 1, characterized in that said comparison consists in carrying out the difference (Id) between said reference signal (Is) and said measurement signal (Ir) and in detecting if said difference (Id) is greater in absolute value at a predetermined threshold value (It). 3. Method according to claim 2, characterized in that said reference signal (Is) and said real current (Ip) are synchronized. 4. Method according to claim 3, characterized in that said detection consists in producing a pulse signal. 5. A circuit for detecting the rotation of a stepping motor (M), said detection circuit comprising a current sensor (4,5) supplying a measurement signal (Ir) of the current (Id) flowing in the windings (La, Lb) of said motor (M), characterized in that it comprises a simulation circuit (6) for producing a reference signal (Is) whose variations with respect to time are representative of the variations of the real current (Ip ) circulating in said windings (La, Lb) when the motor (M) is blocked and comparison means (7,8,9) connected to said detector 4 and to said simulation circuit (6), said comparison means (7, 8,9) providing a signal (s) indicative of the state of rotation of the motor (M) when the difference (Id) between the reference signal (Is) and the measurement signal (Ir) exceeds in absolute value a value predetermined (It). 6. Detection circuit according to claim 5, characterized in that said motor (M) being supplied by a power circuit (2) controlled by pulses (c) of rotation control, said simulation circuit (6) comprises a time constant circuit (18,19,21,23) synchronized by said rotation control pulses (c). 7. Detection circuit according to claim 6, characterized in that said time constant circuit (18,19,21,23) is of the resistance-capacity type, the components of which are dimensioned so as to provide a voltage whose time transition is equal to the rise time of the measurement signal (Ir) when the motor (M) is blocked and whose threshold value (Im) is equal to the threshold value of the measurement signal (Ir). 8. Detection circuit according to claim 7, characterized in that said time constant circuit is supplied by the same voltage source (1) as said power circuit (2). 9. Detection circuit according to claim 8, characterized in that said predetermined value (It) is supplied by a voltage divider (30) supplied by said power source (1). 10. Application of the detection circuit according to one of claims 5 to 9 for controlling the opening angle of a shutter V for adjusting the air flow of an air conditioning system for a motor vehicle.