EP0140089B1 - Process for feeding a stepping motor - Google Patents

Abstract

A method of controlling a stepping motor comprises supplying the motor with a predetermined quantity of energy Eio from whence there will result an automatic adaptation of the width of the motor drive pulse to the voltage Uo at the motor terminals and the resistance Ri of the energy source. At the moment when the internal energy Ei(t) supplied to the motor, as defined by the integral over a time period of the product of Uo-RI and the current It circulating in the motor winding, becomes equal to Eio the drive pulse is cut off (R is the resistance of the motor winding). The circuit for practicing the method according to the invention includes means for measuring the voltage Uo and the energizing current I(t), means for obtaining the difference Uo-RI, a multiplier for forming the product (Uo-RI)I(t) and an integrator the output of which provides a voltage proportional to the internal energy Ei(t).

Description

The present invention relates to a method of supplying a stepping motor, in particular for a timepiece, method in which the quantity of energy E _i (t) is compared during the duration of each control pulse. supplied to the motor at a predetermined amount of energy E; _o , then the control pulse is interrupted as soon as said amounts of energy are equal.

In electronic timepieces that are found today on the market, it is common practice to find a stepper motor to convert the electrical pulses from a quartz time base into mechanical movement for show time. The system is powered by a power source, usually a small battery that will need to be replaced periodically. To save the energy delivered by the battery and therefore make it last as long as possible, adjustment systems have already been proposed which control the duration of the pulse which moves the motor to the load which it must drive; in other words, the pulse is lengthened if the charge increases and it is reduced if the same charge decreases. Such systems are described for example in documents US 4 323 834 and US 4 346 463.

However, we must distinguish several kinds of loads acting on the motor rotor. As we want a well-defined angular position of the rotor between the steps, it will be necessary to exert on it a positioning torque at rest which must be overcome each time we want to make it progress by one step. The rotor will also have to overcome the various friction couples that exist in the bearings. Finally the rotor will have to provide a useful torque to drive the time display mechanism. The energy to be supplied to the engine to overcome these different torques is generally well determined for a given type of watch which, when operating normally, consumes relatively constant energy. However, if the watch has a calendar, it will be understood that once every twenty-four hours the useful torque to be supplied will have to be greater at the change of date. It is then that the servo control mentioned in the paragraph above can intervene and provided of course that the watch is provided with such a system.

The servo systems reacting to the load imposed on the rotor generally assume that the motor is supplied at constant voltage and therefore do not take into account the differences between the voltage delivered by the battery at the beginning then at the end of its service life. At first glance, this simplification might seem legitimate for a silver battery whose voltages at the beginning and end of life are respectively of the order of 1.6 and 1.4 volts. However, as will be seen, the difference indicated already leads to overconsumption of the system if measures are not taken to control the length of the driving pulse to the voltage delivered by the power source as well as the resistance presented. by said source. It is evident, on the other hand, that if this difference increases further, as is the case for example in lithium batteries where the operating limits can be set between 2.4 and 3.6 volts, the consumption in pure loss will be even greater.

EP-A-0 057 663 was cited in the research report (corresponding to US-A-4439717). It is a control device for a stepping motor in which the energy supplied to the motor is compared to a predetermined energy, then the control pulse is cut off when said energies are equal. This device avoids the drawback which the known devices present and which is due to the fact that if the voltage of the power source varies, the power supplied to the motor also varies as indicated in the paragraph above. To overcome this drawback, the cited document proposes means which consist, on the one hand, in supplying the motor by keeping the current in the coil of said motor constant and, on the other hand, in analyzing the voltage signal present on the coil then to provide information on the induced voltage caused by the movement of the rotor. The approach proposed by the present invention is completely different since it does not use the induced voltage any more than it requires keeping the current in the coil constant. As we will see below, the process chosen here is limited to measuring the supply voltage at the motor terminals and the current flowing in its coil, integrating the product of the two measurements with respect to time and then cutting the supply when the energy thus measured is equal to a predetermined quantity of energy.

It is the object of the present invention to automatically adapt the control pulse length to the voltage and resistance of the power source by the means which appear in the claim.

• La figure 1 est un graphique illustrant le principe d'alimentation du moteur pas à pas selon l'invention.
• La figure 2 est un graphique qui montre plus en détail comment varie la longueur d'impulsion d'alimentation du moteur quand la tension d'alimentation et la résistance de la source d'alimentation changent si l'on met à profit le procédé selon l'invention.
• La figure 3 est un schéma bloc du circuit électronique de commande pour mettre en oeuvre le procédé selon l'invention.
• La figure 4 est un exemple de réalisation possible du circuit électronique de principe donné en figure 3.

The invention will now be better understood in the light of the following description and for the understanding of which reference will be made, by way of example, to the drawing in which:

• FIG. 1 is a graph illustrating the principle of supply of the stepping motor according to the invention.
• FIG. 2 is a graph which shows in more detail how the supply pulse length of the motor varies when the supply voltage and the resistance of the supply source change if the method according to the invention is used. 'invention.
• FIG. 3 is a block diagram of the electronic control circuit for implementing the method according to the invention.
• FIG. 4 is an example of a possible embodiment of the electronic circuit in principle given in FIG. 3.

dans laquelle:

• U_o = tension aux bornes du moteur;
• U; = tension induite de mouvement;
• L = self de la bobine du moteur;
• R = résistance de la bobine du moteur;
• I = courant dans la bobine du moteur.

In general, the currents and voltages involved in the operation of the motor are given by the following electrical equation:

in which:

• U _o = voltage across the motor;
• U; = induced movement voltage;
• L = inductor of the motor coil;
• R = resistance of the motor coil;
• I = current in the motor coil.

• _o∫^tU_ol(t)dt _{= o}∫^tRI²(t)dt + _o∫^tLI(t)dl + _o∫^tU_iI(t)dt [2]

By multiplying equation [1] by the term 1 (t) dt and by integrating with respect to time t, we obtain:

• _o ∫ ^t U _o l (t) dt _{= o} ∫ ^t RI ² (t) dt + _o ∫ ^t LI (t) dl + _o ∫ ^t U _i I (t) dt [2]

• _o∫^tU_oI(t)dt = Et = énergie totale fournie au système;
• _o∫^tRI²(t)dt = E_th = énergie dissipée par effet Joule;
• _o∫^tLI(t)dl = Es = énergie emmagasinée par la self;
• _o∫^tU_iI(t)dt = E_m = énergie mécanique fournie par le moteur.

In this equation:

• _o ∫ ^t U _o I (t) dt = Et = total energy supplied to the system;
• _o ∫ ^t RI ² (t) dt = E _th = energy dissipated by the Joule effect;
• _o ∫ ^t LI (t) dl = Es = energy stored by the self;
• _o U ^t U _i I (t) dt = E _m = mechanical energy supplied by the motor.

ce qui signifie que l'énergie interne à fournir au moteur est égale à l'énergie totale fournie par le système (Et) moins l'énergie dissipée par effet Joule (Eth).We want to provide the engine with constant mechanical energy. Since E _s is generally smaller than E _m and practically constant, imposing E _m = constant means that E _m + E _s = constant. The sum E _m + E _s is defined in the present case as representing the internal energy E; supplied to the engine. We can therefore write that:

which means that the internal energy to be supplied to the motor is equal to the total energy supplied by the system (Et) minus the energy dissipated by the Joule effect (Eth).

From equation [3] above, we conclude that there is a value E _i (t) for which we are sure that the rotor will take its pitch, this value depending on the combination of a certain voltage U _o aux motor terminals, of a certain current I circulating in the coil and this for a certain determined time T _i . Let E _{io be} this value which is then expressed by the equation:

This amount of energy E _io can be determined for each caliber of watch that we are dealing with and it will play the role of threshold to limit the duration of the driving pulse sent to the motor. In other words, when the internal energy E _i (t) supplied to the motor reaches the threshold value E _io, the energy is cut. We will thus have sent to the motor just enough energy for its rotor to take the plunge, avoiding unnecessary overconsumption.

The process which has just been indicated is illustrated by the graph in FIG. 1. The integration time t in seconds is plotted on the abscissa and, on the ordinate, the energy E _i (t) in microjoules as well as a energy value E _io for which it is ensured that the rotor will cross its pitch. When the value E _i (t) reaches the threshold value E _io , the pulse is cut off which gives a duration T; for this impulse. Figure 1 also shows the variation of current 1 in the motor coil. We have drawn on the time axis a value t = T; max. It may indeed happen, for abnormally high load cases, that the energy E _i (t) can never reach the threshold E _io . It is then preferable to limit in time the duration of the control pulse, duration which will be chosen to be high, for example 10 ms.

If we refer again to equation [4], we realize that if the voltage U _o at the motor terminals decreases (aging of the battery) it will be necessary to integrate over a time T; longer to reach the threshold value E _io , which corresponds to an elongation of the driving pulse. Similarly, it can be seen that an increase in the internal resistance R _i of the cell causes a drop in U _o and leads to an increase in the pulse duration T _i . Thus the method according to the invention leads to a continuous adjustment of the pulse length as a function of the variations of U _o and consequently of R _i .

FIG. 2 shows an example of simulation of the reaction of this servo-control during a variation of the voltage across the terminals of the motor or during a variation of the internal resistance of the supply battery. In this example, it has been assumed that the minimum energy E _io required to drive the rotor is 1 µJ. This graph shows the same coordinates as those adopted for Figure 1.

The current I and energy E _i curves referenced in 1 present a state in which the motor is supplied by a voltage U _o equal to 1.7 V (new battery). The command pulse length is short, about 4.8 ms. The state referenced 2 is that for which the battery is at a discharge level for which the voltage U _o is _equal to 1.5 V. The coincidence of E; and E _io only intervenes when the pulse duration reaches 6.2 ms. Finally, state 3 is noted for a voltage U _o = 1.5V and for a resistance value R _i which has gone from 100 Ω that it had for states 1 and 2 to 500 Ω. At this time, the command pulse has a duration of 6.8 ms.

In conclusion from the above and according to the present invention, it can be seen that a predetermined quantity of energy is supplied to the motor, from which there will result an automatic adaptation of the length of the control pulse T; at the voltage U _o and at the resistance R _i of the power source.

We will now indicate a means for implementing the method according to the invention.

It is in fact a question of measuring the internal energy E _i (t) supplied to the motor, of comparing this internal energy with a predetermined quantity of energy E _io for which one is assured that the motor will take its step in all cases normal loads that may occur and cut the power to the motor as soon as E _i (t) = E _io . The value of ener internal geometry E _i (t) is known from equation [3] where it follows that:

We will therefore measure the value Uo-RI and the value of the current I (t) flowing in the coil. We will make the product of these two values which we will integrate with respect to time t.

The block diagram in FIG. 3 makes it possible to carry out all the operations mentioned above. The actual control block 4 receives at its input the control pulses (timing) of duration T; max and for which it will be a question of adjusting the width, the supply voltage U _o , the predetermined quantity of energy E _io and the energy supplied to the motor E; (t). This command 4 satisfies the following operating conditions: cut the pulse if E _i (t) becomes larger than E _io or maintain it until a previously defined value T; max if E _i (t) always remains lower than E _io . The circuit 5 is a sensor block which makes it possible to read the value of the current I (t) in the coil of the motor 8 and the value of the voltage U _o at its terminals and to make the difference U _o- RI. Circuit 6 is a multiplier which performs the operation [5] mentioned above. Finally, circuit 7 is an integrator which integrates the product (U _o ―RI) I (t) in time space from which the value of internal energy E _i (t) results.

The block diagram of FIG. 3 is a block diagram allowing the operations necessary to implement the method according to the invention to be carried out. In practice, there are several ways of achieving this and the diagram in FIG. 4 describes a possible embodiment which will be explained now.

The motor M is supplied at its terminals by the voltage U _o . The alternating polarity pulses are sent to the motor by a bridge of transistors 31, 32, 33 and 34. When the transistors 31 and 32 are conductive, the current flows in the direction of arrow 35 while it flows in the direction of the arrow 36 when it is the transistors 33 and 34 which conduct. Between the line 37 and the ground is interposed an operational amplifier 38 at the output 42 from which appears a voltage proportional to the product of the resistance R of the coil and the current 1 flowing in this coil. A second operational amplifier 39 combines, via three resistors r of equal values, the voltage U _o and the voltage R · to provide at its output 40 a voltage U _o- RI. The voltages formed on lines 40 and 42 are sent respectively to the inputs X and Y of a multiplier 43 which, at its output 44, delivers a voltage U proportional to (U _o ―RI) I (t) to a factor of scale close. This voltage U in turn attacks an integrator circuit composed of the operational amplifier 45 to which a network formed by the resistor RM and the capacitor C is applied. There is then at the output of the integrator, on the line 46, a voltage proportional to the value of the internal energy E _i (t). It should also be noted that a transistor 47 is connected in parallel on the capacitor C, which makes it possible to short-circuit the said capacitor as soon as the control pulse has ceased (reset of the integrator). The control of transistor 47 is therefore linked by line 48 to the falling edge of the control pulse.

FIG. 4 also shows two type D flip-flops, 49 and 50 which each receive on their clock input CP the control pulses (timing) coming from the frequency divider (not shown) with which the watch is equipped. These flip-flops rock on the rising edge of the pulse and attack by their outputs Q and Q two NOR gates 51 and 52 and two inverters 53 and 54 to finally supply the signals A, B, C and D which control the transistors 31, 33, 34 and 32 respectively. There is at the output Q of the flip-flop 49 the control pulse 55 which begins with the rising edge (timing) and which ends as soon as the reset input of the same flip-flop is actuated. On the exit Q of the same flip-flop, we will find the same pulse 55 but inverted. The purpose of the flip-flop 50 is to ensure the alternating polarity of the control pulses.

The reset input of flip-flop 49 receives via line 60 and via an OR gate 56 the signal from a comparator 57. The inputs + and-of this comparator respectively receive the signals E _i (t) from which it was question above and E _io which is a predetermined quantity of energy fixed once and for all and which depends on the type of watch to be adjusted. E _io is practically in the form of a stabilized voltage. When E _I (t) = E _io , and according to the invention, the comparator 57 provides a signal 1 which, through the OR gate 56, resets the flip-flop 49 to zero and thus interrupts the motor pulse 55 ( falling edge T;).

However, situations may arise where E _i (t) never reaches E _io , for example when an extraordinarily high torque is applied to the engine. In this case, it will be understood that it is necessary to limit the duration of the motor pulse over time. In the diagram of FIG. 4, the control pulses (timing) appearing at the inputs CP of the flip-flops 49 and 50 are limited in their duration to a length T _i max. If no signal appears at the output of comparator 57, it is the falling edge of the control pulse which, at time T; max, will cause, via the inverter 58 and the gate 56, a reset signal of the flip-flop 49, causing the interruption of the control pulse at the output Q of the same flip-flop.

The circuit of FIG. 4 is produced by means of conventional logic elements with regard to the doors 51, 52, 53, 54, 56 and 58 and the flip-flops 49 and 50. The multiplier 43 can be of the AD 534 type from the manufacturer Analog Devices. We can choose as operational amplifiers 38, 39 and 45 those bearing the reference LF 355 N from the manufacturer National Semiconductor. The comparator 57 may be of the LM 311 type from the same manufacturer National Semiconductor.

Finally, it should be noted that the process which has just been described does not react to the load variation which may occur on the motor shaft. It is therefore necessary Please choose a value E _io which is high enough for the motor to be able to take its step under any circumstances. Thus, if the enslavement of the pulse length to the internal energy of the motor has the advantages which have been described here, it will however be understood that this enslavement will not be sufficient if the motor misses its pitch due to a heavy overload pass or lose a step following a shock. It may therefore be advisable to combine the servo system according to the invention with a system which detects missed steps and which catches up with the accumulated delay as described for example in patent application EP 0 022 270.

Claims (1)

1. Process for feeding a stepping motor specially for timepiece comprising the steps of

- determining a predetermined quantity of energy E_io to be supplied to the motor for which stepping is assured in all cases of normally oc- curing loads,

- furnishing control pulses to the motor,

- measuring the voltage U_o across the motor winding terminals as well as the current I(t) circulating in said winding during each of said control pulses,

- forming the difference U_o-RI where R represents the winding resistance,

- forming the product (U_o-RI)I(t),

- integrating over a time period t, whereby the value of the integral represents the energy E_i(t) supplied to the motor and the limits of said integral being chosen between a first time corresponding to the beginning of the control pulse and a second time corresponding to the end of said control pulse,

- comparing the energy value E_i(t) to the predetermined energy E_io, and

- cutting off the control pulse when E_i(t) = E_io, process characterized by the fact that the duration of the control pulses feeding the motor is limited to a predetermined period T;max in such a manner that when the energy E_i(t) supplied to the motor is not sufficient to reach the predetermined energy E_io, the feeding of the motor is stopped after said predetermined period T;max, the steps skipped by the motor being then detected whereby accumulated losses may be recovered.

Images