Abstract:
In order to produce a phase control device which is free of the constraints of drifts due to the supply frequency or to the temperature of the components forming it, a method is recommended in which a command signal delivered by a control is modified by a converter in order that the activity of the actuator controlled by this device is a linear function, in all cases, of a control signal. As an improvement, account is taken of the variation in frequency of the supply signal by multiplying the convertible signal by a measurement of the duration of the period of the power signal.

Description:
This invention relates to a method of control by phase angle which can be used in both the industrial and domestic fields. It particular, it respectively relates to the variable control of motors and light variators. The principle of the control by phase angle is to use an alternating power signal and to cause the switching in a load of this power signal at an appropriate instant, always the same, within a period of this alternating power signal. The setting of this appropriate instant with respect to an initial instant allows the power to be adjusted by synchronisation. 
     DISCUSSION OF THE BACKGROUND 
     The frequency of the power signal used is an essential characteristic of the adjusting device with uses this principle. A first problem arises from the fact of the variation in the frequencies used: that is to say 60 Hz in America and 50 Hz in Europe and the rest of the world. The design of the corresponding devices must therefore be differentiated. Furthermore, the adjustment range of the phase control is derived according to the utilisation constraints of the integrated circuit which carries out this switching function. Thus, this adjustment range is in particular dependent on the clock frequency of the microcontroller used, on its temperature and on its supply voltage. Particularly if the sensitivity of the adjustment in not linear. 
     This type of disadvantage may not be harmful in the case where the variator used is a manually controlled one. Even so, the ease of use thereof is affected, particularly if the results of the control become much more sensitive at one place in the adjustment range than at another place. 
     On the other hand, in the field of servocontrol, the variation in sensitivity encountered throughout the adjustment range with phase control devices is not tolerable. This sensitivity conditions the activity of an actuator as a function of a control signal. In effect, if the control must be a servocontrol, the open loop gain of the servocontrol can then be different depending on whether the command value is situated in one place of the adjustment range or another. In order to avoid the hunting and oscillation phenomena of the servocontrol, it is necessary to take account of the maximum gain. The consequence of this is that, in the zones of the adjustment range where the gain is low, the reactivity of the servocontrol is also low. In some way the gathering of the activity of the actuator to the command value imposed on it is more or less rapid depending on the sensitivity of the adjustment range in question. 
     SUMMARY OF THE INVENTION 
     The purpose of the invention is to overcome this problem by providing in the control circuit a means of linearizing the activity of the actuator used (a lamp, a heating appliance, a motor or something else) with respect to the available control signal. This means is interposed between the command circuit and the actuator used. It will be shown that this procedure results in the solving of all the problems mentioned above. 
     The invention therefore relates to a method of control in which a periodic electrical signal is applied to an actuator by the intermediary of a switch with automatic cutoff, a control signal is produced, a time difference of an appropriate control instant is determined with respect to a characteristic instant of this periodic signal as a function of this control signal, the periodic closing of the switch is commanded at this appropriate instant in each period by applying this control signal to the switch, the activity of the actuator is adjusted by modifying the control signal, characterized in that the control signal is converted by a conversion into a different converted signal whose application causes a linear behaviour of the activity of the actuator as a function of this control signal and the converted signal is used to determine the time difference. When the activity is speed, the conversion is advantageously of the arcsine type. When the activity is power, the conversion is advantageously of the arcsine squared type. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood on reading the following description and on examining the accompanying figures. The latter are given only by way of indication and are in no way limitative of the invention. The figures show: 
     FIG.  1 : a device which can be used for implementing the method of the invention; 
     FIGS. 2 a  and  2   b:  the representations of a normal transfer function and of the reciprocal transfer function used, in the invention, for correcting the non-linearity of this normal transfer function; 
     FIG.  3 : a diagrammatic representation of a method of generating the reciprocal function; 
     FIG.  4 : a diagrammatic representation of a method of generating the linearity correction as a function of the tabulated expression of the reciprocal function; 
     FIG.  5 : diagrams of the signals which can be used in the method of the invention, plotted against time; 
    
    
     Table  1 : a reciprocal conversion table corresponding to power linearizing. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a device which can be used for implementing the method of the invention. A periodic electrical signal S 1  is supplied by an electrical source  1 . It is applied to an actuator  2  by the intermediary of a switch  3  with automatic cutoff. As will be seen below, the switch  3  essentially comprises triacs, or thyristors. In the latter, conduction is caused at a particular time in the cycle of the periodic signal produced by the source  1 . This conduction cuts off automatically when this electrical signal reverses its polarity. A potentiometer  4 , or a similar device, makes it possible to produce a control signal K. 
     As the method of the invention is a method of control by phase angle, a time difference between an appropriate command instant of the switch  3  is marked with respect to a characteristic instant of the periodic signal S 1 . In practice, the characteristic instant of the signal S 1  is an instant at which the voltage of this signal passes through zero, when polarity changes. It will be seen below that this instant can, in certain cases, be difficult to measure and it is convenient to take account of the deviation, of the measuring error produced, in the detection of this characteristic instant. 
     In other words, the time difference of an appropriate control instant is determined with respect to a characteristic instant of the periodic signal as a function of the control signal K. 
     In a usual way, the signal K is a linear function of the time difference which is required to be imposed. For example, if the value of the signal K is at 100% of its useful value, the delay in switching on the switch will be zero. On the other hand, if the value of K is 0%, the delay in switching on the switch  3  will be total: equal to one half-period of the periodic signal. The switch  3  will not therefore be switched on. In this way, the activity of the actuator  2  is adjusted by modifying the control signal, in practice here by modifying the position of the cursor  5  of the potentiometer  4 . The potentiometer  4  is powered between VCC and earth. Its cursor supplies a voltage proportional to VCC. The signal K in this example is analogue. 
     In the invention, the control signal K is converted by a conversion into a converted signal K′ whose application then causes a linear behaviour of the activity of the actuator as a function of this control signal K. The activity which will be dealt with in the rest of this description is the power developed by the actuator  2 . This can be another parameter: for example its speed. If it is a matter of a heating element, this can be its temperature, or luminosity if it is a matter of electrical lighting. 
     In the invention, a microcontroller  6  is preferably used to convert a control signal K. The microcontroller  6  essentially comprises an arithmetic and logic unit  7  serving as a microprocessor. The microprocessor  7  is connected by an instruction, address and data bus  8  with an input interface  9 , an analogue to digital converter  10 , a program memory  11  and a working memory  12  provided with registers. According to what will be explained below, the conversion of the signal can be calculated on demand, or preferably it can be pre-calculated. In the latter case, the preliminary calculations are recorded in a non-volatile memory  13  of the microcontroller  6  in the form of a table. In order to control the actuator  2 , the microcontroller  6  comprises two more registers  14  and  15  respectively making it possible to have a logic status, relative to the real instant of presence of the characteristic instant in the period, and the control to be imposed on the switch  3 . 
     The switch  3  is preferably a switch of the optocoupler type. It comprises a light emitting diode  16  connected to the register  15  and which emits luminous radiation  17  when the content of the register  15  is active (logic state 1). It does not emit in the opposite case. The light  17  emitted by the diode  16  excites the gates  18  of a set  19  of triacs connected head-to-tail. These triacs  19  are furthermore inserted in series in the circuit connecting the source  1  to the actuator  2 . The instant at which the diode  16  emits is the appropriate instant. At the start of each alternation, when the set  19  of triacs is not conducting, the source  1  is connected to the actuator  2  by a branch comprising a resistor  20  and a set of light emitting diodes  21 . The light emitting diodes  21  excite the base, of a phototransistor  22  whose emitter is connected to an input of a high-gain operational amplifier  23 . The output of the operational amplifier  23  delivers an electrical state representative of the state of conduction of one of the diodes of a set of diodes  21 . 
     In principle, at the start of each alternation, when the set of triacs is in the open circuit state, the signal delivered by the amplifier  23  is conventionally in the 0 state. It changes to the 1 state when the diodes  21  are short-circuited by the conducting triacs  19 . This logic state is stored in the register  14 . The optocoupler  3  is therefore connected on the one hand to the load  2  and to the source  1  and, on the other hand, to the microcontroller  6 . 
     FIGS. 2 a  and  2   b  show the usual effects of a phase control circuit of the prior art. FIG. 2 a  gives information on the monitored activity as a function of the imposed control value K. For example, the monitored activity is power. Taking account of the principles of phase control (application of the power during portions of the period) and of the necessity of squaring the voltage in order to determine the power, the latter can be expressed as a function of K as follows: 
     
       
           P=K− (255/2π). (sin 2 πK /255)  Formula 1 
       
     
     In this formula, K, which in a standardized way can have one of the values between 0 and 1, has been quantified in 8 bits and can thus have a binary value from 0 to 255. It can easily be seen that the gain of the transfer function is maximum for K=128 and that it is very low for K in the region of 0 or 255. The problem of gathering for the value 0 is not very serious, taking account that the dissipated power must be zero; it is easy to gather. On the other hand, for high values, the reactivity of the servocontrol which would use these values would be reduced. The digitized value of K is obtained by sampling the analogue value delivered by the potentiometer to the input  9  and by making it undergo an analogue to digital conversion by the intermediary of the converter  10 . The value K, thus standardized over eight bits, can be stored in one of the working registers of the memory  12 . 
     In the invention, in order to linearize the control phenomenon, provision has been made to replace the value K by a value K′, as shown in FIG. 2 b  which shows K as an abscissa and K′ as an ordinate. 
     FIG. 2 b  in fact shows the reciprocal transfer function corresponding to the preceding formula. Taking account of the complex character of this preceding formula, it is not possible to have a simple analytical reciprocal transfer function K=f(P). K is here the inverse of the delay with which the set of triacs is triggered at the appropriate instant with respect to the characteristic instant. If K is zero, the delay is maximum and the dissipated power is zero. On the other hand, if K is nominal, the delay is zero and the dissipated power is maximum. FIG. 2 b,  in addition to the reciprocal function, carries out, in the present case, an inversion of the value of K. In effect, rather than working on the inverse of the delay, it has been preferred to work on the delay itself: is the typical value which can be used for controlling the switch  3 . 
     The obtaining of the reciprocal (and inverse if necessary) transfer function shown in FIG. 2 b  is tabulated in the memory  13  from the knowledge of the calculation of the transfer function of FIG. 2 a.  The calculation of this transfer function is expressed in the form of a table comprising, on the left, values of K varying from 1 to 255 for each line of the table and, on the right in each line, the value of the power P corresponding to the above formula. From this table, as shown in FIG. 3, it is possible to read backwards, by dichotomy. Thus, during a step  24 , a value P=j (j is equal to 0 at the beginning) is sought in this table. In a step  25 , the table is searched to see if there is a corresponding value of K for this value P=j. If such a value is found, it is said, in a step  26 , that the value K′ is equal to K (or to 255−K if there is inversion). Then, in step  27 , the process is repeated by incrementing j to j+1 until j reaches 255. If the chosen value of P. does not correspond to a value in the table, P is modified to P−1 and the test is repeated. It is repeated until a value of P is found for which there is a value of K in the table. For the starting value of P=j, the value of K is attributed, and therefore the corresponding found value of K′. By doing this, a table is available having as input (addresses) the values of P and as output (in the data stored at these addresses), the values of K′. It is therefore possible to produce a table comprising, as input, the values of K (such as supplied by the potentiometer  4 ) and, as output, the values of K′ whose application as control (or as command in the servocontrol) linearizes the control of the phenomenon. Another formula is used for another phenomenon. Table  1  shows the tabulated conversion carried out in the case of control of power. 
     FIG. 2 b  shows the reciprocal (and inverse) transfer function. It also shows a modification of the calculation of K′ associated with the variation, together, in the conditions of use of the microcontroller  6  and in the frequency of the alternating signal S 1 . In effect, if the period of this alternating signal is longer (its frequency is lower), it is important in order to control the actuator  2  with the appropriate phase, to consequently delay the application of the light pulse  17  to the triacs  19 . In fact, for a calculated value of K′, the delay to be imposed is not K′ but K″ which takes account of the real period of the signal S 1 , measured in the real conditions of use of the microcontroller  6 . 
     For this purpose, this microcontroller has a clock  29  connected to a counter  30 . The clock  29  is for example a 15 KHz clock, the counter  30  being an eight bit counter. FIGS. 4 and 5 make it possible to understand the functioning of this counter. In FIG. 5, signals varying with time can be seen representing, respectively, the electrical power signal S 1 , a signal S 2  passing into the set of diodes  21  and a counting signal S 3 . The signal S 3  is the one which is stored temporarily in the register  14 . 
     The signal S 2  is shown for different values of control phase angle. In the top part, the control is at 33%, in the central part it is at 66% and in the bottom part it is at 100%. The signal S 3  has positive pulses when the current passes into the triacs  19 . It is possible, with the microprocessor  7  receiving the binary state S 3  in the register  14 , to detect the falling edges  31  of S 3  in order to reset the counter  30  to 0 and to initiate the counting. By doing this, the counter  30  is capable, at the end of a half-period, of producing a result, stored in a register of the memory  12 , corresponding to the duration of the half-period of S 1 , and therefore corresponding to the period of S 1 . 
     FIG. 4 shows three registers of the memory  12 . A first register  32  contains the value K such as it is digitized by the analogue to digital converter  10 . A second register  33  shows the value K′ corresponding to the value K, after it has been extracted from table  1  of the memory  13 . In practice, this extraction is carried out by entering the value K as an address of the table of the memory  13  and by reading the value K′ stored at that address in that memory. The binary count corresponding to the duration of the period is stored in a register  34  of the memory  12 . 
     In synchronism and before the resetting to zero of the counter, the microprocessor  7  reads the contents of the registers  33  and  34  and the multiplication of one by the other is caused in order to store the result K″ thereof in a register  35 . In a preferred manner, the registers  33  and  34  are eight bit registers. Register  35  is therefore normally a 16 bit register. In order to standardize the calculation, the resultant value is divided by 255 which is equivalent to a truncation of the eight least significant bits of the register  35 . In the eight most significant bits of the register  35 , there is then a value representative of the duration during which it is necessary to wait before commanding the emission of the luminous excitation  17 . 
     For this triggering, the microprocessor  7  continuously compares the state of the counter  30  with the eight most significant bits stored in the register  35 . This comparison is shown diagrammatically by the signal S 4  of FIG. 5 where the clock pulses are present during a duration corresponding to the opening of the switch  3  (this is the 33% control). When the counter  30  arrives at the count of the register  35 , the microprocessor  7  loads into the register  15  a bit representing the activation of the excitation  17 . This activation stops automatically with the reversal of polarity by switching off the thyristors. The process then starts again for the next period. 
     By operating in this way, the deviations in functioning due to both the microcontroller  6  and the power supply of the source  1  are accounted for in a particularly simple manner. 
     It will be noted that if the control is at 100%, the fact of maintaining the diode  16  permanently on causes a loss of synchronisation and deviation correction information. This is not however a hindrance, considering that in these conditions the actuator is functioning with its maximum activity and it is impossible to do better than this. 
     FIG. 5 also shows a slight delay  36  in the triggering, with respect to the instant  37  at which the alternating signal S 1  passes through zero. It is possible to take account also, in the signal S 4 , of this difference by replacing the comparison of the binary count of the counter  30  with the value K′T by a comparison with the value K′T—the difference at the origin. This means that even this difference will be corrected. As a variant, the table of the memory  13  is corrected consequently. 
     In this case the control will be perfectly linear: there will no longer be any difference at the origin. The measurement of the difference at the origin is preferably carried out when the installation is switched on. This means that, at the moment of putting into service, the switch  3  does not control the actuator  2  during a double alternation of the signal S 1 . On the other hand, the diodes  21  detect the passage of the signal S 1  through zero as very short signals. At the moment of these passages through zero, the signal S 3  rises to 1. In fact, the very short duration during which the signal S 3  is rising to 1 is equal to double the difference  37 − 36 . In effect, it includes the part before the passage through zero and the part after the passage through zero. The duration of this very short pulse is measured with the counter  30 . The result is stored in a register of the memory  12 . In order to obtain a useful correction value, it suffices to divide this result by two. In practice, the least significant bit of the obtained count is dropped. 
     At 50 Hz, the period T of the alternating signal is of the order of 10 milliseconds. The eight bit counter  30 , which is a hardware counter, is provided to count at 125 KHz. The microprocessor  6  will preferably be timed by the clock  29  at a frequency of 4 MHz. The carrying out of the multiplication of FIG. 4 will only take 32 microseconds in practice, that is to say 3/100 of the duration of a cycle. This is negligible. 
     In brief, there is proposed according to the invention a method of servocontrol of an actuator in which a command signal is produced, the activity of the actuator is measured as a measurement signal, the difference between this command signal and this measurement signal is measured as an error signal and the actuator is controlled with this signal, characterized in that the error signal is converted by a conversion into a different converted signal whose application to the actuator causes a linear behaviour of the activity of the actuator as a function of this error signal.