Process and circuit for determining and monitoring the temperature of a bobbin winding.

0001Method and circuit for determining and monitoring the temperature of the winding of a coil

0002The invention relates to a method for determining and monitoring the temperature of the winding of a coil, preferably the solenoid of a solenoid valve. The invention also relates to a circuit suitable for carrying out this method.

0003The dimensioning of coils, whether for electric motors, solenoid valves, transformers or the like, depends on the current flowing through the winding of the coil. If the current exceeds a permissible limit value for a long time, the heating of the winding associated with this can lead to insulation damage. In order to prevent such an impermissibly high heating of a coil, it is known (cf. DE-OS 25 36 375) to sense the coil temperature with the aid of a temperature sensor which also has a safety function.

0004In the case of coils which are subject to high load dynamics, it is known to operate them briefly with excessive excitation in order to be able to dispense with unnecessarily large windings. This technique is known in particular in the case of solenoid valves which briefly require a high excitation current during a stroke phase, while in the remaining time only a so-called holding current is required which z. B. holds against the force of a spring in a certain position. The overexcitation during the lifting phase is usually used to provide more force to accelerate the valve plunger than is needed to hold it in a certain position. Often, load-related inhibitions or additional forces must be overcome as quickly as possible. B. may have their cause in an unfavorable lifting force characteristic of the driving magnet. The operation of a solenoid valve with briefly excessive excitation leads to a particularly economical design of the magnet, since its size can largely depend on the holding excitation and the required number of working cycles per unit of time and not solely on the excitation of the suit.

0005The present invention has for its object to propose a method and a circuit of the type mentioned, which are particularly simple and additional sensor means. B. a temperature sensor, do not need.

0006According to the invention, this object is achieved in that the resistance dependent on the temperature of the winding is used as the measured variable to be monitored. Since the temperature coefficients of the resistances of the wires which are generally used for the construction of coils are known, the resistance can be used as a measure of the temperature of the winding when the current is known and when the voltage drops across the winding.

0007A constant electrical current, or an electrical current which can be determined in terms of its size, is expediently passed through the winding during the measuring time and the voltage which drops across the winding and is dependent on the temperature of the winding is used as the measured variable. On the other hand, there is also the possibility of applying a voltage to the winding and using the current which changes as a function of the temperature of the winding as a measured variable.

0008If the coil is part of a drive for a solenoid valve that requires a holding current, then a particularly expedient development of the invention is that the holding current is used simultaneously as a constant measuring current.

0009Further advantages and details will be explained on the basis of the exemplary embodiments shown in FIGS. 1 to 4.

0010Fig. 1 shows a schematic diagram in which - as in the other figures - a replacement image has been selected for the winding or coil 1 to be monitored for its temperature, namely a temperature-dependent resistor R (T) and an inductance L. A measuring current I flows due to the direct current source 4_M through the winding 1, then a certain voltage U drops across 1_M from. It applies

0011<maths id="math0001" num=""><img file="EP0170833A2_D0001.tif" /></maths>The resistances of most wires used to build coils have positive temperature coefficients in the range of a few parts per thousand Kelvin. Copper has e.g. B. depending on the type 3.9 to 4.0 per mille per degree Kelvin.

0012In the technically interesting area between -50 ° C and + 200 ° C, they follow the relationship with sufficient accuracy<maths id="math0002" num=""><img file="EP0170833A2_D0002.tif" /></maths><ul id="ul0001" list-style="none"><li>R (T) = resistance at temperature T</li><li>R₀ = Resistance at temperature T_O</li><li>α_R = Temperature coefficient of resistance</li></ul>

0013With a known current, the voltage drop across the coil is itself a measure of the temperature of the winding.<maths id="math0003" num=""><img file="EP0170833A2_D0003.tif" /></maths>

0014With known IM, R_O, T_O and α_R can from U_M(T) can be concluded on the temperature T.

0015The measuring current need not be pure direct current. It can also contain a large proportion of alternating current (e.g. current from a one-way or two-way rectifier circuit with or without leading edge). It is crucial that its mean is kept constant.

0016Changes from U_M can be monitored by means of an electronic device generally shown as block 6, which can lead to a warning or shutdown in a suitable manner.

0017Fig. 2 shows an embodiment which is particularly suitable for the temperature monitoring of solenoid valve coils. The coil or winding 1 is assigned two current sources 7 and 8, which can be optionally switched on via the switch 9. The current source 7 supplies a high excitation current that flows during the lifting phase. A brief overexcitation is permitted as long as the coil temperature remains sufficiently low. Following the lifting phase, a switch is made to the current source 8 which supplies the holding current, that is to say a relatively small current which holds the closure element of the valve in a specific position. This holding current can at the same time as measuring current I_M be used so that the temperature of the coil 1 can be inferred from the voltage at 5.

0018In the embodiment according to Fig. 2 becomes the voltage value U_M at 5 via a low-pass filter 11 to a threshold value detector 12. The low-pass filter 11, which can also be implemented as an integrator, ensures that the voltage drop across R must be applied for a minimum time before it triggers the threshold value detector.

0019Very short-term impulses, such as B. caused by the self-induction of the coil or by a short period of overexcitation, thus do not work or are only greatly reduced. A threshold voltage is applied to the threshold value detector 12, which is generated by the voltage source 13 and corresponds to the maximum voltage value UM (and thus the maximum permissible temperature T). The threshold value detector 12 in turn actuates a fault flip-flop 14, which can cause a warning message or a shutdown. The interference flip-flop 14 can be reset by means of the button 15.

0020Fig. 3 shows a corresponding structure with constant voltage supply of the winding 1. The voltage source is designated by 16. The coil or winding 1 is supplemented by the series resistor 17 and the resistors 18 and 19 to form a bridge circuit. The temperature coefficient of the resistors 17 to 19 is expediently in the range of a few 10^-5 per degree Kelvin, which means that it is expediently two orders of magnitude smaller than the temperature coefficient of the copper wires of the winding 1. The comparison points of the bridge circuit are connected to the inputs of the low-pass filter 11. In this exemplary embodiment, the low-pass filter 12 is first followed by a differential amplifier 21, which converts the temperature-dependent bridge voltage to a new reference potential. The following functions (threshold value detector 12, interference flip-flop 14, etc.) correspond to the exemplary embodiment according to FIG. 2.

0021Instead of the DC voltage source 16, a mixed voltage source can also be provided.

0022Finally, FIG. 4 shows an embodiment with an AC voltage source 22 and a subsequent bridge circuit, as shown in FIG^-Fig. 3 is shown. To detect the change in R with temperature, the pure active component of the current T must first be used in this circuit_M be formed. This is done with the aid of a multiplication stage 23, the inputs of which are connected to the comparison points of the bridge circuit. This multiplication stage 23 effects the multiplication of the current-proportional signal, which drops across the resistor 17, by the voltage-proportional signal, which drops across the resistor 19. The same could also be achieved by phase-sensitive rectification of the voltage signals, which depend on the resistors 1.7 and 19. After the formation of the active component of the current I_M low pass 11, threshold detector 12 and interference flip-flop 14 follow again.