Dental Furnace

Abstract

A dental oven for treatment of dental materials having one or more electric heating elements which extends adjacent to a heating space and is controlled by a heating controller. The heating controller regulates the temperature in the heating chamber and includes an output terminal for the heating element and an input terminal for temperature detection, which is electrically connected to a temperature detecting device, wherein the temperature detecting device has the one or more heating elements. The temperature sensing device includes a compensation device for compensation of nonlinearities at temperatures above 700° C. or in nonlinearities of the resistance of the heating element. The heating controller controls its output port based on the detected resistance of the heating element and on the compensation device.

Claims

1. A dental oven comprising at least one electric heating element (16) which extends adjacent to a heating chamber (12) and is controlled by a heating control (44), wherein the heating control (44) regulates the temperature in the heating chamber (12) and has an output connection for the at least one electric heating element (16) and an input connection for detecting the temperature of the at least one electric heating element (16), wherein the input connection is electrically connected to a temperature detecting device (22), wherein the temperature detecting device (22) comprises the at least one electric heating element (16), wherein the temperature detecting device (22) comprises a compensation device (50) configured to compensate for nonlinearities at temperatures above 400° C. detected by the temperature detecting device (22), and wherein the heating controller (44) controls the output connection based on the detected resistance of the at least one electric heating element (16) or a part of the at least one electric heating element (16) or all heating elements (16,18) and based on the compensation device (50).

2. The dental oven according to claim 1, wherein the temperature detecting device (22) detects the current temperature of the at least one electric heating element (16) by determining the resistance of the at least one heating electric element (16), which also includes impendences, inductive and/or capacitive resistances, or equivalent parameters.

3. The dental oven according to claim 1, wherein the heating control (44) comprises at least two control circuits comprising an internal control circuit which controls the heat output based on a current/voltage characteristic field, and an external control circuit which incorporates the temperature detecting device (22) which controls the temperature in the heating chamber (12) based on a measured resistance value of the at least one electric heating element (16) corresponding to a temperature value.

4. The dental oven according to claim 1, wherein the heating control (44) delivers pulsed heating power, wherein the temperature detecting device (22) measures the resistance of the at least one electric heating element (16) and indirectly the temperature in the heating chamber (12) in no-pulse periods or pulse pauses of the heating power, with a pulse length of 10 ms to 20 s and no-pause length of between 2 ms and 10 s.

5. The dental oven according to claim 1, wherein the temperature detecting device (22) measures the resistance of the at least one electric heating element (16) and indirectly the temperature in the heating chamber (12) at the beginning of pulse pauses or no-pulse periods of the heating power in a measuring period which has a length of less than 10 s and has a length of more than 10 ms.

6. The dental oven according to claim 1, wherein the at least one electric heating element (16) is at least partially made of compounds of MoSi, SiC, FeCrAl or FeCrNi, and has a resistance of less than 60 ohms at room temperature.

7. The dental oven according to claim 1, wherein the temperature detecting device (22) emits a measuring current, and measures a voltage drop across the at least one electric heating element (16), or across a part thereof or across all heating elements (16, 18) as a measuring voltage, wherein a product of measuring current and measuring voltage gives a measuring power of less than 10% of the maximum electric heating power of the dental oven or less than 1% of the maximum electric heating power of the dental oven, or the measuring current corresponds to the heating current and the measuring voltage corresponds to the heating voltage, respectively.

8. The dental oven according to claim 1, wherein the temperature detecting device (22) in the compensation device (50) has a look-up table, in which non-linearities of the at least one electric heating element (16) detected in a calibration step are stored, or the temperature detecting device (22) determines a function that represents the relation of resistance and temperature in the heating chamber (12), and wherein the calibration step of the temperature detection device (22) covers temperatures between room temperature and 1900 degrees Celsius over an entire temperature range.

9. The dental oven according to claim 1, wherein the temperature detection device (22) in a run-in firing and/or a regeneration firing of all of the heating elements (16,18) of the heater (32) operates without an additional temperature sensor.

10. The dental oven according to claim 1, wherein the temperature detecting device (22) has a test mode in which it detects the resistance value of the at least one electric heating element (16) at a predetermined state of the oven and the temperature detecting device (22) detects, the aging of the at least one electric heating element (16) or the need to recalibrate/service the furnace when a deviation of the measured resistance value from a value determined during calibration by more than a predetermined tolerance occurs.

11. The dental oven according to claim 1, wherein the temperature detecting device (22) indirectly detects the measuring current through the at least one electric heating element (16), and the uncompensated output value for the temperature measurement, indirectly via an output signal of a power supply unit for the at least one electric heating element (16), which output signal reflects a parameter of the power supply unit.

12. A method of operating a dental oven, comprising at least one electrical heating element (16) extending adjacent a heating chamber (12) and controlled by a heating controller (44), the method comprises the following steps: the heating controller (44) regulates a temperature in the heating chamber (12) and has a temperature detection device (22), the temperature detection device (22) detects the resistance of at least a part of the at least one electrical heating element (16) and comprises a compensation device (50), deviations from the proportionality of the resistance increase of the at least one electrical heating element (16) with the temperature increase of the at least one electrical heating element (16) or nonlinearities, are stored in the compensation device (50) the heating controller (44) controls the temperature of the dental furnace based on the detected resistance of at least a part of at least one electrical heating element (16) and based on the compensation device (50).

13. The method according to claim 12, wherein a limit value for the temperature of the dental oven at which a firing program is configured to begin is determined in advance, and wherein the current temperature of the at least one electrical heating element (16) according to the temperature detection device (22) is compared by the heating control (44) with the limit value temperature and the firing program is started when the limit value is reached.

14. The method according to claim 12, wherein, for the detection of the temperature of the at least one electrical heating element (16), short current pulses comprising pulses with a duty cycle of less than 20 percent are sent through the heating element and during these short current pulses, the temperature detection device (22) measures the resistance of at least a part of the at least one electrical heating element (16).

15. The method according to claim 12, wherein the resistance of the newly installed at least one electrical heating element (16) is detected and stored, and wherein the detection of the resistance of the at least one electrical heating element (16) is repeated at time intervals of at least 3 months, and the measured difference is stored as an offset reflecting the aging of the at least one electrical heating element and/or its-connections of the at least one electrical heating element and is also used in the temperature detection.

16. The dental oven according to claim 1 for predrying dental restoration parts, wherein the nonlinearities at temperatures above 400° C. are nonlinearities of the resistance of the at least one electric heating element (16).

17. The method according to claim 14, wherein the duty cycle is less than 7 percent.

18. The method according to claim 12, wherein the detection of the resistance of the heating element (16) is repeated at time intervals of at least one week.

19. The dental oven according to claim 6, wherein the at least one electric heating element (16) has a resistance of less than 10 ohms at room temperature.

Description

[0120] The following items are shown:

[0121] FIG. 1 shows a schematic representation of a dental oven according to the invention, showing the components essential to the invention;

[0122] FIG. 2 shows the resistance/temperature characteristic of a MoSi heating element of a dental oven according to the invention in one embodiment;

[0123] FIG. 3 shows another resistance/temperature characteristic in standardized form.

[0124] FIG. 4 shows a typical control loop of a temperature control of a dental furnace according to the prior art;

[0125] FIG. 5 shows an embodiment of the invention showing a control loop according to the invention, wherein no thermocouple is used in this embodiment;

[0126] FIG. 6 shows an embodiment of the invention showing a control loop according to the invention, wherein a thermocouple is used in this embodiment;

[0127] FIG. 7 shows a control diagram as part of the control loop, illustrating the interaction of the compensation device in temperature sensing;

[0128] FIG. 8 shows a diagram showing impressed measuring current pulses and the resistance value measured for each as cooling progresses;

[0129] FIG. 9 shows a graph showing the measured heating element resistance, one in new condition and the other in used condition; and

[0130] FIG. 10 shows the two curves shown in FIG. 8 after compensation by offsets.

[0131] From FIG. 1, a dental oven 10 is schematically apparent in its relevant to the invention parts.

[0132] This includes a heating chamber 12, which is heated by a heater 14. The heating device 14 consists of several heating elements, of which from FIG. 1, the heating elements 16 and 18 are visible. The heating elements extend annularly around the heating chamber 12.

[0133] The heating chamber 12 is intended for receiving dental restoration parts 20, which are shown schematically in FIG. Although here the dental oven 10 is formed as a kiln, it is understood that instead of a press furnace with a muffle may be formed according to the invention, or a sintering furnace, a crystallization furnace, a preheating furnace, a predrying, a debinding furnace or a heat curing polymerization furnace/device.

[0134] The illustrated dental oven 10 is multifunctional, so it can be used both as a preheating oven with temperatures around 800°, but also as a high-temperature sintering oven with temperatures around 1800°. Accordingly, the heating elements are temperature resistant. In the illustrated embodiment, they consist of SiC.

[0135] In the illustrated embodiment, the heating element 16 is part of a temperature sensing device 22. It has heating power connections 24 and 26 attached to the heating element 16 at two spaced apart locations. The heating element is traversed by the heating current, which also flows through further heating elements such as the heating element 18.

[0136] In the illustrated embodiment, the heating current line passes through a respective passage 28 and 30, at which the heating current line 32 passes through a heat insulation, not shown, of the furnace.

[0137] The heating current causes a voltage drop across the heating element, and the heating element heats up and thus also heats the heating space 12.

[0138] In the illustrated embodiment, in addition to the heating current terminals 24 and 26 on the heating element 16, a measuring current connection 34 near the heating current terminal 24 and a measuring current connection 36 near the heating current terminal 26 attached to the heating element. The respective measuring lines 38 and 40 can be passed through the passages 28 and 30 with it.

[0139] This embodiment has the advantage that the changing internal resistance of the heating element 16 only completely affects the measuring current in the region of the heating chamber 12.

[0140] The temperature detection device 22 has a control device 42. The control device 42 has an output terminal which is connected to the heating controller 44 and controls and regulates the heating current in the heating power line 32.

[0141] The controller 42 also provides the measurement current in the sense lines 38 and 40. The voltage drop across the heating element is measured. Preferably, the heating element 16 as a measuring resistor in a known per se resistance measuring bridge integrated.

[0142] It is also possible to combine the heating-current connection 24 and the measuring-current connection 34, and correspondingly the heating-current connection 26 and the measuring-current connection 36, each in one connection. However, a part of the heating current flowing through the area of the heating current line is then cold and thus contributes nothing to the measurement result.

[0143] The measuring lines 38 and 40 are preferably of low resistance, and the measuring current terminals 34 and 36 have a very low contact resistance to the heating element 16.

[0144] The measuring current in the measuring circuit is chosen so that it does not or not measurably affect the current temperature of the heating element, i.e. the heating element is not further heated. For example, the measuring current can be 5 mA, and the measuring voltage 20 mV, corresponding to an internal resistance of existing SiC heating element 16 of 4 ohms.

[0145] The heating element 16 is part of the temperature sensing device 22 and serves as a measuring resistor. It basically forms the temperature during heating, namely the temperature of the heating element 16.

[0146] The temperature of the dental restoration part 20 is typically significantly lower than that of the heating element 16, at least during the temperature change.

[0147] In addition, the internal resistance of the heating element 16 increases with increasing temperature, but not exactly proportional, but non-linear.

[0148] The control of the heating control is now carried out so that in principle the measured resistance of the heating element 16 is used as the basis for the control. In addition, however, the output signal of a compensation device 50 is taken into account, which is also part of the temperature detection device 22.

[0149] The compensation device 50 compensates for the non-linearity of the heating element 16. The non-linearity is material-dependent and known per se. If necessary, a measurement curve of the currently used heating element can be run through and stored for the calibration, so that the non-linearities are stored furnace-specific in the compensation device and the control device 22 can determine the exact temperature of the heating element 16.

[0150] Furthermore, it is also possible to mathematically take into account the temperature gradient during the heating, likewise in the temperature detection device 22, in particular in the compensation device 50. A large temperature gradient such as 50° C. per minute or even 80° C. per minute leads to a large temperature difference between the dental restoration part 20 and the heating element 16.

[0151] If the heating element 16 is kept at a constant temperature, based on the control by the temperature sensing device 22, this does not mean following a period of rapid heating that the temperature difference, i.e the local temperature gradient in the heating chamber 12 falls to zero; this occurs only gradually, based on heat transfer via convection and heat radiation.

[0152] These two deviations between the temperature of the heating element 16 and that of the dental restoration part 20 can be considered empirically and/or mathematically and can also be incorporated into the control.

[0153] The measures taken in this respect can be made, for example, according to EP 1 915 972 B1, to which reference is made in its entirety here.

[0154] In the embodiment shown here, it is provided to conduct the heating current through the heating current line 32 in pulse form. In pulse intervals, the heating element 16 is flowed through by measuring current. The pulse break can be e.g. 10 ms long.

[0155] In relation to the pulse pause length of the temporal temperature gradient is low. A long pulse break can be used to record the behavior during the break and thereby draw conclusions about the heating of the furnace. A slow cooling during the break means that the stove is very warm.

[0156] For example, the measurement can take place every third pulse break, and as a precaution, not immediately at the beginning of the pulse break, but, for example, at 10% of the length of the pulse break, the measurement time can then be completed, for example, 20% of the pulse break. This excludes the possibility that the heating element 16 cools by more than a few tens to a hundred milligrams, and thus the measurement result is falsified.

[0157] At 10% of the pulse break, any physical effects produced by the high heating current, such as, in particular, lattice vibrations, self-induction and so on, should decay, so that these too cannot influence the measuring current.

[0158] According to the invention, it is particularly advantageously provided that the form of temperature control shown here is used during the process step of pre-drying. In this process step, a wall section is opened, making it difficult to control via a thermocouple, depending on the design of the furnace even impossible.

[0159] According to the invention, a good temperature control with the features of claim 1 is still possible with an open wall section, and with satisfactory accuracy, and even over the entire operating area.

[0160] It is also possible to use the dental oven according to the invention exclusively or additionally as a kiln, for example at temperatures up to 1800° C. Again, the temperature detection device of the invention is used to control the temperature.

[0161] Furthermore, it is also possible to use a thermocouple or another temperature sensing element in addition to the temperature detection shown here via the temperature sensing device 22. Here it is possible that the two ways of temperature sensing each other control or supplement.

[0162] In FIG. 2 is plotted how the resistivity, plotted against the temperature, nonlinearly developed. The specific resistance Rho of the material measured here, namely molybdenum silicide, initially decreases less strong and then stronger, depending on the material used.

[0163] In an exemplary experiment, temperature measured values of 100° C. to 1550° C. were determined and normalized resistance values were determined. At 650° C., the normalized resistance value is 1.11, which is almost half as high as at 100° C. The normalized resistance increases only slightly, i.e. by 6%, up to 850° C., so that here an area of relatively low nonlinearity or better linearity is present.

[0164] At 1550° C., however, the normalized resistance value has then risen to 1.47, so that the compensation device 50 according to the invention the nonlinearity must compensate.

[0165] In FIG. 3, a normalized resistance is plotted here for silicon carbide. Again, it can be seen that the near-linear region of the non-linearity curve is less than 1100° C., approximately in the region around 700° C.

[0166] The family of curves apparent in the lower temperature range corresponds to different forms of the silicon carbide compound, and depends, for example, on its purity.

[0167] At somewhat higher temperatures, molybdenum disilicide forms a passivating layer of silicon dioxide which is produced, for example, via a burn-in fire and then protects the silicon carbide and makes it durable. Ideally, this break-in fire is controlled directly via the properties of the heating elements, since in this break-in fire, the time sequence of the temperature of the heating elements is relevant to the quality of the SiO.sub.2 layer.

[0168] In an advantageous embodiment, after the run-in firing and the calibration of the oven determines a difference curve for the measured temperature in relation to the heating power introduced and stored, for example in the compensation device 50.

[0169] For example, a test firing is performed automatically or manually after, for example, 50 firing cy-cles. If the ratio between the applied power and the measured temperature differs by more than a predetermined value, for example 5% or 10%, from the reference curve, a recalibration is re-quested; if this is no longer possible, it means that the life of the heating element 16 and thus the other heating elements is reached.

[0170] FIG. 4 shows a control circuit for a well-known and widely used dental furnace. In a manner known per se, the temperature is set as the setpoint. The control circuit 52 includes a PID controller 54, which provides the oven power at its output port. The output terminal of the PID controller 54 is connected to the input terminal of the power electronics 56, which in turn ensures that the heating element 16 or the heating elements are supplied with heating power.

[0171] The heating element 16 is located in a combustion chamber, which is also referred to as the heating chamber 12.

[0172] A temperature sensor 58 is disposed in the combustion chamber 12 or on the thermal insulation surrounding the combustion chamber 12, which ultimately measures the temperature at that location of the heating chamber 12 and is actually intended to sense the temperature of the dental restorations 20.

[0173] Typically, there is a significant distance between the dental restoration 20 and the temperature sensor 58, and therefore, due to the local temperature gradient in the heating chamber 12, the temperature sensor 58 is unable to accurately sense the temperature of the dental restoration 20, if at all.

[0174] The output signal from the temperature sensor 58 is fed to a comparator 60 which dictates the control, i.e., increases the temperature when the temperature set point is greater than the actual temperature value and ultimately reduces the heating power when the temperature set point is less than the actual temperature value.

[0175] A control loop 52 is also provided in the embodiment according to the invention with the block diagram shown in FIG. 5. There, a PID controller 54 is also used, and the power electronics 56 and the heating element 16 in the heating chamber 12.

[0176] However, measuring bridges are arranged on the output side of the power electronics 56, which are to detect the output voltage and the output current. A measured value “voltage” 62 and a measured value “current” 64 are detected and processed.

[0177] The measured values in question can also be tapped in the area of the lines between the power electronics 56 and the heating element 16, or adjacent to the heating element 16.

[0178] According to the invention, the measured values “voltage” 62 and “current” 64 are supplied to the compensation device 50. The compensation device 50 comprises a calculator 66 for calcu-lating the heater element resistance, based on the measured values for voltage 62 and current 64.

[0179] Further, the compensation device comprises a temperature determination device 68, which may be, for example, a lookup table 70.

[0180] The heating element resistance calculated in the computer 66 is supplied to the temperature determination device 68, so that on the output side the lookup table 70 or the temperature determination device 68 is provided with the actual temperature as an actual temperature value.

[0181] This detected actual temperature value is fed to the comparator 60 as in FIG. 4, and the control in this respect is carried out in the usual manner.

[0182] A further embodiment of a block diagram according to the invention is shown in FIG. 6. This is supplemented by the temperature sensor from the prior art and in this respect combines the invention with it.

[0183] As in the block diagram according to FIG. 4, a temperature sensor 59 is associated with the heating chamber 12. However, as can be seen from FIG. 6, it is arranged only partly in the heating chamber 12 and partly outside. In fact, a temperature sensor 59 can be used here that is less temperature resistant and is placed, for example, in the middle of the thermal insulation of the dental furnace. Such a temperature sensor 59 may cost, for example, ⅓ of what the temperature sensor 58 of FIG. 4 costs.

[0184] As in the block diagram of FIG. 5, the combination device 50 has a computer 66 and a temperature determination device 68. These have the same functions.

[0185] However, the output terminal of the temperature determination device 68 is not connected to the comparator 60, but is connected to an optimization device 70. The optimization device 70 is used to optimize the temperature estimation on the firing object.

[0186] The output of the temperature sensor 59 is connected to the optimization device 70.

[0187] In the illustrated embodiment, the optimization device 70 has another input port connected to a computer 72. The computer 72 calculates the actual power from the combination of the measured value “voltage” 62 and the measured value “current” 64, and feeds the calculated actual power to the optimization device 70.

[0188] The optimization device 70 outputs an actual temperature value which is fed to the comparator 60.

[0189] The setpoint/actual control according to the control loop 52 is cyclic in both the embodiment according to FIG. 5 and that according to FIG. 6. The control loop 52 can be cycled at constant time intervals, although it is also possible to shorten the time intervals when the temperature changes rapidly and to lengthen the time intervals when the temperature is more or less constant.

[0190] For example, measurement and control can be performed every 500 milliseconds.

[0191] FIG. 7 shows the control scheme in this respect. Within the compensation device 50, the lookup table 70 is read and thus the current actual temperature is output. The setpoint temperature 72 is set and the control difference between setpoint temperature and actual temperature is fed to the PID controller 54. The heating power is set by the power electronics 56. New current values 64 and voltage values 62 are measured based on the current output power of the power electronics 56 and the heater 16. The measured values are evaluated in the computer 66, and the resistance calculation is performed and the resistance value is updated. This is also done in the computer 66.

[0192] The determined new resistance value is fed to the lookup table 70, and in another 500 milliseconds the controlled system is run through again.

[0193] The lookup table, as an important part of the compensation device 50, is empirically determined in advance and allows a fairly accurate temperature determination An excerpted lookup table 70 is shown below.

TABLE-US-00001 Lookup table Resistance 4 Temperature heating heating elements in elements in micro-ohm degrees C. 26500 100.6 26600 101.8 26700 103.0 26800 104.2 26900 105.3 27000 106.5 160000 1214.6 160100 1215.3 160200 1216.0 160300 1216.7 160400 1217.3 212700 1600.5 212800 1601.3 212900 1602.1 213000 1602.9 213100 1603.7 213200 1604.6

[0194] FIG. 8 shows an example of temperature measurement during cooling according to the invention. It can be seen that a current pulse is regularly impressed into a heating element without heating current. The associated resistance value is measured.

[0195] Thus, after completion of the active firing process, a current pulse is impressed to measure the resistance of the heating elements and thus the temperature in the chamber. While the kiln has not yet fallen below a defined threshold temperature, the fans are set for cooling and no new program can be started.

[0196] FIGS. 9 and 10 show the compensation of the increasing resistance of the heating element connections. Due to aging or other effects—e.g. mechanical load, the resistance can increase.

[0197] This is compensated by periodically (e.g. every 3 months) measuring the resistance of the heating element and comparing it with that of a new heating element. An offset is calculated from the difference. According to FIG. 9, this offset is temperature-dependent. It is ever stored in the compensation device and thus taken into account when the actual temperature is recorded.

[0198] FIG. 10 shows that in this respect, almost complete compensation is also possible for aging effects.

[0199] When introducing an offset, which estimates the actual contact resistance of the connection technology, the following function can be used:

T_heating_elements=f(R_measured−R_offset).

[0200] The function f can again be stored as a lookup table.

[0201] By compensating the increased resistance due to aging (especially the connection technology) by an offset, the curves used and old lie on each other again. Of course, a more complex or complicated compensation than a simple offset can be used, e.g. to compensate for temperature dependent effects etc.