DD287779A5 - CIRCUIT ARRANGEMENT FOR TEMPERATURE MEASUREMENT - Google Patents

Abstract

The invention relates to a circuit arrangement for temperature measurement. The circuit arrangement is used for temperature measurement in exchange assemblies in which temperature sensors can be used for large scattering. They are divided into tolerance groups (measured) and each tolerance group is assigned a different resistance value. The resistor R is connected in parallel with the temperature controller TF and is determinative of the discharge time of the capacitor C, which thus gives an indication of the tolerance group to which the respective temperature controller TF used belongs. Temperature sensor TF and resistor R are housed in the replacement module BG. Adjustment procedures after module replacement are no longer necessary. Fig. 2 {temperature measurement; Exchange assembly; Temperaturfuehler; large spread range; Tolerance group classification; Coding; Recognition of the tolerance group; controllable astable multivibrator; CPU evaluation}

Description

For this 3 pages drawings

Field of application of the invention

The invention relates to a circuit arrangement for measuring temperature, in particular an assembly or a particularly thermally stressed during operation component to control the power consumption in this module so that a certain temperature is not exceeded.

Characteristic of the known state of the art

Thermocouples, platinum resistors, thermistors, but also integrated temperature-current transformers or temperature-voltage transformers are usually used as temperature sensors that convert the temperature into an electrical quantity proportional to the temperature. These sensors convert the temperatures into an analog electrical quantity, e.g. Voltage, current or resistance, which can not be processed directly by a microprocessor (CPU). Either the analog electrical quantity has to be converted into digital signals by means of an analog-to-digital converter, or a circuit following the sensor converts the analog electrical quantity into a time or frequency, thereby making possible the trouble-free further processing in a CPU.

An example of such an application is a printhead for a dot matrix printer, in which very large impulse powers are implemented in a small volume, especially in large nadol number and graphics operation, which is possible for a short time, but would lead to the destruction of the head in the longer term, if the effective printing performance is not generally made small. In order to make optimal use of the possible performance parameters, it is therefore necessary to give the control unit (eg CPU), which controls the head in the printer, information about the current head temperature, so that the effective pressure performance can be controlled accordingly. The head temperature is therefore to be measured and the measurement signal in a usable form for the CPU on this we ererzuloiten.

Many circuit arrangements have been described in which a temperature sensor measures the printhead temperature and converts it into a usable form for a CPU. In DD-PS 252797 a circuit arrangement is described, whose essential components also uses the vorliegem e invention. This circuit is used to control the Heizimpulne the heating elements of a * Thermo Jruckers as a function of the temperature of the print head. In this case, the temperature sensor used is a Temperatu .'- current Wt trader with a capacitor connected in series with him, the temperature-dependent current during the sensing process, the capacitor loads and delivered with the aid of a downstream integrated timer pulses in temperature-dependent width.

However, a precise measurement requires a temperature sensor with high measuring accuracy. Such precise sensors are more expensive than those whose accuracy is associated with tolerances up to ± 20%. The latter can only be used for accurate measuring purposes if an adjustment is made, so that the respective transformer characteristic curve approaches the ideal characteristic as far as possible. The disadvantage of this is when the temperature sensor is permanently installed in the module whose temperature is to be measured, and this module must be easily replaceable without additional adjustment operations, as the z.3. desirable in a printhead.

Object of the invention

It is an object of the invention to allow the Einsati cheaper and scattered within relatively large limits temperature sensor and still to reach high accuracy.

-2- 287 779 Presentation of the Essence of the Invention

It is an object of the invention to provide a circuit arrangement which processes the dispersion of the temperature sensor so that the output signal characterizes the correct temperature of the assembly to be filled with reasonable tight tolerances and thus the interchangeability of the assembly including temperature sensor is ensured without making a comparison without subsequent trim operations.

According to the invention, the object is achieved in that the temperature sensor is connected in parallel in a controllable astable multivibrator with a resistor connected in series with a diode. This determines the discharge time of the capacitor. The resistance value is in each case assigned to the tolerance group into which the respective temperature sensor - after previous measurement - has been grouped. Each tolerance group is therefore assigned a different resistance value. Thus, the discharge time of the capacitor is different for each tolerance module, and the output signal of the downstream timer circuit has low-Zoiten different lengths. The pulse length during the capacitor discharge is thus an indicator of the associated Toleranzgruppa. The charging tent of the capacitor from about 0 volts to the Schv / alfwertspannung on the timer circuit is the indicator for the measured temperature.

The resistance is - as well as the temperature sensor - housed in the replacement module. An advantageous embodiment of the invention is achieved by the application of an integratedi. Type B511N temperature-current converter as temperature sensor and integrated timer type B 555.

Ausfuhrungsbeisplel

The invention will be explained below using an exemplary embodiment. In the accompanying drawing show

Fig. 1: the transducer characteristics of some temperature sensors (B511 N);

2 shows a simplified representation of the solution according to the invention;

3 shows the signal course during the temperature measurement;

Fig. 4: the circuit arrangement according to the invention for the selected embodiment.

As an embodiment of the application of the invention, a dot matrix printer is selected in the printhead, especially in large numbers of needles and graphics operation very large momentum powers are implemented, which bt i longer duration leads to the same overheating of the printhead and the destruction thereof. It is therefore necessary to provide the control unit (CPU) with information about the current head temperature so that the eruptive pressure can be controlled accordingly.

To measure the printhead temperature, a B511N temperature-current converter is used in this example. This integrated circuit provides an output current that is directly proportional to the absolute temperature. The temperature coefficient is 1 μA / K ± 20%. In the trade this IS is delivered in five accuracy classes: B511 Nm and B 511N1 to B511N4. The least scattered class (± 1%) is B 511 Nm with an absolute error of S ± 3 K at 25 ⁰ C (298.2K), ie it spreads between 295.2pA and 301.2uA. The B511N3 scatters between 30O, 2uA and 33OuA at 25 ° C. This scatter is still too high for the present application.

For example, to use this class of accuracy for this embodiment, the temperature sensors of this class of accuracy must be divided by measuring in other finer tolerance groups, for example, in five groups. From Fig. 1, the characteristics of some of these 5 tolerance groups and the ideal characteristic of B511 Nm (1 uA / K) can be seen. Each of these five tolerance groups has a tolerance of ± 1% and scatters, for example, at 25 ⁰ C (298.2K) by an average ± 3uA, the fifth Toloranzgruppe, for example by an average of 327.1 μΑ ± 3uA, ie 324.1 μΑ to 33OuA; at 100 ⁰ C (373.2K), the scatter from 405.5uA to 413μΑ is around an average of 409.2μΑ. In FIG. 1, this is represented by the hatched region of the characteristic curve (approximately 1.1 μA / K). Similar conditions exist in the other tolerance groups. In Fig. 1 z. B. still the characteristic of the second tolerance group (about 1.04 μΑ / Κ) dui ch a dashed line indicated that at 25'C has an average of 309 uA and at 100 ⁰ C has an average of 387 uA. If one now uses temperature sensors of the tolerance groups 0 to 5 for temperature measurement, it must always be taken into account for determining the actual temperature to which the tolerance groups of the temperature sensor used in the specific case belongs. Causes the temperature sensor z. B. at a temperature measurement, a current of 387 μΑ, corresponds to that of a temperature sensor from the tolerance group two a temperature of 100 ⁰ C (373.2 K), when belonging to the tolerance group five but only 79 ⁰ C (352.2K). The circuit arrangement according to the invention must take these conditions into account. Thus, in addition to the measured value, it is also necessary for the evaluating control unit (for example, CPU) to transmit, with each temperature measurement, a logical signal which points to the associated tolerance group of the sensor. FIG. 2 shows a simplified circuit arrangement according to the inventive solution. It represents an oinon controllable astable multivibrator whose essential components are a temperature sensor TF, an integrated timer IZ and a capacitor C. Parallel to the temperature sensor TF a resistor R is geschaltot, which is connected to a diode D in series. Each tolerance group is assigned a specific resistance value. About this resistor R, the capacitor C is discharged during the measuring process. During this time, the temperature sensor TF locks. Durcl. the different resistance values result in different discharge times. Jie are indicative of the tolerance group to which the temperature sensor used TF belongs.

The charging of the capacitor C in the measuring phase takes place by the current proportional to the temperature of the temperature sensor TF, during this time, the resistor R is decoupled by the diode D. Introduced the Moßzeit Im (Fig. 3a) by the Sleuorspannung U ₁ , the transistor T, which assumes such a Wed at the beginning of the measurement phase that the transistor T blocks and thus the capacitor C is no longer short-circuited and can now charge , The charging time tm (Fig. 3c; 3o) until reaching the threshold word voltage U " ₍ is inversely proportional to the measured temperature.) The charging time t _H i and the discharging time t _L are supplied as logical signals to the CPU based on previously stored tables

From the signals, the true temperature is determined, the In the measured assembly BG, in the present example in the

Printhead, prevails, In the table is stored, which unloading times t _L assigned to the tolerance groups used

and which temperatures in the respective tolerance group corresponded to the charging times t _H i.

Figure 3b and 3d show the course of the capacitor voltage U, the measuring process and Flg.3c and 3e the associated Signal curve of the external voltage U, with two different temperature measurements. It can be seen that in both Measurements a temperature sensor of the same tolerance group was used, since the discharge time t _{L is} the same in both cases

or in the same Zeliboreich. In the second case (figures 3d, 3e), however, the measured temperature is higher and consequently the

Charging time t _H) shorter. In Figure 2 it is indicated that the resistor R together with the temperature sensor TF in the assembly to be measured BG

is housed. Both components werc'.in so replaced with the assembly (eg printhead), so that it is ensured that each'm temperature sensor TF is also assigned the correct resistance R. This does not require any replacement after the exchange

Trim operations. The concrete circuit arrangement for the described embodiment is shown in FIG. Therein are as temperature sensor TF

an integrated temperature-current converter type B 511N and used as an integrated timer IZ a type B 555 IS.

The exact completion of the B555 is apparent. The operation of this circuit arrangement in Measurement is as follows. In the idle state, when no temperature is to be measured, the Steuerspannurig U _{11 is} at high level, the transistor T

is turned on, the capacitor C discharged, and the outputs (terminals 3 and 7) of the integrated timer IZ are at high potential.

If the temperature is to be measured, the control voltage U _{11 is} switched to low by the CPU, whereby the Transistor T is blocked. This starts the charging phase of the capacitor C, which with the temperature-dependent Constant current is charged by the temperature sensor TF. The diode D is locked during this phase, so that over the Resistor R no contribution is made to the capacitor charging. The capacitor C is charged from about 0 volts to the threshold voltage U ₁₁ I. If the condition U _c - U "t is fulfilled,

the outputs (terminals 3 and 7), which until then still had high potential, switch to low. This is the the

Constant current and thus the temperature reversed proportional charging time tHi expired. The resistor R1 is used for Current limit for the output 3 of the integrated timer IZ, the resistor R 2 is the load resistance for the open- Collector transistor mounted at output 7 of integrated timer IZ to ground. After expiration of the charging time t _H i, the discharge time t _{lf begins} when U "is still held low and the transistor T is blocked

remains. W / During the discharge time ti. The outputs (terminals 3 and 7) lead from the integrated timer IZ low-potential. Since the

Capacitor C is still charged, the temperature sensor TF is operated with incorrect gopolter operating voltage, causing TF

locks. The capacitor C is connected through the resistor R, the now directionally poled diode D and the ground

Switched output 3 of the integrated timer IZ discharged. Is the capacitor voltage U _c to half the value of U "(dropped, both outputs switch from the integrated timer IZ back to high potential, the discharge time is ti

so that expires, and a recharging process begins, unless, as shown in Fig. 3a to 3e, the measuring process is stopped by U _{11 is switched} to high level. This mode of operation is always used because the charging time of the capacitor C is from about 0 volts to U " ₍ more exact value than the charging times of 0.5 U" ₍ to U " ₍ due to the

How TF works after the polarity reversal. The discharge time ti does not depend on the temperature-dependent constant current, but on the time constant T = R-C and on U t and the diode voltage Uf, and can be influenced separately over the size of R. The discharge time ti the Output voltage U, as well as the charging time t _H i is measured by the CPU. The output voltage U ₁ thus contains in

the charging time t _H) the information about the temperature to be measured and in the discharge time t _L the information which

Resistor R is assigned to the temperature sensor TF, i.e., to which tolerance group the temperature sensor TF belongs. The resistors R are to be chosen so that there is certainly no overlap of the t _L -Zoiten adjacent groups

comes. This requirement limits the number of tolerance groups into which the temperature sensors TF can be divided

The circuit is sensitive to changes in the threshold voltage U " ₍ so that at usual tolerances of Operating voltage U _B of 5 to 10%, the threshold voltage U "» must be generated separately. U " ₍ is as big as possible, but

to choose so that when charged capacitor C still enough voltage across the temperature sensor TF than

Operating voltage drops. It is advantageous to design the circuit for generating so that U _ra i in a small area

is adjustable, which can compensate for the capacitor tolerance in a one-time adjustment process.

Of course, the solution according to the invention is not bound to the use of a type B511N probe. Conceivable

For example, thermistors are more scattered. In this case, a diode would have to be connected in series with the thermistor so that the discharge current does not flow across the thermistor.

Claims (3)

1. A circuit arrangement for measuring the temperature of an assembly with a temperature sensor that converts the temperature into a temperature-proportional electrical variable and is the part of a controllable astable multivibrator whose time-determining capacitor is charged in the charging phase by the temperature-dependent current of the temperature sensor and a the respective temperature of the module characterizing signal generated, characterized in that for determining the discharge time of the capacitor (C) the temperature sensor (TF) with a diode (D) connected in series resistor (R) is connected in parallel, wherein the resistance value of the tolerance group is assigned to which the temperature sensor (TF) was grouped after previous measurement. 2. A circuit arrangement according to claim 1, characterized in that the resistor (R) housed together with the temperature sensor (TF) in the assembly (BG) and with the assembly (BG) is interchangeable, whose temperature is to be measured. 3. A circuit arrangement according to claim 1, characterized in that as temperature sensor (TF) an integrated temperature-current converter of the type B 511N is provided.