Heat-sensitive Record

Abstract

A heat-sensitive recorder using a thermal head, which is furnished with power in accordance with the speed of its movement relative to the recording sheet for the thermal control so as to obtain a uniform record quality.

Description

This invention relates to analog measurement recorders in and, more particularly, to recording apparatus for recording physical quantities converted into electric signals in the observation or recording of natural or man-made phenomena.

There are various prior art recording methods adopted in the recording means of recorders, including one using ink, electrostatic recording method, electrofax process and one using light-sensitive media. One of the major purposes of the recorder is to effect automatic recording of physical phenomema. However, this end of automatic recording cannot be truely attained with any of the above prior art recording methods because of the necessity of replenishing ink or other developing agents and such after-treatment as developing and fixing.

Further, the ink recording method, which is the most generally employed prior art recording method, has such a drawback that the recording sheet would be broken if a signal containing noise is recorded at a low speed (for instance, at a recording sheet feed speed of 5 .times. 10 .sup..sup.-3 mm/sec.). Secondly, in case of high-density recording covering substantially the whole area of the recording sheet, the operator's clothes as well as the instrument are likely to be stained with slowly drying ink. Thirdly, ink is very susceptible to the state of the ink pen and atmospheric pressure, often resulting in excessive or insufficient ink supply.

An object of the present invention is to provide a perfectly automatic recorder without requiring any replenishment of ink or developing agent and any after-treatment such as developing and fixing.

Another object of the invention is to provide a heat-sensitive recorder, which gives records of a uniform quality irrespective of changes of the speed of the thermal head.

A further object of the invention is to provide a heat-sensitive recorder, which gives records in different colors with a single recording head.

In order for the invention to be fully understood, it will now be described in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of the basic arrangement according to the invention;

FIG. 2 is a schematic showing an example of the control circuit for thermally controlling a recording head;

FIGS. 3 to 5 are perspective views, partly broken away, showing basic structures of the head;

FIG. 6 is a view to show the state of contact between head and heat-sensitive sheet;

FIGS. 7 to 9 show contours of the head tip;

FIGS. 10 to 12 are pictorial fragmentary sectional views showing heat-sensitive sheet structures;

FIG. 13 is a schematic showing an example of the control circuit for controlling pulse energy supplied to a head either according to an external signal or manually;

FIG. 14 is a graph showing relative record density and width of the record trace as functions of the recording speed of the recorder according to the invention;

FIG. 15 is a graph showing relative resistance change in various heads plotted against the record length change in various heads plotted against the record length traced; and

FIG. 16 is a graph showing the relation between heat attenuation time and relative heat generating layer area of various heads.

In the drawing, like reference numerals refer to like parts.

The invention is based upon the adoptation of a novel heat-sensitive recording method for the recording means of analog measurement recorders (hereinafter referred to as recorders).

The heat-sensitive recorder according to the invention comprises, as its recording means, a thermal head, a heat-sensitive recording sheet (hereinafter referred to as heat-sensitive sheet) and a control circuit for thermally controlling the thermal head.

While the mechanism for moving the thermal head according to an input signal supplied to the recorder is outside of the scope of the invention, an example of such mechanism will be described in connection with FIG. 1 for the purpose of demonstrating the basic arrangement according to the invention.

Referring to FIG. 1, numeral 1 designates a servo motor for moving a thermal head 6 through a stretched thread 2 attached thereto according to an input signal. Numeral 3 designates a synchronous motor for driving a drum 4 to feed a heat-sensitive sheet 5. Numeral 7 designates a record trace. Numeral 8 designates a circuit to superimpose pulses produced from servo motor and synchronous motor 3 in accordance with the distance traveled by the thermal head, numeral 9 a circuit to produce pulses of a constant pulse length according to the output signal from the circuit 8, the pulse output of the circuit 9 being supplied to the thermal head 6, and numeral 10 a power supply circuit. A disc 1a formed with a number of circumferentially spaced apertures is secured to the shaft of the servo motor 1, and a photoelectric converter 1b which co-operates with the disc 1a detects the speed of the servo motor 1. The speed of the synchronous motor 3 is detected by a detecting means 3a.

In this system, the synchronous motor 3 is run according to an X-axis input signal, or in an automatically balanced strip chart recorder it is run at a constant speed for time sweeping. This motor 3 serves to feed the recording sheet 5. The servo motor 1, on the other hand, is controlled according to a Y-axis input to cause the corresponding movement of the thermal head 6. The speed of the motors 1 and 3 is respectively detected by photoelectric converter 1b and detecting means 3a, whose respective outputs are synthesized in the circuit 8. The output signal of the circuit 8, which is based on the combination of the speeds of the motors 1 and 3, substantially represents the speed of the thermal head 6. This signal is coupled to the pulse generating circuit 9 to supply power to the thermal head 6 according to the speed thereof for the thermal control thereof. In this way, a record with a constant density may be obtained.

FIG. 2 shows an electric circuit used in the system of FIG. 1. A photo-diode 11a which is provided in the photoelectric converter 1b constitutes a photochopper 11, whose output is coupled to a shaper amplifier 12 consisting of transistors 12a and 12b. In case where the recording sheet 5 is fed at a predetermined constant speed, the detecting means 3a shown in FIG. 1 may include a select switch 15a to select a resistance under the control of a feed speed determining operation and a unijunction transistor 15b. A group of resistors 15c which are switched by the select switch 15a constitutes a charging path to charge a capacitor 15d. When a predetermined voltage is built up across the capacitor 15d, the unijunction transistor 15b is turned "on", causing the discharging of the capacitor 15d until the transistor 15b eventually becomes "off." In this way, the circuit 15 provides pulses at a predetermined frequency. The outputs of the circuits 12 and 15 are impressed upon a mono-stable multi-vibrator 13 consisting of transistors 13a and 13b, so that the multi-vibrator produces a pulse output, which is power amplified by an amplifier 14 for coupling across terminals 16 connected to the thermal head 6.

With the recorder of the above construction, it is possible to obtain a record of a uniform quality on the recording sheet at a recording speed ranging between 5 .times. 10.sup..sup.-3 and 3 .times. 10.sup.2 mm/sec. For multi-color recording, a multi-color heat-sensitive sheet to be described hereinafter, a plurality of thermal heads and the same number of thermal control circuits such as shown in FIG. 2 may be employed.

The thermal head (hereinafter referred to as head) is an essential element in this invention, and it will now be described in detail in connection with examples shown in FIGS. 3 to 5.

The head may be made from silicon used as base material. In this case, usual semiconductor techniques are used to diffuse phosphorus or boron into a P- or N-type silicon substrate 19 having a configuration as shown in FIG. 3, 4 or 5 to form a heat generating, resistive layer 18 such that a substrate portion 17 constitutes a heat generating area. The layer 18 is made to have P.sup.+-type conductivity if the substrate 19 is of N-type, or it is made to be of N.sup.+-type if the substrate is of P-type. The thickness of the layer 18 is suitably 1 to 40 microns. Then, electrodes 20 contiguous to the layer 18 are formed by depositing aluminum. Then, nickel and copper are deposited, and the resultant system is finally electroplated with copper.

It is also possible to use ceramics as the base material 19. In this case, 80 to 96 percent pure alumina (Al.sub.2 O.sub.3), forsterite (2MgO.sup.. SiO.sub.2), steatite (MgO.sup.. SiO.sub.2) and beryllium oxide (BeO) are suitable ceramics. In manufacture, heat generating layer 18 is formed in portion 17 by well-known means such as spattering and vapor phase deposition. Alternatively, the heat generating layer 18 may be a hesa coating film formed by spraying an aquaous blend solution containing stannic chloride (SnCl.sub.4) and antimony chloride (SbCl.sub.3) in a molarity ratio of 5 : 1 to 40 : 1 onto the ceramic material heated to a temperature of 400.degree. to 600.degree.C, or it may be formed by imprinting the portion 17 with silver palladium paint (manufactured by Du Pont Corporation) and sintering it a temperature of 600.degree. to 900.degree.C. Then, electrodes 20 contiguous to the heat generating layer 18 may be formed in the same way as in the above case of using silicon.

There often results a state of contact between head 21 and heat-sensitive sheet 22 as shown in FIG. 6. In order to better the record quality it is effective to prepare heads having round tips as shown in FIGS. 7 to 9. In FIGS. 7 to 9, numeral 17 designates a heat generating surface, numeral 19 a base material, numeral 18 a heat generating layer, and numeral 20 an electrode.

The heat-sensitive sheet is required to color or undergo a color change only at its heated portion. Generally, the coloring agent may be prepared by combining a leuco-die and a phenol compound or organic acid or combining an organic metallic soap and an organic reducing agent. As the leuco-die may be used crystal violet lactone (coloring blue) and phenylrhodamine lactone (coloring red). As the phenol compound bisphenol A is suitable. Suitable organic metallic soaps are ferric stearate and silver behenic acid, and suitable organic reducing agents are gallic acid and protocatechuic acid. As the dispersion medium and binder may be used polyvinyl alcohol, polyvinyl acetate and acrylate-vinyl acetate copolymers.

FIGS. 10 to 12 show examples of the heat-sensitive sheet structure. In these examples, the coloring agents are prepared from the combination of leuco-die and phenol compound.

In FIG. 10, numeral 23 designates a base sheet of fine quality paper of the order of 20 to 70 g/m.sup.2, and numeral 24 a binder such as mentioned above. Bisphenol A and crystal violet lactone are dispersed in the binder layer as respectively designated at 26 and at 25. The particles of the components 25 and 26 range between 1 and 5 microns in diameter, and their parts ratio is 1 : 10 to 1 : 25. The total quantity of coating is 3 to 7 g/m.sup.2. This example is a single-color heat-sensitive sheet, and it is rendered blue by one-second heating of it at a temperature of 90.degree.C.

The example of FIG. 11 is a two-color heat-sensitive sheet. It colors red when the one-second heating temperature is 90.degree.C, and it becomes green at a temperature of 100.degree.C. The particle diameter, parts ratio between leuco-die and phenol compound and total quantity of coating of the individual coating layers are similar to those in the FIG. 10 example. In FIG. 11, numeral 27 designates leuco-phenylrhodamine R (coloring red), and numeral 28 leuco-malachite green.

The example of FIG. 12 is a combination of the FIG. 11 and FIG. 10 examples. It is a three-color heat-sensitive sheet coloring blue at 90.degree.C, red at 100.degree.C and green at 110.degree.C.

For the two-color, three-color and other multicolor sheets, it is desirable from the standpoint of improving the color separation to provide between adjacent coating layers an intervening layer containing bisphenol A, stear amide, etc., dispersed in a binder.

It is the basic concept underlying the thermal control of the head according to the invention to make the quantity of heat supplied to a unit area of the heat-sensitive sheet constant irrespective of the recording speed. This end seems to be achievable by supplying thermal energy in the form of pulses to the head at a fixed rate, for instance 1 to 5 pulses per 1 mm of the relative movement of the head in both Y and X directions in FIG. 1. However, from the laws of the conduction of heat it is impossible to have all energy supplied to the head transferred to the heat-sensitive sheet.

Accordingly, it is considered to supply pulses at a greater rate with respect to the relative movement of the heat-sensitive sheet, for instance, in the X direction in FIG. 1 than that in the Y direction. By so doing, the energy supplied to a unit area of the heat-sensitive sheet may be eventually held constant irrespective of the recording speed. The pulse supply rate with respect to the X direction is, for instance, set to 10.sup.2 to 10.sup.5 pulses per one millimeter of progress of the heat-sensitive sheet, which is fed at a speed ranging between 5 .times. 10.sup..sup.-3 and 3 .times. 10.sup.2 mm/sec. The requisite pulse energy differs with the record color. By way of example, with the sheet of FIG. 12, 1 to 2 millijoules per pulse is necessary for coloring blue, 2 3 millijoules per pulse for red, and 3 to 5 millijoules per pulse for green.

The thermal energy transferred to the heat-sensitive sheet is insufficient for a certain initial period until a saturation is reached. This is inevitable even if the head is furnished with pulse energy at a constant rate under fundamentally excellent thermal control of the head insofar as the heat capacity of the head is not zero. The initial time until reaching of the stauration differs with the supplied pulse energy, and it is 5 to 20 seconds with the energy supply of 5 millijoules per pulse and 100 to 500 seconds with 1 millijoule per pulse. The variation of the record density due to this staturation time is negligible in the continuous use of the recorder, but where the recorder is used intermittently the instability of the record quality would give rise to various problems.

According to the invention, the record quality is stabilized through pulse length compensation based on the saturation curve. An example of such compensation for the record quality is illustrated in FIG. 13.

Referring to FIG. 13, block 13 is a circuit the same as the circuit 13 shown in FIG. 2 for controlling the length of pulses supplied to the head. Here the pulse length is determined by the resistances of resistor 32 and thermistor 29. The thermistor 29 is provided to obtain the compensation, and otherwise an ordinary resistor may be used in its place. When the power source voltage is applied to the circuit, the thermistor is heated by a heater 30, so that its resistance is reduced with time. The relation between resistance and time is adapted to make up for the aforementioned saturation time. In effect, the pulse length is increased for an initial stage, and it ultimately settles to a predetermined value when the saturation is reached.

This compensation method, unlike other methods such as one based on the detection of the head temperature and one where a thermistor heater is inserted in series with the head, requires no lead of any movable part and enables free adjustment of the saturation curve.

In addition to the capability of ensuring a constant record quality independently of the recording speed, it is possible with the thermal control of the head according to the invention to selectively switch different colors of recording with the same head. This function of switching recording colors will now be described in connection with the circuit of FIG. 13. In the Figure, numeral 31 designates a comparator to receive external color switch signals, and numeral 33 an amplifier.

In one specific example, the circuit of FIG. 13 was incorporated together with the circuit of FIG. 2 in a recorder having the mechanism of FIG. 1, and the silicon head of FIG. 3 and the two-color heat-sensitive sheet of FIG. 11 were employed. The pulse length of the pulse energy supplied to the head was controlled with thermistor 29 and variable resistor 32 shown in FIG. 13 such as to obtain recording in red color with 1.5 millijoules and recording in green color with 3.0 millijoules. In this example, the thermistor 29 suitably offers a resistance of 80 kiloohms at normal temperature and a resistance of 50 kiloohms after the saturation time is elapsed, and the resistance of the resistor 32 is suitably 50 kiloohms.

In operation, without any signal at input terminal 34 of the comparator the recording proceeds in red color. When a voltage higher than the voltage coupled to the comparator through voltage divider 35 appears at the input terminal 34, transistor 33 is cut off, so that the recording color becomes green. The recording colors may be freely switched by means of manual switch 36. Where a plurality of heads are used in a recorder, this switching function may of course be provided for all the heads.

With the heat-sensitive recorder as has been described in the foregoing, the aims of the recorder mentioned before in connection with the objects of the invention may be attained.

The heat-sensitive recorder having the basic construction as described before in connection with FIGS. 1 and 2 has the important feature that no maintenance is required at all until the recording sheet is used up. In addition, the record quality obtainable with it is superior to that obtainable with any other heat-sensitive recorder.

The recorder refers to the clearness of the record trace and uniformity of the width of the record trace within a recording speed range of 5 .times. 10.sup..sup.-3 to 3 .times. 10.sup.2 mm/sec. By way of example, it may be represented in terms of relative record density and width of the record trace related to the recording speed, as shown in FIG. 14. In the Figure, curves 38 and 40 represent characteristics of well-known heat-sensitive recorders employed for electrocardiographs and the like and using nichrome wire heaters, whereas curves 37 and 39 represent characteristics obtained in accordance with the invention. With these latter characteristics recording with constant resolution and density can be ensured irrespective of whatever changes occur in the magnitude of the signal to be recorded and recording speed.

As have been mentioned earlier in connection with FIGS. 3 to 5, there are various types of heads to be used in the preceding embodiment, and they are classed on the basis of material and structure. Of these heads, those made from silicon and formed with a heat generating superficial resistive layer are superior to others in the half-wave period, which will be described hereinafter.

The half-wave period is an index of the rising performance of the head. It is determined by setting the recorder such that a record density of 1.0 is obtained with the blue-color heat-sensitive sheet of FIG. 10 and the controlled head such as ones listed in Table 1 and at a recording speed of 60 mm/sec. and by interrupting and resuming the recording operation. It is the time required until the reaching of a record density of 0.5 from the instant of resuming the recording after the head has been cooled down to normal temperature.

Table 1 lists heads of various materials and configurations and their half-wave period. The half-wave period depends not only upon the head material but also upon the head structure. In average, it is about 3.2 seconds with the head configuration of FIG. 3, 7.7 seconds with the configuration of FIG. 4 and 10.2 seconds with the configuration of FIG. 5. Considering the average half-wave period with regard to the material of the head, it is 1.4 seconds in case of silicon heads having the configuration of FIG. 3 while it is 5.1 seconds in case of ceramic heads of the same configuration. Thus, superiority of the silicon heads in this respect is evident.

In the light of the fact that the performance of the head depends upon the thermal conductivity of the material of the head, the combination of the silicon material having the configuration of FIG. 3 and the heat generating layer formed by spraying is desirable from the standpoint of improving the performance of the head. However, the manufacture of such a head is industrially difficult. ##SPC1##

Remarks:

1. The material is what is indicated at 19 in FIGS. 3 to 5.

2. The heat generating layer is what is indicated at 18 in FIGS. 3 to 5.

3. The resistance between electrodes 20 in FIGS. 3 to 5 at normal temperature is meant.

4. The half-wave time is the time required until the reaching of half the normal value of the record density.

As have been mentioned above, there are heads of various materials and configurations that may be prepared for use in accordance with the invention, and those made of silicon are advantageous in view of the half-wave time. However, the heads of ceramic materials have a certain merit as will be described hereinafter.

The recorder according to the invention features freedom from maintenance other than the replacement of the recording sheet during recording. However, the tip of the head wears with increase of the number of replacements of the recording sheet, and the resistance of the head is ultimately increased to such an extent that the recording is no longer possible.

The service life of the head before the reaching of this state may be increased by covering the heat tip with a hard material such as silicon carbide and alumina. By so doing, no practical problem will be encountered even after the head has traced beyond a record length of 100 km.

However, it is difficult to know the remaining lifetime of the head in use. Toward the end of the life of the head, the resistance thereof increases progressively sharply, as shown in FIG. 15. This sharp change of the resistance is controllable since it is related to the thickness of the heat generating layer, and in the case of using ceramic materials a heat generating layer with a thickness of up to about 500 microns may be readily formed.

Curves 41 to 43 in FIG. 15 represent characteristics of respective heads, whose respective heat generating layers are 5 microns, 100 microns and 500 microns thick in the mentioned order and are all covered with spattered silicon carbide to a thickness of 4 microns. The resistance of all these heads begins to change when a record length of 102 km is covered, but the lifetime of or record length traced by the head until the recording is no longer possible differs with the thickness of the heat generating layer; the greater the thickness the ss the more the head is useful for the recorder. However, it is practically impossible to form as thick a heat generating layer as mentioned above in the case of silicon heads because of impurity diffusion techniques. In contrast, with ceramic materials it is very easy to form a heat generating layer having a sufficient thickness. By way of example, a heat generating layer with a thickness of 0.5 mm, which had been obtained by sintering a 0.58 -mm thick dry film of silver palladium (manufactured by Du Pont Corporation) formed by the screen printing method on alumina, showed the characteristic of curve 43.

The service life of the head may also be greatly extended by rounding the tip of the head. This effect has no bearing upon the material of the head, and this aspect will now be discussed in connection with FIGS. 7 to 9 and the graph of FIG. 15.

Curve 44 in FIG. 15 represents the characteristic of a silicon head having the configuration of FIG. 3 and a heat generating layer with a thickness of 1 micron. On the other hand, curve 45 represents the characteristic of different silicon heads, whose heat generating layer is also 1 micron thick, but which are manufactured by shaping the substrate into the contours of FIGS. 7 to 9 by the well-known electrolytic polishing technique before the impurity diffusion treatment. It will be evident from these curves 44 and 45 that the service life of the head greatly differs with the shape of its tip. It is thought that this effect depends for its analysis upon the state of contact between head and heat-sensitive sheet. With the head shape of FIG. 3 a contact state as shown in FIG. 6 would result, and only the edges of the heat generating surface would selectively wear, whereas with the head shapes of FIGS. 7 to 9 the whole heat generating surface will uniformly wear. This could be readily observed by magnifying the weared head tip.

In the grading or evaluation of the performance of the head in the recorder, the heat attenuation time at the time of suddenly reducing a certain high recording speed to zero is an important factor. It is a time interval from the instant of cutting the input to the head in use with the basic construction according to the invention by reducing the chart bias to the head to zero, in other words, only applying a trigger corresponding to the speed of the head and not applying a trigger corresponding to the speed of the recording sheet, until the record density in the normal recording at the recording speed, V = 200 mm/sec is reduced to one half. The shorter this time interval is, the higher the thermal efficiency of the head and hence the easier the thermal control of the head.

FIG. 16 shows values of the heat attenuation time that were obtained for various ratios of heat generating layer area to contact area of the heads listed in Table 1. In the figure, the number attached to each plot is the same as the rererence number of the corresponding head listed in Table 1. The empirical upper limit of the permissible heat attenuation time is 2.5 seconds, so that the area ratio should be less than 3 to obtain effective results.

Experimental two-channel and three-channel recording was conducted with the recorder according to the invention and using the multi-color heat-sensitive sheets of FIGS. 11 and 12. In the two-channel recording, records in red color were obtained at the low head temperature and records in a blend of red and green colors were obtained at the high head temperature. In the three-channel recording, the records obtained were in blue color at the low head temperature, in a blend of blue and red colors at the medium head temperature, and in a blend of blue, red and green colors at the high head temperature. In either case, the clearness was slightly inferior to that obtainable with two-channel or three-channel recorder using ink. However, the record quality was excellent because the record density and width of the record trace were steady and stable even with changes of the recording speed as are evident from FIG. 14.

As mentioned above, it is very advantageous to use multi-color heat-sensitive sheets, which constitute an essential element of the invention. It has also been mentioned earlier that leuce-dies and organic metallic soaps are useful as the main composition of the coloring agent used in the preparation of the uni-color or multicolor heat-sensitive sheets. The following description will concern with which one of these two types of materials is superior.

As mentioned earlier, with the heat-sensitive sheets having the structures of FIGS. 11 and 12 a particular color of recording is selectively obtained at a corresponding head temperature. When the recording is made at the low head temperature, only the uppermost or outermost layer undergoes coloring, so that a clear record may be obtained. However, when the recording is made at the medium or high head temperature, the record obtained is in a blend or colors of the individual excited layers, so that the clearness inferior to that of the ink recording.

In order to prevent this blending of colors, efforts have been paid to obtain heat-sensitive sheets, with which the uppermost layer in case of the structure of FIG. 11 or the uppermost layer and intermediate layer in case of the structure of FIG. 12 are decolored so that coloring of only the lowermost layer results in the recording at the high head temperature.

Of the two types of dies, leuco-dies are found to be effectively susceptible to the decolorizing action of polyethylene glycol, which is incorporated in each layer or applied between adjacent layers. With this decolorizer excellent results were obtained. For example, a heat-sensitive sheet having the structure of FIG. 11 and using leuco-phenylrhodamine for the lower coating layer and crystal violet lactone for the upper coating layer enabled to obtain clear records in blue color at the low head temperature and in red color at the high head temperature. Also, a heat-sensitive sheet having the structure of FIG. 12 and using the afore-mentioned dies respectively for the lower and intermediate layers and leuco-methylene blue for the upper layer enabled to obtain clear records in green color at the low head temperature, in blue color at the medium head temperature and in red color at the high head temperature.

As has been mentioned earlier in connection with the basic arrangement for the heat-sensitive recorder according to the invention, the head 6 shown in FIG. 1 is furnished with pulses provided on the basis of the signal produced from the servo motor 1 in accordance with the speed of the relative movement of the head.

When the size of the head is taken into consideration, the suitable rate of the pulse supply is 1 to 5 pulses per millimeter of the movement of the head. Meanwhile, the suitable width of the record trace is in a range of 0.3 to 0.5 mm. This means that the size of the heat tip, which may be either square or circular, is about 0.4 mm on the side or in diameter, respectively. With this size, solid record trace may be obtained at a pulse supply rate within the afore-mentioned range of 1 to 5 pulses per mm. At a pulse supply rate below this range, for instance at a rate of 0.5 pulse per mm, a broken record trace would result. On the other hand, at a pulse supply rate of 6 pulses per mm or above the output frequency of the photo-chopper 11 in FIG. 2 would reach 1.8 KHz at a head speed of 300 mm/sec. Such a high frequency, to which the head cannot respond, would lead to no effect.

The pulse frequency of the output signal of the circuit 15 in FIG. 2 ensures the supply of pulses to the head to cause the coloring of the heat-sensitive sheet even when the speed of the head in the Y direction becomes zero. Its suitable value is 10.sup.2 to 10.sup.5 pulses per mm when the afore-mentioned pulse supply rate according to the relative movement of the head ranges between 1 and 5 pulses per mm. If it is outside of this range, the record density in the X direction would be too high or too low in comparison to that in the Y direction when the input to the recorder becomes zero even if the pulse energy is so adjusted as to obtain a suitable record density in the presence of the input. Although such circumstances are only prone to fluctuations of the record density in the case of using a single-color heat-sensitive sheet, in the case of using a multi-color heat-sensitive sheet the color or hue of recording would also change, so that the correspondence between input signal and record would become indistinct.

As mentioned earlier, according to the invention clear records may be obtained by using single-color and multi-color heat-sensitive sheets. The selection of the record density and color of recording can be achieved by adjusting the pulse energy supplied to the head. To this end, a pulse energy range of 1 to 5 millijoules per pulse is enough for any one of the afore-mentioned heads. For example, with a silicon heads as shown in FIG. 1 and a multi-color heat-sensitive sheet using leuco-die as shown in FIG. 12 clear records could be obtained in green color with 1 to 2 millijoules per pulse, in blue color with 2.5 to 3.5 millijoules per pulse and in red color with 4 to 5 millijoules per pulse.

As mentioned earlier, the record density and color of recording are affected by the head temperature in the initial stage of recording before the normal recording condition sets in, and this drawback can be overcome by having the pulse length and pulse energy slightly increased during the initial recording stage. By this means, it is made possible to obtain a constant record quality without using any extra lead leading to the head and irrespective of whether the recording is made intermittently or continuously.

The basic arrangement according to the invention as mentioned earlier has a further merit in that threshold values of the input signal and time marks may be additionally recorded in a separate color in course of the recording by positively controlling the pulse energy.

With this function it is possible to know exact threshold values even where the recorder is controlled from a somewhat remote place, so that the monitoring efficiency can be improved. Also, when applied to a time marker it is possible to record time marks without causing any time delay of recording, so that records of improved reliability may be obtained.

In addition to the afore-mentioned various features of the invention, it is to be noted that it is very easy to change the number of heads used in the recorder according to the invention. This means that it is possible to set any given number of heads to the recorder.

Claims

What we claim is:

1. A heat-sensitive recorder comprising

a. a thermal head furnished with electrical power for heating,

b. a heat-sensitive recording sheet disposed in contacted with a heat generating surface of said thermal head,

c. means to move said thermal head and said heat sensitive recording sheet respectively,

d. first detecting means to detect the distance traveled by said thermal head and to produce first trigger signals at intervals proportional to said detected distance,

e. second detecting means to detect the distance traveled by said heat-sensitive recording sheet and to produce second trigger signals at intervals proportional to said detected distance,

f. a pulse generator receiving said first and second trigger signals and producing a pulse output at controlled average pulse frequencies, and

g. means to control power supplied to said thermal head according to the pulse output from said pulse generator.

2. A heat-sensitive recorder according to claim 1, wherein said thermal head consists of silicon and is provided with a superficial heat generating resistive layer formed by selectively diffusing such an impurity as phosphorus and boron into the silicon material.

3. A heat-sensitive recorder according to claim 1, wherein the heat generating surface of said thermal head is formed in the shape of a smooth curved-surface having a U-shaped cross section.

4. A heat-sensitive recorder according to claim 1, wherein the area of the heat generating surface of said thermal head is no greater than about three times the area of contact between said thermal head and heat-sensitive recording sheet.

5. A heat-sensitive recorder according to claim 1, wherein said heat-sensitive recording sheet is capable of changing the color of recording or record density, and which further comprises a control means to control power supplied to said thermal head such as to obtain records in various colors or with various record densities.

6. A heat-sensitive recorder according to claim 5, which further comprises means to compare the level of signals to be recorded and a particular reference level, the output signal from said comparing means being used to control said control means.

7. A heat-sensitive recorder according to claim 5, wherein said heat-sensitive recording sheet is a multi-color heat-sensitive sheet consisting of a base sheet and one or more coating layers formed on said base sheet and containing a leuco-die and a phenol compound or an organic acid as heat-sensitive coloring components.

8. A heat-sensitive recorder according to claim 5, which further comprises one or more additional thermal heads, and means to cause the movement of the individual thermal heads according to different input signals and supply energy of different levels to the individual thermal heads for recording in different colors.

9. A heat-sensitive recorder according to claim 1, wherein the rate of pulse supply with respect to the movement of said thermal head is within a range between 1 and 5 pulses per millimeter of movement of said thermal head and the rate of pulse supply with respect to the progress of said heat-sensitive recording sheet is within a range between 10.sup.2 and 10.sup.5 pulses per millimeter of progress of said recording sheet, the energy in each pulse being within a range between 1 and 5 millijoules.

10. A heat-sensitive recorder according to claim 1, which further comprises means to increase the pulse energy supplied to the thermal head during an initial recording period until normal recording conditions set in.