DE102011112455A1 - Method for supplying power to LED in e.g. camera for image processing system, involves controlling output voltage of switching regulator by measured voltage drop as feedback signal during measuring period - Google Patents

Abstract

The method involves controlling supply voltage (Vs) by a switching regulator (8). Voltage drop occurred at a transistor (4) is measured on a lightning impulse of an illumination source (1) based on current flow through the transistor during a measuring period using a differential voltage amplifier, where the measuring period corresponds to duration or portion of duration of the current flow through the transistor. Output voltage of the switching regulator is controlled by the measured voltage drop as a feedback signal (FB) during the measuring period. The transistor is designed as a bipolar transistor, a FET or a MOSFET. The switching regulator is designed as a downward controller, an upward controller, a transformer, a push-pull controller or a single-ended primary-inductor converter (SEPIC) circuit. An independent claim is also included for an electronic circuit.

Description

The invention relates to a method and an electronic circuit for power supply for a pulsed illumination source, in particular for an image processing system with a pulsed LED illumination.

In industrial image processing, a pulsed light source, often called a stroboscope or stroboscopic light source, is often used for illumination. With exposure times of the image sensors of typically 1 millisecond and below and image repetition times of typically several 10 to 100 milliseconds, there are a number of advantages, such as better energy efficiency, lower power consumption, lower heat generation, longer life of the light source, during the pulse duration higher light output, more compact design, etc For example, in a typical dimensioning of the periodic illumination flashes with a length of 41 μsec and a pulse frequency of 2.4 kHz using LED lighting, the energy consumption is only 10% compared to the continuous light. The heat loss of the lighting drops and its life increases accordingly.

For example, in the DE 10359723 A1 described the stroboscopic illumination of a wafer for evaluation with an image processing system.

As a flash source for stroboscopic lighting, a variety of light sources can be considered. Preferably, the illumination flashes are generated by means of an LED as a flash light source, which may preferably be current-controlled or current-controlled.

In order to keep the luminous flux as constant as possible during a flash, regardless of specimen scatters and other parameters such as temperature, the prior art usually makes use of current regulation of the current flowing through the LEDs by means of a transistor (bipolar transistor or field effect transistor) ,

More modern light-emitting diodes now have power consumption of 3 W, 5 W, 10 W and as multiple elements, ie. H. with several in series and / or parallel operated light-emitting diodes, sometimes up to 30 W. Mainly because of the specimen scattering and the temperature drift of the LEDs must be used for safety with a considerably higher than nominally necessary supply voltage for the LEDs, so that the current control thus constructed despite the Component tolerances works. The increased supply voltage for this reason, however, leads to considerable power dissipation on the transistor.

As an alternative, there are a number of integrated circuits that can be used to build switching regulators with significantly better efficiency. These circuits also have a current control for the LEDs. In light emitting diodes a pulse width (PWM) method for brightness control is known, which works with periodic pulses. An integrated module with which such a method can be realized is, for. For example, the LM3402 from National Semiconductor.

The disadvantage of these circuits is that because of the use of a coil (storage choke) and a capacitor at the output of the switching regulator of the current available for the LED current for the application as pulse lighting can not be built up fast enough. If the capacitor is dispensed with, the circuits work faster, but have an unwanted current ripple and therefore no constant illuminance.

Based on this prior art, the present invention has the object to provide a method and a corresponding electronic circuit for powering a pulsed illumination source, in particular for an image processing system with a pulsed LED illumination, which is suitable as a stroboscope for image processing , The illumination sources should be able to be pulsed in the range of a few microseconds to a few seconds. The current through the illumination sources should be as constant as possible for the entire pulse duration. In addition, the entire circuit of a corresponding electronic circuit should have the lowest possible own power loss.

This object is achieved by a method or an electronic circuit with the features of the attached independent claims. Preferred embodiments and further developments of the invention will become apparent from the dependent claims and the following description with accompanying drawings.

An inventive method for powering a pulsed illumination source for pulse illumination for an image processing system, wherein an object is illuminated by means of the pulsed illumination source with flash pulses and the image processing images of the illuminated with the flash pulses object are recorded by means of an image sensor, wherein the illumination source by means of a supply voltage with Powered and to trigger a Lightning impulse is driven by means of a transistor comprehensive current control or current control, so has the peculiarity that the supply voltage is controlled by a switching regulator, which at a light source emitted by the lightning pulse due to the current flow through the transistor at the transistor voltage drop during a measurement period, which corresponds to the duration or a portion of the duration of the current flow through the transistor, is measured with a differential voltage amplifier, and the output voltage of the switching regulator is controlled during the measurement period by means of the measured voltage drop as a feedback signal.

A corresponding electronic circuit according to the invention for supplying power to a pulsed illumination source for pulse illumination for an image processing system in which an object is illuminated with flash pulses by means of the pulsed illumination source and images of the object illuminated with the lightning pulses are taken by means of an image sensor for image processing, wherein the illumination source by means of a Power supply voltage and is driven to trigger a lightning pulse by means of a transistor comprising current control or current regulation, comprising a Vorspannungspannungsschaltung and a current control or current control, comprising a transistor for triggering a lightning pulse to a connected to the electronic circuit illumination source has the particularity on, that it comprises a switching regulator for regulating the supply voltage, a differential voltage amplifier, with which at a voltage pulse emitted by the illumination source due to the current flow through the transistor at the transistor voltage drop during a measurement period corresponding to the duration or a portion of the duration of the current flow through the transistor, and is designed such that during the measurement period means the measured voltage drop as the feedback signal, the output voltage of the switching regulator is controlled.

In the invention, therefore, the supply voltage can be regulated by the voltage regulator and a feedback signal controlling it to the actual value required for the operation of the illumination source by measuring the difference voltage across the transistor as a voltage drop across the transistor during the current flow through the transistor and measuring this voltage as Feedback signal is used. As a result, the power loss of the power electronics can be set to a minimum value during the measurement period during the generation of a light flash. Outside the measurement period, the supply voltage need not be particularly regulated, because during this time no or no high current flow through the transistor and therefore no significant power loss occurs.

Accordingly, it is provided according to an advantageous feature that the switching regulator generates an output voltage when the illumination source is switched off, which is provided with a safety margin as a voltage increase over the voltage required for triggering a flash with the illumination source, and during the measurement of this excessive output voltage the feedback signal is reduced.

Advantageously, it is further provided that by means of the feedback signal, the output voltage of the switching regulator is controlled during the measurement period such that the voltage drop across the transistor assumes a predetermined, required for the function of the transistor for current regulation residual voltage. In other words, it can also be provided that the output voltage of the switching regulator is controlled during the measurement period by means of the feedback signal in such a way that a minimum residual voltage required as a current source is maintained across the transistor. In this way, the minimum power loss is achieved.

The invention can be used in an intelligent image processing sensor or image processing sensor with ergonomic illumination. This includes a camera or smart camera which has an electronic circuit according to the invention integrated in the camera and optionally one or more integrated pulsable illumination sources.

In the prior art cameras are known, which are referred to as smart cameras or as smart cameras. They are characterized by the fact that they contain one or more imaging chips in the form of CCDs or CMOS sensors, a processing unit with memory, for. B. in the form of a microprocessor or DSPs, as well as various interfaces for input / output such. Eg serial interfaces, network connection, PLC interface and video output. If such a camera combined with optics and lighting or firmly connected in a housing, it is called an image processing sensor or "Vision Sensor". Since the measurement or object illumination in such cameras is often in the form of pulsed LEDs, the invention can be used advantageously here.

• – der Schaltregler bei ausgeschalteter Beleuchtungsquelle eine Ausgangsspannung erzeugt, die mit einer als Sicherheitsreserve dienenden Spannungserhöhung über der Spannung liegt, die für das Auslösen eines Blitzes mit der Beleuchtungsquelle erforderlich ist, und während der Messdauer diese überhöhte Ausgangsspannung durch das Rückkopplungssignal reduziert wird.
• – Die Schaltung ist derart ausgebildet, dass mittels des Rückkopplungssignals die Ausgangsspannung des Schaltreglers während der Messdauer derart geregelt wird, dass der Spannungsabfall an dem Transistor eine vorgegebene, für die Funktion des Transistors zur Stromregelung erforderliche Restspannung einnimmt.
• – Die Schaltung ist derart ausgebildet, dass mittels des Rückkopplungssignals die Ausgangsspannung des Schaltreglers während der Messdauer derart geregelt wird, dass über dem Transistor eine minimale zur Funktion als Stromquelle erforderliche Restspannung erhalten bleibt.
• – Die Schaltung ist derart ausgebildet, dass die Messdauer gleichzeitig mit dem Auslösen eines Blitzimpulses oder hierzu geringfügig verzögert beginnt.
• – Die Schaltung ist derart ausgebildet, dass der Spannungsabfall direkt an dem Transistor gemessen wird.
• – Die Schaltung ist derart ausgebildet, dass der elektronische Schalter außerhalb der Messdauer die Ausgangsspannung des Schaltreglers und während der Messdauer die von dem Differenzspannungsverstärker gelieferte Spannung als Rückkopplungssignal zum Regeln der Ausgangsspannung des Schaltreglers an den Schaltregler schaltet.
• – Der Transistor ist ein Bipolartransistor oder ein Feldeffekttransistor.
• – Der spannungsgesteuerte Schaltregler ist als Abwärtsregler, als Aufwärtsregler, als Aufwärts-/Abwärtsregler, mit Transformator und Gegentaktansteuerung, als Flyback-Schaltung oder als SEPIC-Schaltung ausgebildet.

Further advantageous features of an electronic circuit according to the invention can be the following:

• - The switch regulator generates an output voltage with a backup power serving voltage increase, which is required for the triggering of a flash with the illumination source with the backup source, and during the measurement period, this excessive output voltage is reduced by the feedback signal.
• - The circuit is designed such that by means of the feedback signal, the output voltage of the switching regulator is controlled during the measurement period such that the voltage drop across the transistor assumes a predetermined, required for the function of the transistor for current regulation residual voltage.
• - The circuit is designed such that by means of the feedback signal, the output voltage of the switching regulator is controlled during the measurement period such that above the transistor a minimum required for functioning as a residual current source voltage is maintained.
• - The circuit is designed such that the measurement period starts simultaneously with the triggering of a lightning pulse or slightly delayed.
• - The circuit is designed such that the voltage drop is measured directly on the transistor.
• - The circuit is designed such that the electronic switch switches the output voltage of the switching regulator and during the measurement period supplied by the differential voltage amplifier voltage as a feedback signal for controlling the output voltage of the switching regulator to the switching regulator outside the measurement period.
• - The transistor is a bipolar transistor or a field effect transistor.
• - The voltage controlled switching regulator is designed as a buck regulator, as a boost regulator, as an up / down controller, with transformer and push-pull control, as a flyback circuit or SEPIC circuit.

The invention will be explained in more detail with reference to an embodiment shown in FIGS. The features described therein may be used alone or in combination with each other to provide preferred embodiments of the invention. Identical or equivalent parts are denoted by the same reference numerals in the various figures and usually described only once, although they can be used advantageously in other embodiments. Show it:

1 an electronic circuit according to the prior art and

2 an inventive electronic circuit.

The 1 shows an electronic circuit for powering a pulsed illumination source 1 , The illumination source 1 here includes several LEDs connected in series 2 , They are supplied by a supply voltage circuit, not shown, with a supply voltage Vs. At low power consumption of the LEDs 2 , small pulse / pause ratio or low efficiency requirements of the drive circuit is used to trigger a lightning pulse from current regulation by the LEDs 2 flowing electricity i made use of. The current regulation takes place by means of a measuring resistor 3 for the current measurement, of a transistor 4 , in this example a MosFet, and a differential amplifier 5 , at whose one input a control signal 6 is applied for the setpoint of the current i. In some cases it will also affect the differential amplifier 5 waived.

Mainly because of the specimen scattering of the LEDs 2 and the temperature drift of the LEDs 2 For safety's sake, the supply voltage Vs for the LEDs must be considerably higher than the nominal voltage required 2 be worked, so that the thus established current regulation works. The increased supply voltage Vs leads to partially considerable power dissipation at the transistor 4 ,

The following calculation example illustrates this. When the lighting source 1 four LEDs in series 2 with 3.5 V voltage drop per LED 2 and the current i through the LEDs is 2 A, the nominal power consumption of the LEDs is 4 × 3.5 V × 2 A, ie a total of 28 W. With a planned safety reserve for the supply voltage Vs of 1 V per LED 2 plus 2 V for the MosFet, ie a total of 6 V for safety reasons voltage overshoot of the supply voltage Vs, the circuit is thus supplied with 20 V instead of 14 V. In the MosFet this results in a power loss of 2 A × 6 V = 12 W, which is converted into heat. The efficiency of the circuit for current regulation is 28 W / (28 W + 12 W) = 28 W / 40 W = 70%.

The 2 shows an electronic circuit according to the invention 7 for powering a pulsed illumination source 1 , As a source of illumination 1 is preferably an LED 2 used. The illumination source 1 also includes several LEDs connected in series 2 , which are supplied by a supply voltage circuit, not shown, with a supply voltage Vs. In general, a lighting source 1 can be used, which comprises a plurality of illumination sources, which are connected in series and / or in parallel. The transistor 4 for controlling the current i through the LEDs 2 can in particular a bipolar transistor or a field effect transistor.

As in 1 takes place at the electronic circuit 7 a current control of the through the LEDs 2 flowing current i by means of a measuring resistor 3 for the current measurement, of a transistor 4 , in this example also a MosFet, and a differential amplifier 5 , at whose one input a control signal 6 is applied for the setpoint of the current i.

The circuit 7 also includes a switching regulator 8th for regulating the supply voltage Vs. Advantageously, this can be a voltage-controlled switching regulator 8th which is designed as a buck regulator, as a boost regulator, as an up / down controller, with transformer and push-pull drive, as a flyback circuit or SEPIC circuit. Such switching regulator 8th are referred to with associated power supply Vs as a switching power supply.

The switching regulator 8th generates an output voltage, initially with a safety margin over that actually for the illumination source 1 required voltage is. In the calculation example 1 if this were 6 V overvoltage, so the switching regulator 8th with LEDs off 2 provides an output voltage of 20V. The LEDs 2 are initially turned off, z. B. in that the SOllwertvorgabe for the current i, ie the control signal 6 for the setpoint of the current i, is at 0.

As soon as the setpoint value increases to the nominal value, it causes a flash pulse with the LEDs 2 the illumination source 1 immediately trigger the nominal current through the LEDs 2 with a delay time only due to the transit times through the differential amplifier 5 and the transistor 4 (MosFet) is limited. These terms can technically be well under a microsecond.

Because the circuit 7 initially with an overvoltage works as a safety reserve, while maintaining the high output voltage of the switching regulator 8th also when triggering a lightning impulse the power loss at the transistor 4 getting too high in the long run. To prevent this, an electronic switch is activated simultaneously with the beginning of the light pulse or slightly delayed 9 in the circuit for the feedback signal FB (feedback) in the form of a voltage feedback from the pure voltage measurement of the output voltage (left position) switched to the differential voltage measurement between the source and drain of the MosFet. For this purpose, a further differential voltage amplifier is as a measuring amplifier 10 present, which measures this voltage on the MosFet and so far amplifies that the signal as a feedback signal FB for the switching regulator 8th suitable is. The output voltage of the switching regulator 8th is then controlled by means of the feedback signal FB so that remains above the MOSFET a minimum required to function as a residual current residual voltage.

The triggering of a lightning pulse on the transistor 4 occurring voltage drop is preferably directly on the transistor 4 measured. This can for example be like in 2 shown in such a way that the difference voltage between the corresponding inputs and outputs of the transistor 4 (eg emitter / collector or source / drain) is measured, ie both poles of the voltage measurement are directly at the transistor 4 , In other embodiments, the direct measurement of the voltage drop across the transistor 4 also take place in such a way that only one pole or both poles of the voltage measurement include a measuring resistor, which lies in the current path i. In 2 For example, the bottom pole of the differential voltage measurement could not be applied to the transistor 4 but instead connected to earth, causing the voltage drop across the measuring resistor 3 would be included in the differential voltage measurement by the current i.

The supply voltage Vs is thus from the switching regulator 8th regulated. The at one of the illumination source 1 emitted lightning pulse due to the current flow through the transistor 4 on the transistor 4 resulting voltage drop is during a measurement period, the duration or a portion of the duration of current flow through the transistor 4 corresponds to a differential voltage amplifier 10 measured. During the measuring period, the output voltage of the switching regulator is determined by means of the measured voltage drop as the feedback signal FB 8th regulated. The switching regulator 8th generated when the illumination source is switched off 1 an output voltage that is above the voltage with a margin of safety increase, which is used to trigger a flash with the illumination source 1 is required, and during the measurement period, this excessive output voltage is reduced by the feedback signal FB. The electronic switch 9 switches the output voltage of the switching regulator outside the measuring period 8th and during the measurement period, those from the differential voltage amplifier 10 supplied voltage as a feedback signal FB for controlling the output voltage of the switching regulator 8th to the switching regulator 8th ,

The measurement duration may preferably start simultaneously with the triggering of a lightning pulse or slightly delayed. Because of the usually at the output of the switching regulator 8th This capacitor does not regulate immediately, but with a small delay to the new, from the Feedback signal FB regulated lower output voltage. But this is not a problem, because in this and the rest of the time the current i through the LEDs 2 always kept constant by the MosFet circuit.

The power loss in the MosFet is also low, because only at the beginning of the pulse for a few microseconds increased power loss is generated. For the remaining duration this is much lower. Integral, the short initial increase is negligible.

For the calculation example of 1 arises with the circuit of 2 assuming a 0.3 V over the MosFet when triggering a lightning pulse, a power loss of 0.6 W. The efficiency of the current control is therefore 28 W / 28.6 W = 98%. The over the prior art improved efficiency and lower power loss make it possible to construct corresponding circuits and devices compact.

LIST OF REFERENCE NUMBERS

1

lighting source

2

LED

3

measuring resistor

4

transistor

5

differential amplifier

6

control signal

7

electronic switch

8th

switching regulators

9

electronic switch

10

Differential voltage amplifier

FB

Feedback signal

i

electricity

vs

supply voltage

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10359723 A1 [0003]

Claims (13)

Method for supplying power to a pulsed illumination source ( 1 ) for pulse illumination for an image processing system, in which an object is detected by means of the pulsed illumination source ( 1 ) is illuminated with flash pulses and for image processing, images of the object illuminated with the lightning impulses are recorded by means of an image sensor, wherein the illumination source ( 1 ) is supplied with current (i) by means of a supply voltage (Vs) and for triggering a lightning impulse by means of a transistor ( 4 ) comprehensive current control or current control is controlled, characterized in that - the supply voltage (Vs) from a switching regulator ( 8th ), - at one of the illumination source ( 1 ) emitted lightning pulse due to the current flow through the transistor ( 4 ) on the transistor ( 4 ) resulting voltage drop during a measurement period, the duration or a portion of the duration of the current flow through the transistor ( 4 ), with a differential voltage amplifier ( 10 ) is measured, and - during the measurement period by means of the measured voltage drop as a feedback signal (FB), the output voltage of the switching regulator ( 8th ) is regulated. Method according to the preceding claim, characterized in that the switching regulator ( 8th ) with the illumination source switched off ( 1 ) generates an output voltage which, with a voltage increase serving as a safety margin, is above the voltage which is required for triggering a flash with the illumination source ( 1 ), and during the measurement period this excessive output voltage is reduced by the feedback signal (FB). Method according to one of the preceding claims, characterized in that by means of the feedback signal (FB) the output voltage of the switching regulator ( 8th ) is controlled during the measurement period such that the voltage drop across the transistor ( 4 ) a predetermined, for the function of the transistor ( 4 ) occupies the residual voltage required for current regulation. Method according to one of the preceding claims, characterized in that by means of the feedback signal (FB) the output voltage of the switching regulator ( 8th ) is controlled during the measurement period such that above the transistor ( 4 ) a minimum required to function as a current source residual voltage is maintained. Method according to one of the preceding claims, characterized in that the measuring duration begins simultaneously with the triggering of a lightning pulse or slightly delayed. Method according to one of the preceding claims, characterized in that the voltage drop directly at the transistor ( 4 ) is measured. Method according to one of the preceding claims, characterized in that an electronic switch ( 9 ) is used, the outside of the measurement period, the output voltage of the switching regulator ( 8th ) and during the measuring period that of the differential voltage amplifier ( 10 ) supplied voltage as a feedback signal (FB) for controlling the output voltage of the switching regulator ( 8th ) to the switching regulator ( 8th ) switches. Method according to one of the preceding claims, characterized in that a lighting source ( 1 ) comprising a plurality of illumination sources connected in series and / or in parallel. Method according to one of the preceding claims, characterized in that as illumination source ( 1 ) an LED ( 2 ) is used. Method according to one of the preceding claims, characterized in that a transistor ( 4 ), which is a bipolar transistor or a field effect transistor. Method according to one of the preceding claims, characterized in that a voltage-controlled switching regulator ( 8th ), which is designed as a buck regulator, as a boost regulator, as an up / down controller, with transformer and push-pull control, as a flyback circuit or SEPIC circuit. Electronic switch ( 7 ) for powering a pulsed illumination source ( 1 ) for pulse illumination for an image processing system, in which an object is detected by means of the pulsed illumination source ( 1 ) is illuminated with flash pulses and for image processing, images of the object illuminated with the lightning impulses are recorded by means of an image sensor, wherein the illumination source ( 1 ) is supplied with current (i) by means of a supply voltage (Vs) and for triggering a lightning impulse by means of a transistor ( 4 ) current control or current regulation, comprising a supply voltage circuit and a current control or current regulation, which is a transistor ( 4 ) for triggering a lightning pulse on one of the electronic circuit ( 7 ) connected lighting source ( 1 ), characterized in that - they have a switching regulator ( 8th ) for regulating the supply voltage (Vs), - a differential voltage amplifier ( 10 ) with which the at one of the illumination source ( 1 ) emitted lightning pulse due to the current flow through the transistor ( 4 ) on the transistor ( 4 ) resulting voltage drop during a measurement period, the duration or a portion of the duration of the current flow through the transistor ( 4 ), can be measured, and - is designed such that during the measurement period by means of the measured voltage drop as a feedback signal (FB), the output voltage of the switching regulator ( 8th ) is regulated. Camera or smart camera, characterized in that it has an integrated into the camera electronic circuit ( 7 ) according to the preceding claim.

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