Microprocessor Controlled Thermoelectric Cooler and Laser Power Controller

This invention relates generally to thermo electric coolers and temperature sensors and more particularly concerns a circuit for a thermoelectric cooler and a temperature sensor which is able to monitor and provide accurate temperature control of the thermoelectric cooler.

A single beam laser diode assembly has a single diode and usually, in a scanning system, the diode is driven by a train of image pixel information. The pixel information is used to drive the diode and therefore stimulate laser flux emission where there is a white pixel in a write white system. In a write white system, a laser is turned on to create white space on a page. Intensity of the light beam is directly proportional to the output power of the laser. In order to keep the output power of the diode constant, the temperature of the diode should be kept at a constant level. However, due to the structure of the laser diode assembly, as the pixel information changes, which causes the diode to turn on and off, the temperature of the diode fluctuates, which in turn causes the output power of the diode and the intensity of the light beam to fluctuate. In a printing, system fluctuation in the intensity of light beams causes fluctuation in the exposure of a printed pixel.

A multiple beam diode assembly has at least two diodes in close proximity on a common substrate. Each diode is driven by a separate train of image pixel information. Again, as the pixel information changes, the temperature of each diode fluctuates. However, in a multiple diode system, the changing temperature of a diode also causes a temperature fluctuation in adjacent diodes. The temperature fluctuations of the adjacent diodes cause the output power and the intensity of the light beams in those adjacent diodes also to fluctuate.

A tri-level system may use one or more diodes with at least one diode operating at full on, full off, and partially on. One example of an application using a single diode tri-level system is the printing of black and white documents with a highlight color. Tri-level systems suffer from the same heating effects both in the full on and the partially on modes of the laser.

Accordingly, it is the primary aim of the invention to provide a method for quickly compensating for a variety of thermally induced effects. Further advantages of the invention will become apparent as the following description proceeds.

Briefly stated and in accordance with the present invention, there is provided a microprocessor controlled electronic subsystem for monitoring either a thermoelectric cooler or a laser diode and adjusting the thermoelectric cooler to compensate for temperature effects.

FIG. 1 is a circuit diagram of a linear servo controlled electronic circuit to monitor and correct for problems of laser drift.

FIG. 2 is a circuit diagram of a pulse width modulation electronic circuit to monitor and correct for problems of laser drift.

FIG. 3 is a temperature curve showing rising temperature of a thermoelectric cooler.

FIG. 4 is a flow chart showing a program in a microprocessor in the electronic circuit shown in FIG. 1.

While the present invention will be described in connection with a preferred embodiment and method of use, it will be understood that it is not intended to limit the invention to that embodiment/procedure. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

curve C1

curve C2

range R

initital temperature value T0

temperature value T1

temperature value T2

target temperature value T3

thermoelectric cooler 20

stabilizing circuit 22

microprocessor 24

8-bit bus 26

analog converter 28

line 30

thermelectric cooler circuit 32

time constant operational amplifier 34

power operational amplifier 36

time constant operational amplifier 38

resistor 40

resistor 42

resistor 44

capacitor 46

resistor 48

resistor 50

resistor 52

resistor 54

resistor 56

capacitor 58

capacitor 60

line 62

feedback circuit 64

thermoelectric cooler sensor 66

line 68

thermoelectric cooler 70

stabilizing circuit 72

microprocessor 74

line 76

buffer 78

line 80

switch 82

line 84

sensor 86

line 88

Turning now to FIG. 1 a circuit diagram of an electronic circuit to monitor and correct for problems of laser drift by monitoring temperature of a thermoelectric cooler is shown. A thermoelectric cooler 20 is used to stabilize the temperature of a laser diode (not shown). In order to more accurately stabilize the temperature the thermoelectric cooler 20 must be monitored and adjusted. A stabilizing circuit 22 is used to perform this funtion. A microprocessor 24 outputs a digital target value on 8-bit bus 26. The target value is then received by a digital to analog converter 28. After conversion to an analog signal the target value is passed to a thermoelectric cooler circuit 32 on line 30.

The thermoelectric cooler circuit 32 has 3 operational amplifiers. Two of the operational amplifiers 34, 38 are used in the configuration to provide forward and feedback time constants, respectively, to the thermoelectric cooler circuit 32. The third, power operational amplifier 36 is used to provide the correct power output to the thermoelectric cooler 20. The three operational amplifiers 34, 36, 38 are all configured to work in inverting mode. Each operational amplifier 34, 36, 38 is also connected in parallel with a resistor 40, 42, 44 respectively. Additionally, operational amplifier 34 is also connected in parallel with a capacitor 46. Each of the three operational amplifiers 34, 36, 38 also has a resistor 48, 50, 52 respectively connected at its input. Each of the three operational amplifiers 34, 36, 38 also has a resistor 50, 54, 56 respectively connected at its output. The elements for the thermoelectric cooler circuit 32 are completed by including two capacitors 58, 60 connected to operational amplifier 58. Capacitors 58, 60 provide power supply filtering functions to the power operational amplifier 36.

The capacitor 46 and resistors 40, 48 provide the forward time constant function for the thermoelectric cooler circuit 32. The capacitor 46 and resistors 40, 56 provide the feedback time constant for the thermoelectric circuit 32.

An alternative embodiment, shown in FIG. 2, uses a pulse width modulated electronic circuit. A thermoelectric cooler 70 is used to stabilize the temperature of a laser diode (not shown). To more accurately stabilize the temperature, the thermoelectric cooler 70, a stabilizing circuit 72 is used.

In stabilizing circuit 72, a microprocessor 74 outputs a driving pulse width voltage on line 76. The driving voltage is received by buffer 78 and passed along line 80 to a switch 82. The driving voltage is then passed along line 84 to the thermo electric cooler 70. The stabilizing circuit 72 also includes a feedback path comprising a sensor 86 connected to the microproccessor 74 through line 88.

The forward time constant provided by the capacitor 46 and resistors 40, 48 in thermoelectric cooler circuit 32 and the feedback time constant provided by the capacitor 46 and resistors 40, 56 in thermoelectric circuit 32 is provided within the microprocessor 73 in the pulse width modulation circuit shown in FIG. 2.

When the target value is passed to a thermoelectric cooler circuit 32 on line 30 it first encounters resistor 48. Resistor 48 is connected to the convertor 28 on one end and the parallel configuration of the operational amplifier 34 with resistor 40 and capacitor 46. Also connected to the same end as the operational amplifier 34, resistor 40, and capacitor 46 is resistor 56. Resistors 48, 40 along with capacitor 46 form an RC circuit with an RC time constant which provides a delay to compensate for the reaction time of thermoelectric cooler 20 and stabilizes the feedback portion of the thermoelectric cooler circuit. Operation amplifier 34 generates a time delayed signal which is then passed along line 62 to resistor 50. This function would be performed within the microprocessor 72 in the pulse width modulation circuit shown in FIG. 2.

Resistor 50 is connected between the parallel configuration of operational amplifier 34 with capacitor 46 and resistor 40 and the parallel configuration of operational amplifier 36 with resistor 42. Resistor 42 and resistor 50 work in concert with operational amplifier 36 for gain control. The operational amplifier 36 is then used to drive the thermoelectric cooler 20. Resistor 54 is inserted between the output of the operational amplifier 36 and the input of thermoelectric cooler 20 and is used as a current limiting resistor to avoid overdriving the thermoelectric cooler 20. This function would be performed by the switch 82 in the pulse width modulation circuit shown in FIG. 2.

The thermoelectric cooler circuit 32 also contains a feedback circuit 64 connected to a thermoelectric cooler sensor 66. The thermoelectric cooler sensor 66 senses the temperature of the thermoelectric cooler. Alternatively the thermoelectric cooler sensor 66 could sense the temperature of the laser, or could be replaced with optical sensor to sense the power output of the laser. Alternatively, the thermoelectric cooler sensor 66 could sense the temperature of the laser, or could be replaced with an optical sensor to sense the power output of the laser.

The thermoelectric cooler sensor 66 in FIG. 1 then passes the sensed information to the resistor 52 and the microprocessor 24 on line 68. The resistor 52 functions along with resistors 44, 56, 48, and 40 to form an RC time constant with capacitor 46 for the operational amplifier 38. Operational amplifier 38 and its RC time constant provide the feedback circuit 64 for the stabilizing circuit 22.

The sensed information on line 68 is also sent to the microprocessor 24. This information is useful to microprocessor 24 as it controls the ramp up voltage or target voltage outputted by the microprocessor 24 on the 8-bit bus 26. When the unit is first turn on, the microprocessor 24 will ouput a target voltage higher which represents a temperature greater than the actual target voltage temperature. This higher voltage will drive the thermoelectric cooler 20 harder causing it to change its temperature faster than if the actual target voltage temperature were used. As the thermoelectric cooler 20 approaches the actual target voltage temperature the microprocessor 24 will output a lower target voltage which represents a temperature closer to the actual target voltage temperature. This will continue until the microprocessor 24 outputs the actual target voltage temperature and the thermoelectric cooler circuit 32 has stabilized.

In the pulsewidth modulation circuit shown in FIG. 2, the sensor 86 passes the sensed information directly to the microprocessor 74. The function of the RC time constant is then performed within the microprocessor 74 according to a set of training rules contained within the microprocessor 74.

This effect of the stabilizing circuit, whether it is the linear servo controlled electronic circuit shown in FIG. 1 or the pulse width modulated circuit shown in FIG. 2, is illustrated in FIG. 3. FIG. 3 is a graph of the temperature of the thermoelectric cooler 20 over time with temperature represented along the vertical axis and time along the horizontal axis. A target temperature value T3 surrounded by a range R is shown. The target temperature value T3 is the desired temperature for the thermoelectric cooler 20. If the microprocessor 24 or 74 were to ouput a voltage value representative of target value T3, the thermoelectric cooler circuit 32 or 72 would drive the thermoelectric cooler 20 or 70 to that target temperature value T3 along a curve C1. If however, the microprocessor 24 or 74 outputs a driving voltage which represents a temperature that is much higher than the target temperature value T3 on the 8-bit bus 26 it can cause the thermoelectric cooler circuit 32 or 72 to climb at a much faster rate as shown by the portion of another curve C2, as it appears between an initial temperature value T0 and temperature value T1. If the thermoelectric cooler 20 or 70 were allowed to stabilize at this driving voltage then it would overshoot the target temperature T3.

Once the thermoelectric cooler 20 or 70 has reached temperature value T1, that temperature will be sensed by the thermoelectric cooler sensor 66 or 86 respectively and feed back to the microprocessor 24 or 74 respectively. The microprocessor 24 or 74 respectively will then output a new value. The new value will still be greater than the voltage that represents target temperature value T3 but less than the inital value. The temperature of the thermoelectric cooler 20 or 70 will change at a slightly slower rate as represented by the portion of curve C2 between temperature value T1 and temperature value T2 but still at a faster rate than it would have if a voltage that represented the target temperature value T3 were used. If the thermoelectric cooler 20 or 70 were allowed to stabilize at this driving voltage then it would overshoot the target temperature T3 although by less than the initial driving voltage.

When the thermoelectric cooler 20 or 70 reaches a termperature value T2 that temperature will again be sensed by the thermoelectric cooler sensor 66 or 86 respectively and the temperature information will be sent back to the microprocessor 24 or 74 respectively. When the microprocessor 24 or 74 respectively has received this information it will again change the value output. This time the microprocessor 24 or 74 will output a value representative of the target temperature value T3. The temperature of the thermoelectric cooler will change at a slower rate and finally stabilize at the target temperature value T3 as shown on the portion of the curve C2 between temperature valueT2 and target temperature value T3.

As long as the temperature of the thermoelectric cooler 20 or 70 as sensed by the thermoelectric cooler sensor 66 or 86 respectively remains within range R the system is stabilized and the microprocessor 24 or 74 respectively will not change the value output. However, should the temperature of the thermoelectric cooler 20 or 70 change, the thermoelectric cooler sensor 66 or 86 respectively will sense the changed termperature and tell the microprocesser 24 or 74 respectively. The microprocessor 24 or 74 must then respond to correct the problem. The microprocessor 24 or 74 will output a new value that either represents a warming voltage or a cooling voltage depending on whether the thermoelectric cooler 20 or 70 is below or above the target temperature. The magnitude of the output voltage will depend on how far outside the range R the temperature of the thermoelectric cooler 20 or 70 has drifted. If the temperature of the thermoelectric cooler 20 or 70 is close to the target temperature value then only a small change is necessary. However, if the temperature of the thermoelectric cooler 20 or 70 has drifted into the temperature range represented between temperature value T1 and temperature value T2 for instance, then the microprocessor 24 or 74 must again pick a large enough value to quickly drive the thermoelectric cooler 20 or 70 towards the target temperature value T3. Then as the temperature of the thermoelectric cooler 20 or 70 approaches the target temperature value T3, the microprocessor 24 or 74 respectively will ouput a voltage more representative of the target temperature value T3. In all cases, whether the stabilizing circuit 22 or 72 has been turned on for the first time or is adjusting to changes in the thermoelectric cooler 20 or 70 respectively, the ouput value from the microprocessor 24 or 74 respectively will always be dependent on how large the difference is between the temperature of the thermoelectric cooler 20 or 70 respectively and the target temperature value T3. In this example, the microprocessor 24 or 74 utilizes a look up table contained in memory to choose an appropriate output value, however, the microprocessor 24 or 74 respectively could have used an appropriate algorithm and calculated a new appropriate output value. It should be noted that this example uses only three discrete ranges of temperatures where the microprocessor 24 or 74 evaluates and chooses an output value. However, this was done for explanatory purposes only and any number of ranges or a continuous function could have been used in actuality.

The software with the microprocessor for monitoring and correcting temperature deviations is represented by the flow chart in FIG. 3. So long as the voltage received from the thermoelectric cooler sensor 66 is neither less than a specified minimum voltage nor greater than a specified maximum voltage the status of the thermoelectric cooler 20 is reported as fine and the microprocessor does nothing. However, when the voltage received from the thermoelectric cooler sensor 66 deviates from the specified range, the microprocessor retrieves a new output value.