Voltage Ramp Temperature Controller

This invention relates to temperature control circuits and in particular to a control circuit for supplying a programmed voltage to a power supply connected to a heater, in order that a predetermined temperature from the heater occurs over an elapsed time period.

In the prior art, devices used to control the voltage to a heater develop an error signal and then reference the error signal with an externally generated sweep voltage. As a result, the amount of voltage applied to the heater is dependent on an outside variable and is not accurately responsive to deviations from the predetermined temperature sweep voltage requirement.

By summing a portion of the sweep voltage used to control the heater voltage to develop the error signal required and then summing the error signal with the sweep voltage a control signal is generated that accurately responds to deviations from the predetermined temperature sweep.

The present invention is an apparatus for providing a control signal to a heater power supply whereby the temperature linearly follows a predetermined sweep voltage. A portion of the sweep voltage signal and an output voltage developed at the heated device by a thermocouple are summed in an operational amplifier to develop an error signal. The error signal and the sweep voltage signal are then summed in a second operational amplifier to provide the control signal supplied to the heater power supply. By this method a thermocouple emf is generated that is linear to the sweep voltage signal.

FIG. 1 is a block diagram of an embodiment of the present invention;

FIG. 2 is a schematic diagram of an embodiment of the present invention.

Referring now to the drawings, more particularly to FIG. 1, the present invention has a sweep generator circuit 20 coupled to a first and a second operational amplifier 41 and 61. The system input on line 15 and the sweep generator circuit output from line 29 are coupled to first operational amplifier 41 for providing an error signal output over line 45. Error signal output connected over line 45 and the sweep generator circuit output over line 29 are coupled to second operational amplifier 61 which provides the system control signal output at 79.

Referring to FIG. 2, a schematic diagram of the embodiment, including sweep generator circuit 20, first and second operational amplifiers 41 and 61, an adjustable clamp circuit 90 and the system output 79, is shown.

In the embodiment, a programable direct current power supply is coupled to a heater element 80. Heater element 80 is coupled via conduction or convection to a device 81. A thermocouple 13 attached to device 81 is coupled to the input of isolation amplifier 11. Isolation amplifier 11 has its output coupled via line 15 and impedance matching resistor 39 to the first operational amplifier 41.

The sweep generator circuit 20, shown in this embodiment as a linear ramp generator, has a mode switch 27 with two sets of contacts and is coupled to operational amplifier 21. One set of mode switch 27 contacts couples the feedback of operational amplifier 21 through capacitor 25 and resistor 26. The other set of switch 27 contacts couples the feedback of operational amplifier 21 through resistor 23. A switch 22 is coupled in parallel with feedback capacitor 25 and resistor 26. An input resistor 28 is coupled between operational amplifier 21 and ground.

The sweep generator output over lead 29 is coupled through a current limiting resistor 31, to junction 32. A branch from this junction leads via current setting resistor 35 to first operational amplifier 41. An input and gain setting resistor 37 couples one side of operational amplifier 41 to ground potential. Another branch couples junction 32 to an input of a second operational amplifier 61 through current line 53 and resistor 55. Resistor 57 couples from one side of resistor 55 and ground potential.

The error signal output via lead 45 is coupled to the second operational amplifier 61 through an input resistor 51 and junction 64. A feedback resistor 43 is coupled between the first operational amplifier output and the input to the first operational amplifier 41.

A feedback resistor 63 is coupled between the second operational amplifier 61 output and its input through junction 64. Impedance matching resistor 57 couples one side of the input of second operational amplifier to ground potential via junction 66.

The output, via line 67, of the second operational amplifier 61 is coupled through a diode 65, a voltage dividing resistive network 71 and 75 and a current limiting resistor 73 to the control terminals of a programable direct current power supply as control signal output at 79. In the depictive embodiment, an adjustable clamp circuit 90 is shown having an operational amplifier 91 with an output lead 101 coupled to junction 96, a feedback resistor 93 coupled between junction 96 and an input to operational amplifier 91 through junction 94, a current supply resistor 97 coupled between junction 96 and ground potential, a potentiometer 92 coupled to an input of operational amplifier 91 and a clamping diode 95 coupled between junction 96 and junction 32.

In operation switch 27 is positioned to place resistor 23 in the feedback path of sweep circuit 20, thus causing operational amplifier 21 to be configured as a unity gain amplifier to set the initial or start point condition into the system. Upon reaching the start point, which is a predetermined temperature equal to or greater than the ambient temperature, switch 27 is repositioned placing capacitor 25 and resistor 26 in the sweep circuit 20 feedback loop and configuring operational amplifier 21 as an integrator to produce a linear voltage ramp output over lead 29. A portion of the sweep generator output is fed via lead 33 into the first operational amplifier 41 which operates as a non-inverting summation device.

A thermocouple 13 is attached to the device whose temperature is to be raised to provide an output voltage from the thermocouple that is proportional to the device temperature. This voltage is coupled to the input of amplifier 11 which operates as an isolation amplifier having a fixed predetermined gain. The output of amplifier 11 is coupled to first operational amplifier 41 via line 15. A portion of the sweep generator ramp voltage output from operational amplifier 21 appears across input and gain setting resistor 37 which has one side coupled to the input of operational amplifier 41 and its other to ground potential. Both of these voltages are algebraically summed for providing an output that is a non-inverted error signal. The output of operational amplifier 41 is a voltage which will be a positive direct current voltage if the thermocouple voltage is higher than the ramp voltage appearing across resistor 37. On the other hand, if the thermocouple voltage is lower than the ramp voltage across resistor 37 than there will be a negative voltage output from operational amplifier 41. This output voltage is fed to the input of operational amplifier 61 where it is summed with the magnitude of sweep voltage appearing across input resistor 55 that is coupled between junction 32 and the input to operational amplifier 61. The impedance of operational amplifier 61 and its input are matched by resistor 57 that is coupled between input and ground potential.

The error signal and sweep voltage are summed in operational amplifier circuitry 61 which operates in an inverting configuration to furnish an output signal that is coupled across a resistor diode network and ground potential. This voltage appearing at 79 is a programming voltage output that is applied to a programable direct current power supply for controlling its output voltage. Normally the voltage is coupled to a resistive heater device for supplying variable power to the heater to either increase or decrease the temperature of the heater thus causing a device exposed to the heat to increase or decrease linearly in temperature by a predetermined amount during an elapsed period of time.

Operational amplifier 91 with its associated resistor 97 and diode 95 network provides an adjustable clamp voltage at junction 32 to control the upper limit of the ramp voltage. Variable resistance 92 controls the magnitude of clamp voltage and is adjusted to a predetermined level before operation of the temperature control circuit.

The time required to heat a device by the heater element or elements to a specific temperature is determined by the slope of the ramp voltage and the type of thermocouple used. The slope of the ramp voltage can be positioned so as to have a number of predetermined fixed values with at least one variable value. Sweep time of the ramp can be selected to vary over a wide range of times. Thus time selection together with the adjustable upper limit determined by setting variable resistor 92 enables the control circuitry to operate over a wide range of heating rates. In addition, the adjustable resistors in sweep generator circuit 20 allows the starting current to be adjusted so that it is possible to program the heating over any desired range.

The circuitry is capable of producing a linear temperature rise of a device to be heated with very negligible variation in linearity.

Although an embodiment has been described, it will be understood that within the perview of this invention various changes may be made in form, details, proportion and arrangement of parts, the combination thereof and mode of operation which generally stated consist in a device capable of carrying out the objects set forth, as disclosed and defined in the appended claims.