Arc voltage estimation and use of arc voltage estimation in thermal processing systems

Abstract

A system and method is featured for controlling a process parameter of a thermal processing system by estimating an arc voltage between a plasma arc torch tip and a metallic workpiece and controlling the process parameter based on the estimated arc voltage. Particular embodiments include adjusting the height of a plasma torch based on the estimated arc voltage. A system and method is also featured for estimating an arc voltage in a thermal processing system in which a switch mode power supply provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece.

Description

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a plasma arc system.

FIG. 2 is a diagram that illustrates a torch height control system that measures arc voltage using a voltage divider board.

FIG. 3 is a diagram that illustrates a torch height control system that obtains the arc voltage through an arc voltage estimation technique.

FIG. 4 is a circuit diagram of the power control block that includes an arc voltage estimation module.

FIGS. 5A and 5B are signal diagrams that represent a gate signal over one cycle for exemplary error and carrier signals.

FIG. 6 is a flow diagram illustrating a method of arc voltage estimation according to the first embodiment.

FIG. 7 is a flow diagram illustrating a method of arc voltage estimation according to the second embodiment.

FIG. 8 is a flow diagram illustrating a method of arc voltage estimation according to the third embodiment.

DETAILED DESCRIPTION

According to one aspect, a system and method is featured for controlling a process parameter of a thermal processing system by estimating an arc voltage between the tip of the plasma arc torch and a metallic workpiece and controlling the process parameter based on the estimated arc voltage. For example, particular embodiments can include adjusting the height of a plasma torch based on an estimated arc voltage.

According to another aspect, a system and method is featured for estimating an arc voltage in a thermal processing system that includes a switch mode power supply for providing an arc current to generate a plasma arc between the tip of the plasma arc torch and a metallic workpiece.

FIG. 3 is a diagram that illustrates a torch height control system that obtains the arc voltage through an arc voltage estimation technique. Although torch height control is one application of arc voltage estimation, other embodiments can use arc voltage estimates to control other process parameters in a thermal processing system. A difference between FIG. 3 and FIG. 2 is that the plasma arc controller 50 includes a modified power control block 60 that incorporates an arc voltage estimation module 62. The arc voltage estimation module 62 utilizes information obtained from the power block 58 and the power control block 60 to estimate the arc voltage.

FIG. 4 is a circuit diagram of the power control block that includes an arc voltage estimation module. As shown, the circuit 100 includes a Pulse Width Modulation (PWM) control circuit block 200 coupled to a power circuit block 300. The power circuit block 300 is a switched mode power supply that includes an unregulated dc input voltage source V_IN, a power transistor switch-diode combination Q1, D1, an output filter inductor L1 and a plasma arc load R_LD. The power circuit block 300 operates as a standard chopper such that the output current I_ARC through the arc load R_LD depends on the duty cycle of the switch Q1. Although the power circuit block 300 shown is a buck converter, other embodiments can include other circuit topologies, including boost, buck-boost and variations thereof. For example, an inverter is a form of a buck converter.

The PWM control circuit block 200 provides a gate signal T3PWM to the switch Q1 to control its duty cycle, and thus the output current I_ARC through the plasma arc load R_LD. As shown, the PWM control block 200 includes a current reference block 210, an error control block 220, a feedback current sensor 240, a PWM comparator block 230, and an arc voltage estimation module 250.

An operator of the system manually sets block 210 to a desired current reference I_REF at which to maintain the output current I_ARC. The output current I_ARC is monitored using the current sensor 240, such as a Hall current sensor. The current sensor 240 transmits a feedback current I_FB to an input of the error control block 220. The error control block 220 can be implemented, for example, as a standard proportional-integral-derivative controller (PID controller) known to those skilled in the art. The error control block 220 compares the feedback current I_FB against the desired current reference I_REF and outputs a modulating error signal, Error.

The error signal, Error, is then input to the PWM comparator block 230 where it is sampled and used to generate the appropriate gate signal T3PWM that adjusts the duty cycle of the switch mode power supply 300, thereby correcting for the error in the output current. The PWM comparator block 230 and the arc voltage estimation module 250 can be realized using a digital signal processor (DSP), such as TMS320LF2407A from Texas Instruments. These control blocks can also be realized using a combination of one or more suitably programmed or dedicated processors (e.g., a microprocessor or microcontroller), hardwired logic, Application Specific Integrated Circuit (ASIC), or a Programmable Logic Device (PLD) (e.g, Field Programmable Gate Array (FPGA)) and the like.

In order to generate the appropriate gate signal T3PWM, the PWM comparator block 230 compares an instantaneous error sample T3CMPR with a carrier wave signal T3CNT. The carrier wave signal can be generated as a sawtooth or triangular carrier wave with its frequency ranging anywhere from hundreds of Hertz to MegaHertz depending on the application. In a plasma cutting application, the frequency of the carrier wave signal is typically around 15 kHz. The comparator amplifies the difference between the two signals and produces a gate signal T3PWM whose average value over one switching cycle of the carrier wave signal T3CNT is equal to the value of the instantaneous error sample T3CMPR. Application of the gate signal to the switch Q1 adjusts the duty cycle to drive and maintain the output current I_ARC at a desired steady state value.

FIGS. 5A and 5B are signal diagrams that represent a gate signal T3PWM over one cycle for exemplary error and carrier signals, T3CMPR and T3CNT. The peak of the timing signal T3CNT is identified as T3PR. In this example, the output current I_ARC is less than the current reference I_REF, resulting in an instantaneous error sample T3CMPR as shown in FIG. 5A. In order to correct for this error, the PWM comparator block 230 compares the error sample T3CMPR against the carrier wave signal T3CNT. For one switching cycle, the comparator block 230 generates pulses T3PWM, while the value of the error sample T3CMPR is more than the incrementing value of the carrier wave signal T3CNT. These pulses, as shown in FIG. 5B, are used to enable and disable the switch Q1 of the switch mode power supply 300. By turning the switch Q1 ON and OFF in this manner, the duty cycle of the switch mode power supply can be adjusted to correct and maintain the output current at the desired current reference.

According to a first embodiment, the method for arc voltage estimation is based on the principle that inductor voltage drop is zero at constant arc current I_ARC. This implies that the average dc voltage at the input of the inductor L1 is equal to the average value of the arc voltage V_ARC. Thus, an estimate of the average arc voltage V_ARC can be determined by calculating the product of the steady state duty cycle D_SS of the switch Q1 and a dc input voltage V_IN according to equation (1):

V _ARC =D _SS *V _IN  (1)

As known to those skilled in the art, it is also possible to implement the product as a summation.

FIG. 6 is a flow diagram illustrating a method of arc voltage estimation according to the first embodiment. At step 400, the arc voltage estimation module 250 obtains the steady state duty cycle D_SS of the switch mode power supply. The steady state duty cycle D_SS can be calculated as the ratio of an instantaneous error sample T3CMPR to the peak of the carrier wave signal T3PR. For example, if the peak of the carrier wave signal equals 1070 counts and the instantaneous error sample T3CMPR corresponds to 535 counts, the steady state duty cycle D_SS is 50%.

At step 410, the arc voltage estimation module 250 obtains the dc input voltage V_IN. As shown in FIG. 4, the arc voltage estimation module 250 can obtain the dc input voltage V_IN through a tap 300a. Because the unregulated dc input voltage V_IN can have a magnitude in the range of hundreds of Volts, signal conditioning circuitry 310 can be used to scale down the voltage V_IN to a voltage suitable for processing by the arc voltage estimation module 250.

The dc input voltage V_IN can also be determined from the input ac voltage V_ACIN (not shown). The input ac voltage V_ACIN is an ac voltage from which the dc input voltage V_IN can be derived, for example, through a rectifier stage. The arc voltage estimation module 250 can determine the dc input voltage V_IN from the peak value of the input ac voltage V_ACIN. The dc input voltage V_IN can also be derived from the root mean square (RMS) value of the input ac voltage V_ACIN. Other methods for translating an input ac voltage to a dc input voltage can also be implemented. Because the input ac voltage V_ACIN can have a magnitude in the range of hundreds of Volts, signal conditioning circuitry 310 is used to scale down the input ac voltage V_ACIN to a voltage suitable for processing by the arc voltage estimation module 250.

At step 420 of FIG. 6, the arc voltage estimation module 250 calculates the arc voltage estimate V_ARC as the product of the duty cycle D_SS of the switch mode power supply and the dc input voltage V_IN. Although this estimate of arc voltage V_ARC may or may not provide the same accuracy as would direct measurement of the arc voltage, it is suitable for the purpose of particular applications, including torch height control, in that it filters out the transient noise that could produce jitter or other instability. In other applications, such as in arc current control applications, more accuracy in the estimation of arc voltage may be required. An example of current control that uses an arc voltage estimation is disclosed in co-pending U.S. patent application Ser. No. ______, (Attorney Docket No. HYP-068) entitled “LINEAR, INDUCTANCE BASED CONTROL OF REGULATED ELECTRICAL PROPERTIES IN A SWITCH MODE POWER SUPPLY OF A THERMAL PROCESSING SYSTEM,” filed concurrently herewith. The entire teachings of the above application are incorporated herein by reference.

According to a second embodiment, the method for arc voltage estimation additionally accounts for the voltage drop in the inductor according to the Equation (2) below.

$\begin{array}{cc}{V}_{\mathrm{Arc}}=D*V_{\mathrm{IN}}-L\frac{i}{t}& \left(2\right)\end{array}$

FIG. 7 is a flow diagram illustrating a method of arc voltage estimation according to the second embodiment. Steps 500, 510 and 520 are similar to steps 400, 410 and 420, respectively, as previously described in FIG. 6. At step 530, the arc voltage estimation module 250 calculates the voltage drop across the inductor L1 from the product of its inductance and the change in output current I_ARC Equation (2), which is a continuous linear equation, can be discretized using currents and voltages sampled on a regular basis. For example, Equation (2) can be transformed into a discrete representation for arc voltage by making the following substitution:

$\begin{array}{cc}L\frac{i}{t}=L*\left(I_s-I_s*z^{-1}\right)=L*\left(I_s*\left(I-z^{-1}\right)\right)& \left(3\right)\end{array}$

where current sample I_s is a present sample of the inductor current, current sample I_s*z⁻¹ is a preceding sample of the inductor current, and L is the inductance of the inductor L1. In this example, Equation (2) is discretized using a backwards Euler transform. However, other discretization transforms known to those skilled in the art can also be used. For example, another discretization transform is the Tustin transform (also referred to as the “Bilinear Z” transform) Other substitutions can be possible.

At step 540, the arc voltage estimation module 250 calculates the estimate of the arc voltage V_ARC based on the difference between the voltage applied to the input of the inductor L1 from step 520 and the calculated voltage drop across the inductor from step 530. For example, after substitution of Equation (3) into Equation (2), the arc voltage estimate V_ARC can be obtained from the following:

$\begin{array}{cc}{V}_{\mathrm{Arc}}=D*V_{\mathrm{IN}}-L\frac{i}{t}=D*V_{\mathrm{IN}}-L*I_s*\left(1-z^{-1}\right)& \left(4\right)\end{array}$

Equation (4) provides an accurate estimate of arc voltage but in practice is sensitive to noise in the current measurement I_s and requires low pass filtering that significantly affects the estimate. Also Equation (4) implicitly assumes that the output voltage changes so slowly as to be essentially constant throughout the PWM switching period and makes a sudden step change at the sampling instant. In the case of plasma arc loads, this assumption generally does not hold. Rather, the voltage across a plasma arc can be highly dynamic with rapid changes relative to typical PWM switching periods.

According to a third embodiment, the accuracy of the arc voltage estimate can be further improved by starting with the assumption the arc voltage V_ARC changes throughout the PWM switching period. Many different profiles can be assumed for the change in arc voltage V_ARC, including linear, parabolic, exponential profiles, for example.

FIG. 8 is a flow diagram illustrating a method of arc voltage estimation according to the third embodiment. Steps 600, 610 and 620 are similar to steps 400, 410 and 420, respectively, as previously described in FIG. 6. At step 630, the arc voltage estimation module 250 obtains a time varying profile representing expected variation in the arc voltage. Such variations may be modeled as linear, parabolic, exponential, or using any other mathematical or statistical representation.

At step 640, the arc voltage estimation module 250 models the change in arc current through the inductor based on the voltage applied to the input of the inductor and the time varying profile of the expected variations in arc voltage. At step 650, the arc voltage estimation module 250 derives a model of the arc voltage based on the model of the change in arc current through the inductor. At step 660, the arc voltage estimation module 250 calculates the arc voltage estimate from the model derived in step 650.

Although not so limited, the following is an example of a method for estimating the arc voltage according to the third embodiment. For the purpose of example only, the arc voltage is assumed herein to vary linearly throughout the PWM switching period. However, as previously discussed, the variation in arc voltage over time can be modeled as linear, parabolic, exponential, or using any other mathematical or statistical representation.

The following table includes a description of terms discussed in following example for estimating the arc voltage according to the third embodiment.

TABLE 1
Isample or Is or Is * z0 present current sample
z−1                      time delay operator that denotes a time
                         delay of ‘T’ seconds
Is * z−1                 current sample preceding present current
                         sample
D                        duty cycle of the current switching period
L                        inductance in henries.
T                        switching period in seconds.
Varc                     arc voltage estimate
VIN                      input dc voltage
Vapplied                 Average voltage applied to the input of the
                         inductor and load

A single switching period begins with a current sample and ends with a current sample. The average voltage applied to the output circuit (i.e., the inductor and the load) is:

V _applied =D*V _IN  (1)

The basic equation for the voltage across an inductor is:

$\begin{array}{cc}v=L\frac{i}{t}& \left(2\right)\end{array}$

Converting to a discrete form we obtain:

$\begin{array}{cc}{V}_L=L*\left(\frac{I_s-I_s*z^{-1}}{T}\right)=I_s*\left(1-z^{-1}\right)*\left(\frac{L}{T}\right)& \left(3\right)\end{array}$

The change in the Arc Voltage between sampling instants is:

ΔV_arc=V_arc−V_arc*z⁻¹=V_arc*(1−z⁻¹)  (4)

The rate of change of the Arc Voltage is:

$\begin{array}{cc}{V}_{\mathrm{rate}}=\frac{\Delta V_{\mathrm{arc}}}{T}& \left(5\right)\end{array}$

The change in the Arc Current between sampling instants is:

ΔI_s=I_s−I_s*z⁻¹=I_s*(1−z⁻¹)  (6)

Assuming a linear uniformly changing Arc Voltage:

$\begin{array}{cc}\Delta I_s={\int }_0^{D*T}\frac{V_{\mathrm{IN}}-\left(V_{\mathrm{arc}}+V_{\mathrm{rate}}*t\right)}{L}t+{\int }_{D*T}^T\frac{0-\left(V_{\mathrm{arc}}+V_{\mathrm{rate}}*t\right)}{L}t& \left(7\right)\end{array}$

Simplifying:

$\begin{array}{cc}\Delta I_s=\left(D*V_{\mathrm{IN}}-V_{\mathrm{arc}}-\frac{V_{\mathrm{rate}}*T}{2}\right)*\left(\frac{T}{L}\right)& \left(8\right)\end{array}$

$V_{\mathrm{rate}}=\frac{\Delta V_{\mathrm{arc}}}{T}\text{:}$

Back substituting

$\begin{array}{cc}\Delta I_s=\left(D*V_{\mathrm{IN}}-V_{\mathrm{arc}}-\frac{\Delta V_{\mathrm{arc}}}{2}\right)*\left(\frac{T}{L}\right)& \left(10\right)\end{array}$

Back substitutingΔV_arc=V_arc*(1−z⁻¹)  (11)

$\begin{array}{cc}\Delta I_s=\left(D*V_{\mathrm{IN}}-V_{\mathrm{arc}}-\frac{V_{\mathrm{arc}}*\left(1-z^{-1}\right)}{2}\right)*\left(\frac{T}{L}\right)& \left(12\right)\end{array}$

Solving for V_ARC:

$\begin{array}{cc}{V}_{\mathrm{arc}}=\left(\frac{2}{3}\right)*\left(\frac{D*V_{\mathrm{IN}}-I_s*\left(1-z^{-1}\right)*\left(\frac{L}{T}\right)}{1-\left(\frac{1}{3}\right)*z^{-1}}\right)& \left(13\right)\end{array}$

Solving for a recursive, implementable form:

$\begin{array}{cc}{V}_{\mathrm{arc}}=\left(\frac{1}{3}\right)*V_{\mathrm{arc}}*z^{-1}+\left[\left(\frac{2}{3}\right)*\left(D*V_{\mathrm{IN}}-I_s*\left(1-z^{-1}\right)*\left(\frac{L}{T}\right)\right)\right]& \left(14\right)\end{array}$

This technique is extendable to other models of arc voltage behavior including, for example, parabolic models in which the arc voltage varies with t².

With respect to all of the embodiments, one or more of the steps described can be combined as known to those skilled in the art.

While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

1. A method of controlling a process parameter of a thermal processing system in which a switch mode power supply provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the method comprising: estimating an arc voltage between the plasma arc torch tip and the metallic workpiece; andcontrolling a process parameter of the thermal processing system based on the estimated arc voltage.

2. The method of claim 1 wherein controlling the process parameter comprises: adjusting the height of a plasma arc torch based on the estimated arc voltage.

3. The method of claim 1 wherein the switch mode power supply includes an output inductor, the method further comprising: estimating the arc voltage based on an average voltage applied to the input of the inductor.

4. The method of claim 1 wherein the switch mode power supply includes an output inductor, the method further comprising: estimating the arc voltage based on the difference between an average voltage applied to the input of the inductor and a voltage drop across the inductor.

5. The method of claim 1 wherein the switch mode power supply includes an output inductor, the method further comprising: obtaining a time varying profile of expected variations in arc voltage; andestimating the arc voltage from a model representing changes in arc current through the inductor, the model based on an average voltage applied to an input of the inductor and the time varying profile of expected variations in the arc voltage.

6. A method for estimating an arc voltage in a thermal processing system in which a switch mode power supply provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the method comprising: obtaining a duty cycle of the switch mode power supply;obtaining a value representing a dc input voltage of the switch mode power supply; andestimating the arc voltage between the plasma arc torch tip and the metallic workpiece based on a combination of the duty cycle of the switch mode power supply and the value representing the dc input voltage of the switch mode power supply.

7. The method of claim 6 further comprising: calculating the duty cycle of the switch mode power supply based on a ratio of a sampled error signal to a peak value of a carrier wave signal, the sampled error signal comparing a measured value of the arc current to a preset current reference.

8. The method of claim 6 further comprising: measuring the value representing the dc input voltage of the switch mode power supply.

9. The method of claim 6 further comprising: deriving the value representing the dc input voltage of the switch mode power supply from an ac input voltage.

10. The method of claim 6 further comprising: scaling a value representing an input voltage of the switch mode power supply.

11. A method for estimating an arc voltage in a thermal processing system in which a switch mode power supply including an output inductor provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the method comprising: obtaining an average voltage applied to an input of the inductor;obtaining a value corresponding to a voltage drop across the inductor; andestimating the arc voltage based on the difference between the average voltage applied and the voltage drop.

12. The method of claim 11 further comprising: obtaining a duty cycle of the switch mode power supply;obtaining a value representing a dc input voltage of the switch mode power supply; andobtaining the average voltage applied to the input of the inductor based on the product of the duty cycle of the switch mode power supply and the value representing the dc input voltage of the switch mode power supply.

13. The method of claim 11 further comprising: obtaining the voltage drop across the inductor based on time varying changes in current through the inductor.

14. A method for estimating an arc voltage in a thermal processing system in which a switch mode power supply including an output inductor provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the method comprising: obtaining a time varying profile of expected variations in arc voltage;obtaining an average voltage applied to an input of the inductor; andestimating the arc voltage from a model representing changes in arc current through the inductor, the model based on an average voltage applied to the input of the inductor and the time varying profile of expected variations in the arc voltage.

15. The method of claim 14 wherein the time varying profile is a mathematical or statistical representation of expected variations in arc voltage;

16. A system for controlling a process parameter of a thermal processing system, the system comprising: a switch mode power supply that provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece;an arc voltage estimation module that estimates an arc voltage between the plasma arc torch tip and the metallic workpiece; anda process controller that controls a process parameter of the thermal processing system based on the estimated arc voltage.

17. The system of claim 16 wherein the process controller is a torch height controller that adjusts the height of a plasma arc torch based on the estimated arc voltage.

18. The system of claim 16 wherein the switch mode power supply includes an output inductor and the arc voltage estimation module estimates the arc voltage based on an average voltage applied to the input of the inductor.

19. The system of claim 16 wherein the arc voltage estimation module estimates the arc voltage based on the difference between an average voltage applied to the input of the inductor and a voltage drop across the inductor.

20. The system of claim 16 wherein the switch mode power supply includes an output inductor and the arc voltage estimation module estimates the arc voltage from a model representing changes in current through the inductor, the model based on an average voltage applied to an input of the inductor and a time varying profile of expected variations in the arc voltage.

21. An apparatus that estimates an arc voltage in a thermal processing system in which a switch mode power supply provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the apparatus comprising: processing means for obtaining a duty cycle of the switch mode power supply;processing means for obtaining a value representing a dc input voltage of the switch mode power supply; andprocessing means for estimating the arc voltage between a plasma arc torch tip and the metallic workpiece based on a combination of the duty cycle of the switch mode power supply and the value representing the dc input voltage of the switch mode power supply.

22. An apparatus that estimates an arc voltage in a thermal processing system in which a switch mode power supply including an output inductor provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the apparatus comprising: processing means for obtaining an average voltage applied to an input of the inductor;processing means for obtaining a value corresponding to a voltage drop across the inductor; andprocessing means for estimating the arc voltage based on the difference between the average voltage applied and the voltage drop.

23. An apparatus that estimating an arc voltage in a thermal processing system in which a switch mode power supply including an output inductor provides an arc current to generate a plasma arc between a plasma arc torch tip and a metallic workpiece, the method comprising: processing means for obtaining a time varying profile of expected variations in arc voltage;processing means for obtaining an average voltage applied to an input of the inductor; andprocessing means for estimating the arc voltage from a model representing changes in arc current through the inductor, the model based on an average voltage applied to the input of the inductor and the time varying profile of expected variations in the arc voltage.

24. The method of claim 1 wherein the switch mode power supply is based on a boost, buck or buck-boost circuit topology.

25. The method of claim 6 wherein the switch mode power supply is based on a boost, buck or buck-boost circuit topology.

26. The method of claim 11 wherein the switch mode power supply is based on a boost, buck or buck-boost circuit topology.

27. The method of claim 14 wherein the switch mode power supply is based on a boost, buck or buck-boost circuit topology.

28. The method of claim 6 wherein the combination is one of a summation or product.

29. The method of claim 21 wherein the combination is one of a summation or product.