METHOD AND APPARATUS FOR STABILIZING COMBUSTION SYSTEM PERFORMANCE

Abstract

A combustion system includes an ionizer configured to eject charges (or accept charges) for uptake by a combustion reaction to cause a combustion reaction to carry a majority charge or voltage. The ionizer includes an inner electrode, a dielectric body surrounding the inner electrode, and one or more conductive or semi-conductive inner electrodes disposed on the surface of the dielectric body. The inner and outer electrodes are configured to be in a capacitive relationship.

Description

BACKGROUND

Combustion systems typically include a fuel source and oxidant source. The fuel and oxidant are mixed together in a combustion chamber and a combustion reaction is initiated and sustained. The heat from the combustion reaction can be used to generate electricity, to heat materials in industrial processes, to drive endothermic chemical reactions, and many other applications. The characteristics of a combustion reaction determine how effectively these purposes can be carried out. It is desirable to be able to manipulate a combustion reaction in a selected manner to improve the effectiveness of the combustion reaction.

SUMMARY

One embodiment is a combustion system including a fuel source and burner for initiating and maintaining a combustion reaction in a combustion volume. An ionizer is positioned adjacent the combustion reaction, separated from the combustion reaction by a gap including a dielectric gas. The ionizer includes an inner electrode coupled to a high voltage power source. The inner electrode is covered by a dielectric body. An electrode is positioned on an outer surface of the dielectric body and electrically insulated from the inner electrode by the dielectric body. The electrode is nevertheless capacitively coupled to the inner electrode. When the power source supplies a high voltage to the inner electrode, a high voltage is similarly induced on the electrode via the capacitive coupling. The high voltage on the electrode can be used to manipulate a characteristic of the combustion reaction.

In one embodiment, the combustion system includes a counter electrode positioned in or near the combustion reaction. The counter electrode is coupled to the power supply and configured to receive a second voltage from the power supply. The second voltage is imparted to the combustion reaction by the counter electrode, which is electrically coupled to the combustion reaction. In one embodiment, the second voltage is ground. By applying respective voltages to the counter electrode and the inner electrode, the combustion reaction can be manipulated to obtain a desired effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a combustion system including an ionizer to apply an electrical potential to a combustion reaction, according to an embodiment.

FIG. 2 is a diagram of a combustion system including an ionizer to apply an electrical potential to a combustion reaction, according to another embodiment.

FIG. 3 is a diagram of a combustion system including an ionizer to apply an electrical potential to a combustion reaction, according to another embodiment.

FIG. 4A is a cross sectional diagram of the ionizer of FIG. 1, according to one embodiment.

FIG. 4B is a cross sectional diagram of an ionizer, according to one embodiment.

FIG. 5 is a cross sectional diagram of an ionizer, according to one embodiment.

FIG. 6 is a diagram of a combustion system including an ionizer and an annular counter electrode, according to one embodiment.

FIG. 7 is a diagram of a combustion system, according to one embodiment.

FIG. 8 is a flow diagram of a process for operating a combustion system, according to one embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure.

FIG. 1 is a diagram of a combustion system 100 including an ionizer 101 to apply an electrical potential to a combustion reaction 110, according to an embodiment. As used herein, the term “ionizer” refers to an apparatus configured to generate charged particles, which can be ions (atoms including an atomic nucleus and missing or additional electron(s)) or electrons. The ionizer 101 for a combustion system 100 includes an inner electrode 112 and a dielectric body 104 having an inside surface 106 and an outside surface 108. The dielectric body 104 is configured to maintain high electrical resistance in the presence of a combustion reaction 110. The inner electrode 112 is disposed inside the dielectric body 104, the inner electrode 112 being electrically insulated from the combustion reaction 110. One or more outer electrodes 114 are disposed outside the dielectric body 104 in capacitive communication through the dielectric body 104 with the inner electrode 112. A high voltage power supply 116 has first and second voltage output nodes 118, 122 operatively coupled to the inner electrode 112 and to a conductive combustion support structure 124, respectively.

The high voltage power supply 116 can apply a periodic voltage signal to the inner electrode 112 via the first voltage output node 118. The periodic voltage signal can be selected to cause ejection of electrical charges between the one or more outer electrodes 114 and a dielectric gap 120 disposed between the outer electrodes 114 and the combustion reaction 110. In some embodiments, the dielectric gap 120 includes a gas that acts as a dielectric to prevent direct electrical continuity between the combustion reaction 110 and the outer electrodes 114. In some embodiments, a source of cool gas can maintain a flow of cool gas in the dielectric gap 120. For example, the cool gas can include combustion air. In some embodiments, the ejection of electrical charges can be periodic and synchronous with the periodic voltage.

The periodic voltage signal can include a first portion characterized by a positive voltage. The outer electrodes 114 can receive electrons from the dielectric gap 120 during the positive voltage portion of the periodic voltage signal, resulting in ejection of a positive charged particle. The periodic voltage signal can include a second portion characterized by a negative voltage. The outer electrode 114 can eject electrons into the dielectric gap 120 during the negative voltage portion of the periodic voltage signal.

The high voltage power supply 116 can be configured to output a periodic voltage signal having a peak-to-peak difference of 40,000 volts or more. In some embodiments, the high voltage power supply 116 can be configured to output a periodic voltage signal having a peak-to-peak difference of 100,000 volts or more. Optionally, the high voltage power supply 116 can apply an asymmetric waveform including a first portion having one polarity configured to eject charged particles of the same polarity, and a second portion of opposite polarity at a voltage insufficient to eject charged particles of the opposite polarity. Moreover, as will be described below, the ionizer 101 can be structured to preferentially eject charged particles having a selected polarity (e.g., by doping the outer electrodes 114).

The periodic voltage signal can include an alternating current (AC) voltage waveform. Additionally or alternatively, the periodic voltage signal can include a direct current (DC) chopped voltage waveform. The DC chopped voltage waveform can be DC offset from voltage ground. The DC chopped voltage waveform can include a square or a sawtooth waveform, for example.

In one embodiment, the dielectric body 104 can include fused quartz. Alternatively, another suitable dielectric material can be used for the dielectric body 104.

In one embodiment, the conductive combustion support structure 124 is a fuel nozzle configured to emit fuel and hold the combustion reaction 110. Alternatively, the conductive combustion support structure 124 can include a flame holder disposed adjacent to or in a fuel jet and configured to hold the combustion reaction 110. The conductive combustion support structure 124 can be disposed for at least periodic electrical continuity with the combustion reaction 110 by which a voltage can be imparted to the combustion reaction 110 from the high voltage power supply 116. The conductive combustion support structure 124 can receive ground voltage, or another voltage signal, from the high voltage power supply 116 via the second voltage output node 122. Additionally or alternatively, the conductive combustion support structure 124 can be electrically isolated from electrical ground.

In various embodiments, the inner electrode 112 can include a solid conductor, a metal mesh, a stranded structure, stainless steel, and/or a superalloy such as Inconel.

The one or more outer electrodes 114 can be shaped to cause an electric field curvature in the dielectric gap 120 disposed between the outer electrode 114 and the combustion reaction 110. In some embodiments, the one or more outer electrodes 114 can be shaped to have a lateral extent less than about 0.10 inch. In some embodiments, the one or more outer electrodes 114 can be shaped to have a lateral extent less than about 0.02 inch in at least one dimension along the outside surface 108 of the dielectric body 104. The one or more outer electrodes 114 can include a metal, stainless steel, and/or Inconel.

FIG. 2 is a diagram of a combustion system 200 including an ionizer 201 to apply an electrical potential to a combustion reaction 110, according to another embodiment. The one or more outer electrodes 114 can include a semiconductor. The semiconductor can include germanium, doped germanium, silicon and/or doped silicon.

The one or more outer electrodes 114 can include a p-doped semiconductor. The one or more p-doped semiconductor electrodes 114 can be configured to receive electrons from the dielectric gap 120 adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a positive voltage. Additionally or alternatively, the one or more p-doped semiconductor electrodes 114 can be configured to minimize an ejection of electrons to the dielectric gap 120 adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a negative voltage. Additionally or alternatively, the one or more p-doped semiconductor electrodes 114 can be configured to eject positive charges to a dielectric gap adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a positive voltage.

The one or more outer electrodes 114 can include an n-doped semiconductor. The one or more n-doped semiconductor outer electrodes 114 can be configured to eject electrons to the dielectric gap 120 adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a negative voltage. Additionally or alternatively, the one or more n-doped semiconductor electrodes 114 can be configured to minimize an ejection of positive charges to the dielectric gap 120 adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a positive voltage.

The one or more outer electrodes 114 can include both p-doped semiconductor outer electrodes 114 and n-doped semiconductor outer electrodes 114. The one or more p-doped semiconductor outer electrodes 114 can be configured to receive electrons from the dielectric gap 120 adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a positive voltage. Additionally or alternatively, the one or more n-doped semiconductor outer electrodes 114 can be configured to eject electrons into the dielectric gap 120 adjacent to the inner electrode 112 during a time interval when the inner electrode 112 is held at a negative voltage. The p-doped and n-doped semiconductor outer electrodes 114 can be arranged in an interleaved pattern on the outside surface 108 of the dielectric body 104. In this embodiment, the n-doped semiconductor outer electrode(s) 114 act(s) to increase electric field curvature around the p-doped semiconductor electrode(s) 114 during a time interval when the inner electrode 112 can be held at a positive voltage. Additionally or alternatively, the p-doped semiconductor outer electrode(s) 114 act(s) to increase electric field curvature around the n-doped semiconductor electrode(s) 114 during a time interval when the inner electrode 112 can be held at a negative voltage.

The dielectric body 104 can include shapes other than tubular. For example, the inner electrode 112 can be configured as a planar element. The dielectric body 104 can be formed from a planar material such as two fused quartz sheets. The fused quartz sheets can be arranged superjacent and subjacent to the planar inner electrode 112 with some margin around three or more edges of the inner electrode 112. A metal lead operatively coupled to the inner electrode 112 can optionally be placed to emerge from between a margin in the quartz sheets along a fourth edge of the inner electrode 112. The edges of the subjacent and the superjacent quartz sheets can be heated to fuse together, leaving an inner electrode 112 that is insulated. In an embodiment, the outer electrodes 114 can be disposed around one or more of the fused quartz edges. Placing the outer electrodes 114 in this location can, for example, help to reduce electric field shadowing of the electrodes 114 by the inner electrode 112. Other shapes may be substituted for a planar and rectangular inner electrode 112 and planar and rectangular quartz sheets.

FIG. 3 is a diagram of a combustion system 300 including an ionizer 301 configured to apply an electrical potential to a combustion reaction 110, according to an embodiment. The ionizer 301 is substantially similar to the ionizer 101 of FIG. 1 except that the outer electrodes 314 of FIG. 3 have a circular cross section.

In one embodiment the outer electrodes 314 are individual electrodes physically separated from each other. Alternatively, the outer electrodes 314 can all be a same thin wire wound around the ionizer 301.

FIG. 4A is a cross section of the ionizer 101 of FIG. 1, according to one embodiment. The ionizer 101 includes a cylindrical inner electrode 112. The cylindrical inner electrode 112 is covered by a layer of the dielectric body 104. The inside surface 106 of the dielectric body 104 is in contact with the inner electrode 112. The outer electrode 114 is positioned on the outside surface 108 of the dielectric body 104.

In the embodiment of FIG. 4A, the outer electrode 114 is a sharp electrode. The outer electrode 114 is electrically insulated from the inner electrode 112 by the dielectric body 104 such that an electrical current will not flow between the outer electrode 114 and the inner electrode 112. However, the outer electrode 114 is capacitively coupled to the inner electrode 112 via the dielectric body 104 separating the inner electrode 112 from the outer electrode 114. Due to the capacitive coupling between the inner electrode 112 and the outer electrode 114, when a voltage is applied to the inner electrode 112, the voltage on the outer electrode 114 will also change. By applying a high voltage to the inner electrode 112, a high voltage can be induced on the outer electrode 114.

When a second voltage (for example, ground voltage) is applied to a structure near the combustion reaction 110, such as the conductive combustion support structure 124, a high charge density will accumulate at the outer edges of the outer electrode 114 and particularly at the outer corners of the outer electrode 114. The high charge density can correspond to a particularly high density of electrons or the absence of electrons at the outer edges of the outer electrode 114 depending on the polarities of the voltages on the inner electrode 112 and the conductive combustion support structure 124. For example, if the high voltage on the outer electrode 114 has a negative polarity with respect to the combustion reaction 110, then a high density of electrons will accumulate at the outer edges of the outer electrode 114. If the high voltage on the outer electrode 114 has a positive polarity with respect to the combustion reaction 110, then electrons will flee the outer edges of the outer electrode 114 resulting in a high density of positive charges at the outer edges of the outer electrode 114.

The high charge density at the outer edges of the outer electrode 114 results in a very strong electric field near the outer edges of the outer electrode 114. The strong electric field near the outer electrode 114 can affect the combustion reaction 110 in various ways. The outer electrode 114 can eject charge into the combustion reaction 110 or the dielectric gap 120. The outer electrode 114 can also induce ionization of gases in the dielectric gap 120. Additionally, the electric field from the outer electrode 114 can influence the combustion reaction 110 without ejecting charges or ionizing material in the dielectric gap 120. By selecting the respective voltage polarities, respective voltage magnitudes, the width of the dielectric gap 120, and, in the case where multiple outer electrodes 114 are present, the relative positioning of the outer electrodes 114, the characteristics of the combustion reaction 110 can be manipulated in a desired manner. For example, the combustion reaction 110 can be manipulated to more thoroughly combust the fuel, to reduce pollutants, to stretch the length of the combustion reaction 110, to contract the length of the combustion reaction 110, to change color, to make the combustion reaction 110 not apparent, etc.

In one embodiment, the outer electrode 114 can be electrically connected to the high voltage power supply 116. Prior to applying the high voltage to the inner electrode 112, both the inner electrode 112 and the outer electrode 114 can be connected to ground voltage to establish a voltage relationship between the inner electrode 112 and the outer electrode 114. A switch can then electrically decouple the outer electrode 114 from the high voltage power supply 116. Due to the established capacitive relationship between the inner electrode 112 and the outer electrode 114, when the high voltage is applied to the inner electrode 112, a high voltage will appear on the outer electrode 114.

In one embodiment, the outer electrode(s) 114 can be produced by depositing a conductive material on the dielectric body 104 such that the dielectric body 104 is covered by the conductive material. A mask is then placed on the conductive material. The mask has a pattern according to which the outer electrode(s) 114 will be formed. With the mask covering the surface of the conductive material, the ionizer 101 is placed in a liquid etchant such as potassium hydroxide (KOH) or another suitable etchant that will selectively etch the conductive material in those areas not covered by the mask without significantly etching the dielectric body 104. When the ionizer 101 is removed from the liquid etchant and the mask is removed, the outer electrode(s) 114 remains. The particular etchant can be selected based on the particular materials from which the dielectric body 104 and the outer electrode(s) 114 are made.

FIG. 4B is a cross section of an ionizer 401, according to one embodiment. The ionizer 401 is substantially similar to the ionizer 101 of FIG. 4A, except that a sharp outer electrode 414 has a triangular cross section. In particular, the sharp outer electrode 414 includes a sharp point on a side of the outer electrode 114 furthest from the inner electrode 112. The ionizer 401 operates in a substantially similar manner as the ionizer 101 of FIG. 4A.

While an outer electrode having a triangular cross-section and an outer electrode having a rectangular cross-section have been disclosed, other shapes are possible for the outer electrodes 114 as will be understood by those of skill in the art in light of the present disclosure. For example, the outer electrodes 114 can have a cross-section corresponding to that of a thin rounded wire. All such other electrode shapes fall within the scope of the present disclosure.

FIG. 5 is a cross section of an ionizer 501, according to an alternate embodiment. The ionizer 501 has a substantially rectangular cross-section. In particular, an inner electrode 512 and a dielectric body 504 of the ionizer 501 have a rectangular cross-section. An outer electrode 414 is positioned on the on the dielectric body 504. Though not shown in the figures, those of skill in the art will understand, in light of the present disclosure, that many other shapes and configurations can be implemented for an ionizer in accordance with principles of the present disclosure. All such other shapes and configurations fall within the scope of the present disclosure.

FIG. 6 is a diagram showing a combustion system 600 including an ionizer 101 and a counter electrode 628, according to an embodiment. The counter electrode 628 is proximate to the combustion reaction 110. The ionizer 101 includes an inner electrode 112 (see FIGS. 1-3) and a plurality of outer electrodes 114 isolated from the inner electrode 112 by the dielectric body 104. A high voltage power supply 116 is operatively coupled to the counter electrode 628 by a voltage output node 622, and to the inner electrode 112 of the ionizer 101 by a voltage output node 118.

In the combustion system 600 of FIG. 6, the counter electrode 628 has the shape of a torus or a toroid surrounding the combustion reaction 110 and positioned a selected distance above the conductive combustion support structure 124. In one embodiment, the counter electrode 628 lacks the sharp features of the outer electrode 114. Therefore, the strength of the electric field in the immediate vicinity of the counter electrode 628 is not as great as the strength of the electric field in the immediate vicinity of the sharp outer electrode 114. The counter electrode 628 does not tend to eject charge into or induce ionization of the surrounding dielectric medium.

In one embodiment, the counter electrode 628 is configured so that the electric field adjacent to it is about equal to or less than the average electric field magnitude in the region between the outer electrodes 114 and the counter electrode 628.

The counter electrode 628 is operatively coupled to the high voltage power supply 116. In one embodiment, the counter electrode 628 can be held substantially at ground potential, or can be configured to be driven to an instantaneous voltage substantially the same as the instantaneous voltage applied to the outer electrodes 114. Alternatively, the counter electrode 628 can be configured to be galvanically isolated from ground and from other electrical potentials.

FIG. 7 is a diagram of a combustion system 700, according to one embodiment. The combustion system 700 includes a combustion wall or enclosure 730 defining an inner furnace volume 732. An ionizer 101 extends into the furnace volume 732 through an aperture in an upper portion of the wall 730. The conductive combustion support structure 124 sustains a combustion reaction 110 within the furnace volume 732. A fuel source 734 provides fuel to the conductive combustion support structure 124. A high voltage power supply 116 is electrically coupled to the conductive combustion support structure 124 into the ionizer 101.

The conductive combustion support structure 124 is a conductive flame holder or fuel nozzle that supports the combustion reaction 110. According to one embodiment, ground voltage is applied to the conductive combustion support structure 124 by the high voltage power supply 116. Because the combustion reaction 110 is conductive, the ground voltage is imparted to the combustion reaction 110 by the conductive combustion support structure 124.

In one embodiment, the ionizer 101 is configured substantially as described in relation to FIG. 1. The ionizer 101 can include an inner electrode 112 covered in a dielectric body 104. Outer electrodes 114 are positioned on the outside of the dielectric body 104. The outer electrodes 114 are electrically insulated from the inner electrode 112 by the dielectric body 104. Nevertheless, the outer electrodes 114 are capacitively coupled to the inner electrode 112. The outer electrodes 114 are separated from the combustion reaction 110 by a dielectric gap 120 containing a dielectric gas such as air or flue gas.

The high voltage power supply 116 is configured to supply a high voltage to the inner electrode 112 of the ionizer 101. Due to capacitive coupling between the inner electrode 112 and the outer electrodes 114, when the high voltage power supply 116 supplies the high voltage to the inner electrode 112, a high voltage is also induced on the outer electrodes 114.

The combustion reaction 110 can be manipulated by applying respective voltages to the inner electrode 112 and to the combustion reaction 110. In particular, the combustion reaction 110 can be manipulated to change the color of the combustion reaction 110, to make the combustion reaction 110 not apparent, to stretch the length of the flame, to contract length of the flame, to more thoroughly combust the fuel, to reduce pollutants, etc.

Typically, as shown in FIG. 7, the ionizer 101 is separated from the combustion reaction 110 by dielectric gap 120. However, it is possible in some circumstances that direct contact will occur between the combustion reaction 110 and one or more of the outer electrodes 114. In some circumstances, it is even possible that the ionizer 101 will be intentionally positioned within the combustion reaction 110. In the event that the ionizer 101 is in direct contact with the combustion reaction 110, a short circuit of the high voltage power supply 116 is prevented because the outer electrodes 114 are electrically insulated from the high voltage power supply 116 by the dielectric body 104 covering the inner electrode 112. Thus, in the event of accidental or intentional contact between the ionizer 101 and the combustion reaction 110, a short circuit will not occur.

FIG. 8 is a flow diagram of a process 800 for operating a combustion system, according to one embodiment. At 802, a combustion reaction is implemented and sustained in the combustion system. The combustion reaction can include combustion between a fuel and oxygen source injected into the combustion volume.

At 804, a high voltage is applied to an inner electrode of an ionizer. The inner electrode of the ionizer is covered by a dielectric body. An electrode is positioned on the outside of the dielectric body. The outer electrode is electrically insulated from the inner electrode by the dielectric body. Nevertheless, the outer electrode is capacitively coupled to the inner electrode by the dielectric body.

At 806, a high voltage is induced on the outer electrode by the capacitive coupling between the outer electrode and the inner electrode. Thus, when the high voltage is applied to the inner electrode, a high voltage is induced on the outer electrode by capacitive coupling with the inner electrode.

At 808, a second voltage is applied to a counter electrode electrically coupled to the combustion reaction. The counter electrode can be a fuel nozzle from which fuel for the combustion reaction is emitted, a conductive mesh on which a solid fuel rests, a flame holder configured to hold the combustion reaction, or a conductor otherwise positioned in or near the combustion reaction. Because a flame conducts electricity, the second voltage is imparted to the flame by the counter electrode. At 810, a characteristic of the combustion reaction is manipulated by controlling the high voltage. The high voltage induces a strong electric field adjacent to the electrode of the ionizer. The strong electric field can eject charges from the electrode, can attract charges to the electrode, can cause ions or charged particles within the flame to behave in a certain way, etc. In this way, a desired effect can be introduced in the combustion reaction by applying respective voltages to the inner electrode and the counter electrode.

In the foregoing description, an ionizer or ion source has been described. Nevertheless, in some embodiments the ionizer or ion source may not, in fact, be a source of ions, but may instead merely manipulate a combustion reaction by influencing via electric field/electric potential ions or free charges already present in the combustion reaction. Nevertheless, the terms “ionizer” and “ion source” still apply to such other embodiments, even if the function is not to ionize or act as an ion source.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. A method, comprising: applying a first high voltage signal to an inner electrode of an ionizer positioned adjacent to a combustion reaction, the ionizer including a dielectric body covering the inner electrode and one or more outer electrodes disposed on the dielectric body;causing the one or more outer electrodes to form a charged particle in a dielectric gap between the ionizer and the combustion reaction via capacitive coupling of the outer electrodes with the inner electrode; andproviding the charged particle to the combustion reaction to influence a characteristic of the combustion reaction.

2. The method of claim 1, wherein applying the first high voltage signal includes applying a periodic voltage signal.

3. The method of claim 2, wherein the first high voltage signal has a peak-to-peak voltage greater than 1000V.

4. The method of claim 2, wherein the first high periodic voltage signal has a peak-to-peak voltage greater than 40,000V.

5. The method of claim 1, further comprising applying a second voltage signal to a counter electrode separate from the ionizer.

6. The method of claim 5, further comprising: outputting fuel for the combustion reaction from a combustion support structure, wherein the combustion support structure includes the counter electrode.

7. The method of claim 6, wherein the counter electrode is a conductive ring positioned proximate to the combustion reaction.

8. The method of claim 5, wherein applying the second voltage signal includes applying a ground voltage.

9. The method of claim 1, wherein the one or more outer electrodes are metal.

10. The method of claim 1, wherein the one or more outer electrodes include a doped semiconductor material.

11. The method of claim 1, wherein the first high periodic voltage signal is an AC voltage signal.

12. The method of claim 1, wherein the first high periodic voltage signal is a sawtooth voltage signal.