Pulsed Supersonic Jet with Local High Speed Valve

Abstract

A pulsed supersonic jet excavator uses a short duration blast of air to excavate using a minimum amount of air and a minimum reaction force. This device uses a fast acting valve to create a jet of air that lasts about as long as it takes to develop. The result is a device that works much more efficiently than existing air jet excavators. This device can be mounted on a small robot and allow it to dig, whereas a normal backhoe type excavator would just lift the robot when attempting to dig in packed earth.

Description

BACKGROUND

This application relates to excavators, particularly to portable pneumatic excavators.

PRIOR ART

Excavators which use a jet of air are well known, and they may be used to excavate mines, gas lines and such. These devices may be pulsed by the operator, or they may be pulsed by valves located in the handle, as in U.S. Pat. No. 5,966,847. These devices are cumbersome and bulky because they use powerful air compressors and large quantities of air. Prior devices waste air while building up to a supersonic jet and tapering down to zero pressure. The pressure during the rise and fall time is not sufficient to dig earth. In a previous application 20090044372, the inventor describes a device for cleaning surfaces which use supersonic jets. During the development of that device I discovered that short pulses of air are just as effective as long ones. Therefore a device which incorporates a fast acting valve located nearby a De Laval nozzle will provide the best excavation rate for the least amount of air consumption.

SUMMARY

In accordance with one embodiment an excavator includes a source of compressed air or gas such as a tank or compressor, an air conduit leading to a pulse jet, a nozzle to accelerate the air to maximum velocity, and at least one valve to let the air out through the nozzle in a sharp pulse. In some embodiments an electric or pneumatic circuit controls the operation of the valve or valves.

At present I believe the pressure of the air should be approximately 300 psi in order to create a supersonic jet of reasonable length, such that the air reaches the surface and recompresses in order to provide the maximum pressurization and shear force on the earth, but higher or lower pressures are also satisfactory. A pressure regulator may be used in between the tank or compressor and the valve. A heater or heat exchanger may be used upstream of the valve in order to keep the specific volume of the air at a high level.

I have found that the digging action occurs as the air pressure rises in proximity to the ground during the initial formation of the supersonic jet. The air pressure before the supersonic jet formation is not sufficient to dig. In one embodiment, the pulse duration is only long enough for the jet to form. This conserves air consumption. This is achieved by using a fast-acting valve which is close coupled to the nozzle. The time required to pressurize the nozzle ahead of the system is minimized. Such a valve is described in U.S. Pat. No. 5,271,226. This technology is common in the art of cold gas thrusters. At present I have found that a Marotta MV78C valve operates most efficiently, but other fast-acting valves are also satisfactory. The Marotta valve is an aerospace poppet valve which is actuated by a small balanced pilot valve.

In some circumstances, the flow of air to the valve may be limited. In this case a reservoir of air may be located close to the valve. The valve may be designed with hysteresis to open at a given pressure and close at a lower pressure. For example the valve may open a 300 psi and close at 100 psi. This type of valve converts a steady low flow of air to a pulsatile flow of air with a tapering off of pressure in each pulse. This allows the device to loosen soil with the initial pressure pulse and then blow it away with a lower pressure, thereby achieving effective excavation with a minimum of air consumption.

Another embodiment has a continuous supply of air to the valve and nozzle, an air storage chamber near the nozzle, and a dump valve which opens when the pressure in said chamber reaches a given value. This embodiment allows for the use of a smaller air conduit and pressure regulator leading to the pulse jet.

In some other embodiments, the air is heated before it reaches the nozzle. This has the advantage of generating more pulses for a given amount of air. It also helps counteract the cooling of the air which occurs as an air tank is consumed. This may be accomplished by means of a heat exchanger which uses the outside air, or a fuel driven heater to heat the air. It is well known that air cools when it expands through a supersonic nozzle, so the air can be heated up above ambient temperature and still result in a pulse jet that is at ambient temperature.

The nozzle is a standard De Laval type, which accelerates the air using a contraction and expansion section. The exit area can be determined based on the upstream pressure and local ambient pressures. This type of nozzle is common in rocket engines and the equations to design them are well known in the art.

In order to achieve the required short pulse, the valve must actuate very quickly, or the air supply must be limited. In some embodiments the air supply will be from a pressurized tank of air at 2000-5000 psi. In some embodiments in order to get the most energy out of the air, a valve which operates very quickly is used to utilize the full pressure of the tank. In some embodiments, adjusting the on time of the valve controls the impulse generated. This way, all the energy of the tank is used. Another possible scenario is to use a spring loaded accumulator in between the two valves. That way the air pressure does not need to taper off and more of the energy in the air can be utilized. In some embodiments the pressure in the air storage container is controlled by controlling the on time of the inlet valve. In this way, and adjustable pressure pulse is delivered.

ADVANTAGES

Several advantages of one or more aspects are to provide an excavator that is more efficient, more portable, inexpensive, has less reaction force, and provides more effective excavation with high pressure and less air consumption. Other advantages of one or more aspects are to provide an excavator that may use a local compressed air source tank and is not tethered to a compressor. Other advantages of one or more aspects are to provide a robotically controlled excavator wherein the robot is small and light-weight, and could not readily operate any other type of digging tool. Other advantages of one or more aspects will be apparent from a consideration of the drawings and ensuing description.

DRAWINGS

Figures

In the drawings, closely related figures have the same number but different alphabetic suffixes.

FIG. 1 is a schematic drawing of one embodiment with a local air chamber in the shape of a cylinder.

FIG. 2 is a drawing of a quick opening poppet valve in the closed position.

FIG. 3 is a drawing of a quick opening poppet valve in the open position.

FIGS. 4A to 4C show the dimensions determined for a given nozzle throat diameter.

FIGS. 5A and 5B show the results of one test of the stagnation pressure and duration of each pulse of air.

FIG. 6 shows a schematic of the logic used to control one embodiment with two valves.

FIG. 7 shows a graphical representation of the air conserved with a sharp supersonic pulse instead of a gradual pressure pulse.

FIG. 8 shows one embodiment including a pulsed supersonic jet with local high-speed valve attached to a robot.

FIG. 9 shows one embodiment including a pulsed supersonic jet with local high-speed valve and local video camera

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of one embodiment of the device. The nozzle 14 and a valve 13 are located is close proximity so that as the valve opens, little air is wasted filling the duct in between the nozzle and the valve. A local air chamber 12 holds a volume of air to be dispensed. In operation the prefill valve 11 lets high-pressure air into local air chamber 12. Valve 11 then shuts and valve 13 opens, dumping the air through the nozzle. In one embodiment, operation of the valves is controlled by an electric or pneumatic circuit. In some embodiments, the high pressure air is be supplied by a compressor or a tank of compressed air upstream of conduit 10.

In FIG. 2 a nozzle and valve combination is illustrated. This includes a volume of air in toroidal prefill chamber 21 in close proximity to valve seat 27. The poppet 25 keeps the valve closed until the pressure applied over the outer ring 23 overcomes the pressure in dome load compartment 22, which is augmented in some embodiments by a spring (not shown). Once the pressure in chamber 21 pushes the poppet 25 up, the air then fills in the entire area 28 under the poppet, forcing it up. The throat 26 of the nozzle becomes the flow limiting feature. As the restricted inlet flow is not sufficient to maintain the high pressure continuously, the pressure behind the nozzle 14 falls to a lower pressure, such as 100 psi, at that point, the pressure in compartment 22 is enough to push the poppet 25 back on the seat 27. At that point the valve closes and does not reopen until the pressure rises to 300 psi in prefill chamber 21. The poppet may be limited to a purely up and down movement by poppet guide 24.

In FIG. 3, the same valve is shown in the open position. In this position the pressure in chamber 21 is applied to the whole diaphragm 23 and poppet 25.

FIG. 4 shows the nozzle dimensions for one embodiment for ideally expanded supersonic flow. The nozzle exit diameter for various feed pressures was determined using adiabatic and isentropic relations for supersonic flow. The calculations assume air as the working fluid and a throat diameter of 0.635 cm (0.25 inch). My present nozzle has a 0.635 cm (0.25 inch) throat diameter, a 15° cone angle, and a 1.105 cm (0.435 inch) diameter exit nozzle, but other nozzle dimensions are also acceptable.

FIG. 5 shows a stagnation pressure test of one embodiment which included a single Marotta MV78C valve. The valve was commanded to open for 40 ms and the resulting stagnation pressure pulse width was 63 ms, but other pulse durations are also satisfactory.

FIG. 6 shows the control sequence of one embodiment including two valves in order to produce a short pulse width pressure burst. The fill length 30 is the period of time the upstream valve 11 is open. The fire delay 32 is the period of time that both valves remain closed. The firing length 34 is the period of time the downstream valve 13 is open and exhausting the air in the volume between the two valves. The delay until fill 36 is the period of time from when the downstream valve 13 is closed and the upstream valve 11 is opened again.

FIG. 7 shows a graphic representation of a sharp supersonic pulse 44 and a gradual pressure rise and fall 42. The pressure where digging occurs is at pressure level 38 and above. The sharp burst 44 provided with the local high speed valve uses nearly all of the air in the digging region 38. The gradual pressure pulse 42 wastes air during time spent in the low pressure area 40 where no digging occurs.

FIG. 8 shows one embodiment of the pulsed supersonic nozzle 14 with local high-speed valve 13 attached to a robot 46. In this embodiment, a tank of compressed air 48 supplies the high pressure air, but a compressor would also be satisfactory. In this embodiment, the high-speed valve 13 is controlled by a pneumatic circuit 50, but an electric circuit would also be satisfactory.

FIG. 9 shows one embodiment of the pulsed supersonic nozzle 14 with local high-speed valve 13 attached to a robot 46 including a video camera 52.

Claims

1. A pulsed supersonic jet excavator consisting of A supersonic nozzle including a converging portion and a diverging portion.A high-speed valve located within 10 nozzle diameters of said nozzle,Said nozzle and valve being attached to a remotely controlled manipulation means.

2. The device as in claim 1, including a local air storage chamber which is periodically dumped through the valve

3. The device as in claim 1, including a heater to heat the air.

4. The device as in claim 1, including a poppet valve wherein the downstream pressure motivates the valve towards an open position.

5. The device as in claim 1, including a robot to carry said device.

6. The device as in claim 1, including a video camera to feed data to a remote location.

7. The device as in claim 1, including an attachment to a vehicle.

8. The device as in claim 1, with including a metal detector to detect buried objects.

9. The device as in claim 1, wherein the volume of the fluid connection between said high speed valve and said nozzle is less than 5 times the volume of said nozzle

10. The device as in claim 2, including a pressure transducer to measure the pressure in said storage chamber.

11. The device as in claim 1, Said valve open duration being less than 10 times to the time required for the supersonic jet to develop to its full supersonic length

12. The device as in claim 1, with a local video camera with positioning means to observer the excavation.