Machine Design
Absolute encoders let water cannon douse flames accurately

Absolute encoders let water cannon douse flames accurately

Water cannons, called monitors in the firefighting industry, let firefighters direct water by aiming a spray nozzle mounted on a set of swivels that allow horizontal and vertical motion. Elkhart Brass manufactures over 20 models of monitors such as the Sidewinder unit pictured here. Sidewinders can be remotelly controlled from up to a quarter-mile away using RF communications. They are also used in fields ranging from the maritime industry for antipiracy to washdowns of mining and construction equipment, as well as trucks and airplanes. Water streams from a monitor can reach more than390 ft. The rate at which water is applied, not total volume of water, is what determines success. Firefighters calculate gallons-per-minute (gpms) of spray per estimated Btu of energy from the fire, not gallons per Btu. That is one reason why Elkhart Brass makes spray nozzles with precise flow rates. The unusual shape of the monitors serves to balance reaction force as high-velocity water shoots out of a monitor. It prevents the monitor from spinning on either axis when it is used.

Remotely controlled water cannons stay focused on their target thanks to their use of absolute position sensors on their X and Y axes.

The encoders are Vert-X 1300 noncontact rotary position sensors from Novotechnik, Southborough, Mass. The sensors use the Hall effect to note rotary position. They go on Sidewinder EXM water cannons from Elkhart Brass Mfg. Co., Elkhart, Ind.

One sensor goes on a thrust rod that takes axial hydraulic pressure off the ball bearings on the joint that permits Y-axis rotation of the water stream. The other sensor is on the thrust rod performing a similar role on the X-axis joint. Both sensors give accuracies to below 0.75° over the travel range of 0 to 350°.

Shock and vibration ratings of 50 g and 20 g, respectively, and Ingress Protection Ratings of 67 let the sensors handle the pressure levels routinely seen in water cannons.

Rotational limits and block-out zones for the cannons are controlled via position information from the encoders. Absolute encoding lets the cannons be accurately positioned even in the event of power loss. Moreover, firefighters or other users may move the cannons manually, so the positioning system must track where the device is aimed to prevent a high-pressure water stream from being inadvertently directed at bystanders.

In typical uses, water pressure in these systems can go from zero to as high as 1,500 psi, and water flow is 15 to 3,000 gpm. The time it takes for pressure to build from zero to a maximum level can be as short as a few seconds.

Visible in this exploded view of the Sidewinder EXM water cannon (monitor) are the Vert-X 1300 Hall-effect rotary sensors that feed back position of the X and Y axes. The thrust rods on which they sit absorb some of the load that would otherwise concentrate on the roller bearings at the joints.

Users control the cannon from the safety of a fire truck using a joystick.

The EXM has a travel range of ±175° horizontally and 145 to 175° vertically. The devices can also mount on stands, fire hydrants, and elsewhere. Rotational limits can be set to avoid directing high velocity water where it isn’t wanted, such as outside a cone of operation for a particular fire.

The Sidewinder EXM has a plug-and-play digital communication and control architecture that operates on the CANbus protocol. The bus also lets operators control the engine, pump, valves, and lights, as well as cameras and aerial ladder on the fire truck, via a central controller. Besides the joystick, firefighters view an LED display of the spray-nozzle position.

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