Apparatus and method for controlling the introduction of chemical foamant into a water stream in fire-fighting equipment

Abstract

A fire-fighting system in which a liquid chemical foamant is introduced into the water stream in varying amounts so as to maintain a predetermined percentage mixture irrespective of variations in the water flow includes a motor-driven pump where the motor is controlled over a first speed range by pulse width modulating the drive to the motor and over a second speed range, producing intermittent motion, by burst width modulating the drive signal when it is operating at its minimum pulse width modulated duty cycle. Using this technique the range of available motor speeds is extended well beyond that which can be achieved using pulse width modulation of the motor drive signal alone.

What is claimed is: 
     
       1. In a fire extinguishing system of the type including a water supply means for normally delivering water at varying flow rates through a hose member, means for monitoring .[.the flow of.]. water .Iadd.flow .Iaddend.through said hose member and producing an electrical signal related to .[.the rate of water flow.]. .Iadd.a characteristic of the water flowing .Iaddend.through said hose member, a supply tank for containing a supply of a liquid chemical foamant, positive displacement pump means having an input port coupled to said supply tank, an output port coupled to said hose member.Iadd., .Iaddend.and pump speed sensing means for sensing .[.the.]. .Iadd.a .Iaddend.speed at which said pump .Iadd.means .Iaddend.is being driven .Iadd.and generating a corresponding output signal.Iaddend., a variable speed electrical .Iadd.drive .Iaddend.motor for driving said pump means at a speed determined by a control signal applied .[.thereto.]. .Iadd.to said drive motor.Iaddend., computing means coupled to receive said electrical signal .[.relating to flow.]. and .[.a.]. .Iadd.said output .Iaddend.signal from said pump speed sensing means, said computing means determining the .Iadd.determined .Iaddend.speed at which said .Iadd.drive .Iaddend.motor should be driven to introduce a metered quantity of said chemical foamant into said hose member.[., depending on the rate of flow of water through said hose member.]., the improvement comprising: (a) motor drive means coupled to said computing means for generating said control signal for said drive motor, said .[.motor drive means producing.]. .Iadd.control signal comprising .Iaddend.a pulse width modulated signal of a predetermined frequency whose duty cycle is determined by said computing means for varying the speed of said drive motor between a maximum and a lower intermediate speed; and   (b) burst width modulating means for turning said pulse width modulated signal on and off at predetermined time intervals determined by said computing means for varying the speed of said drive motor between said lower intermediate speed and a predetermined minimum speed whereby .[.the.]. .Iadd.a .Iaddend.flow rate of said .[.liquid concentrate.]. .Iadd.chemical foamant .Iaddend.into said hose member is controllable over an extended range.   
     
     
       2. The fire extinguishing system as in claim 1 and further including: (a) level sensing means for sensing .[.the.]. .Iadd.a .Iaddend.level of chemical foamant in said supply tank and providing a signal to said computing means for disabling said motor drive means following .[.the.]. .Iadd.a .Iaddend.fall of said chemical foamant in said supply tank below a predetermined level. .[.3. The fire extinguishing system as in claim 2 and further including:   (a) air compressor means for injecting compressed air into said hose member; and   (b) interlock circuit means operatively coupled between said computing means and said motor drive means to detect the presence and absence of said signal from said pump speed sensing means and said signal from said level sensing means for controlling the on/off state of said air compressor..]..[.4. The fire extinguishing system as in claim 3 wherein said interlock circuit means puts said air compressor in said off state when either said signal from said pump speed sensing means or said signal   
     
     
        from said level sensing means is absent..].5. The fire extinguishing system as in claim 1 and further including display means coupled to said computing means for visually indicating any one of the current flow of water and chemical foamant per minute, the total amount of water and chemical foamant pumped, the chemical foamant injection rate setting in the percent mode.Iadd., .Iaddend.and .Iadd.the .Iaddend.total amount of 
     
     
        chemical foamant pumped. 6. The fire extinguishing system as in claim 5 and further including non-volatile RAM memory means operatively coupled to said computing means and manually operable switch means coupled to said computing means for entering operating information into said RAM memory 
     
     
        means. 7. The fire extinguishing system as in claim 6 wherein said operating information includes calibration parameters relating to a known quantity of chemical foamant pumped from said supply tank to a displayed 
     
     
        quantity on said display means. 8. The fire extinguishing system as in claim 1 wherein said predetermined frequency is 476 Hz and said duty cycle varies between 100 percent and about 30 percent to 35 percent in 
     
     
        establishing said maximum and lower intermediate speeds. 9. The fire extinguishing system as in claim 8 wherein said burst width modulating means provides a burst on-time to an off-time ratio varying between 100 
     
     
        percent and about five percent. 10. The fire extinguishing system as in claim 9 wherein said hose member is of a sufficient length to ensure complete mixing of said chemical foamant with said water when said pump means is driven intermittently by said motor.