Patent Publication Number: US-6705361-B2

Title: Apparatus and method for filling a painting robot canister

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional patent application serial no. 60/331,968 filed Nov. 21, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to automated spray painting or coating equipment and, in particular, to an apparatus for controlling a paint canister filling operation. 
     In spray painting of various types of products, such as automobile bodies or automobile body panels, automated machinery has been developed to spray a succession of products in a continuous and rapid manner. One type of prior art spray painting device includes a canister that is adapted to be releasably attached to a robot arm. When the canister is empty, or when a change in paint color is required, the robot moves the canister to a docking station where the canister is exchanged for a full canister. While the robot is using paint from the full canister, the empty canister is being filled for the next job at the docking station. 
     The canisters include a piston slidably disposed therein whereby the position of the piston defines a desired volume required for “filling” the paint canister. A first step in the filling operation is to extend the piston so that no paint remains in the canister. A supply valve is opened and paint under pressure from a paint supply forces the paint into the canister, which paint displaces the piston within the canister. One prior art method of determining the amount of paint in the canister is to monitor the flow of paint to the canister with a flow monitoring instrument. Another method is to monitor the position of the piston with a sensor, such as a potentiometer, which provides an analog signal to a paint supply control. When the control detects that the piston has moved to the position that represents the desired volume of paint in the canister, the supply valve is closed and the paint filling operation is complete. The filled canister is now ready to be exchanged when the robot returns to the docking station. Alternatively, an encoder, rather than the potentiometer, can be used to detect when the piston is at the desired position. 
     The sensor, whether the potentiometer or the encoder, however is a potential source for electrical and/or mechanical failure. In addition, the sensor must be made intrinsically safe or explosion proof as it must operate in hazardous environments, such as a paint booth or the like. 
     It is desirable, therefore, to provide a method and an apparatus for monitoring a paint filling operation that reduces the risk of failure and does not require intrinsically safe or explosion proof construction in order for the apparatus to operate. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a method and an apparatus for monitoring a paint canister filling operation. The apparatus includes a paint canister having a paint receiving interior with a piston slidably movable therein. A position of the piston in the canister interior determines a selected amount of paint to be received by the canister. The canister interior is connected to a paint supply through a supply valve. An actuator is connected to the piston for applying a force to the piston, which tends to move the piston in the canister interior. A sensor is connected to the actuator for sensing a force applied by the actuator to the piston. A control means is connected to the supply valve, the actuator and the sensor. The control means operates the actuator to move the piston to and maintain the piston at a predetermined position in the canister interior and opens the supply valve to cause paint to flow into the canister interior. The control means is responsive to a signal generated by the sensor representing a force applied by the actuator to maintain the piston at the predetermined position to close the supply valve when the force applied indicates a selected amount of the paint has been received by the canister. Preferably, the actuator is an electric motor and the sensor senses a value of torque generated by the electric motor representing said force applied to the piston. 
     The method for controlling a volume of paint in a canister during a filling operation according to the present invention, the canister including a piston movable in an interior of the canister, comprises the steps of: applying a force to a piston to move the piston to a predetermined required position in an interior of a canister corresponding to a total volume of paint required for a painting operation; supplying paint to the canister interior; applying a force to the actuator to maintain the piston in the required position; comparing the force being applied to the piston to a set point force representing the force required to maintain the piston at the required position; and stopping the supply of paint to the canister interior when the force being applied to the piston exceeds the set point force. The method can include, prior to performing said step a., moving the piston to an extended position to expel any paint in the canister interior, determining the total volume of paint required and calculating the required position for the piston. 
     The present invention advantageously provides efficiency to the paint filling process by determining when sufficient material has entered the canister. In the prior art, the control software waits for predetermined time interval. When the time interval expires, the assumption would be that all the material has been loaded into the canister. 
     The present invention eliminates this assumption by providing confirmation that material has entered the canister by the motor torque achieving the specified threshold torque value. The present invention also advantageously provides a means for air contamination detection by monitoring the rate of change of torque. Paint contaminated with air will be more compressible and, therefore, will have a slower increase of observed torque. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
     FIG. 1 is a schematic view of a prior art apparatus for controlling the filling of a canister for a painting robot with a piston extended; 
     FIG. 2 is a schematic view of the prior art apparatus shown in FIG. 1 with the piston in a predetermined paint volume filled position; 
     FIG. 3 is a schematic view of the apparatus for controlling the filling of a canister for a painting robot in accordance with the present invention; and 
     FIG. 4 is a flow diagram of the method of filling the canister in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, there is shown a prior art paint canister  10  having a generally hollow interior  11  in which a piston  12  is slidably moveable. The interior  11  is connected through a paint supply valve  13  to a paint supply  14  by a paint supply conduit  14   a . The piston  12  is coupled to a potentiometer  15  that, in cooperation with a power supply (not shown), generates an electrical signal representing a position of the piston  12  in the canister interior  11 . For example, the potentiometer  15  can generate a zero volt d.c. signal when the piston  12  is in a fully extended position shown in FIG. 1 wherein no paint is present in the canister interior  11  and generate a ten volt d.c. signal when the piston is fully retracted. In FIG. 2, the piston  12  is shown in a partially retracted position wherein a predetermined volume of paint  16  has entered the canister interior  11  through the supply valve  13  that has been opened to permit paint under pressure to flow from the paint supply  14  through the conduit  14   a  to the canister  10 . The potentiometer  15  will generate a signal between zero volts and ten volts representing the position of the piston  12  as it moves from the fully extended position shown in FIG. 1 to the position shown in FIG. 2 whereupon the valve  13  is closed. However, the potentiometer  15  is susceptible to mechanical and electrical failures that require a robotic painting operation to be shut down. 
     Referring now to FIG. 3, there is shown a robotic painting system  20  including an apparatus  21  for controlling the filling of a paint canister  22  with a desired volume of paint in accordance with the present invention. The generally hollow paint canister  22  defines a canister interior  23  connected at one end to a paint supply  24  via a paint supply conduit  25 . The flow of paint from the paint supply  24  to the canister  22  is controlled by a supply valve  26  connected in series in the conduit  25 . A piston  27  is slidably disposed in the canister interior  23  and is operable to move between a fully retracted position  28  and a fully extended position  29 . As the piston  20  moves toward the fully extended position  29 , paint in the canister  22  is forced into a spray apparatus  30  to be applied to a product (not shown). 
     The piston  27  is moved in the canister  22  by a suitable actuator. For example, the piston  27  is connected to a ball screw  31  driven in rotation by a first pulley  32  mounted on a shaft of the ball screw. The pulley  32  is coupled to a second pulley  33  by a timing belt  34 . The second pulley  33  is mounted on a shaft of an electric motor  35  that is operated to rotate the second pulley  33  to actuate the ball screw  31  thereby moving the piston  27  in the canister interior  23 . The operation of the motor  35  is controlled by a control means such as a controller  36  connected to the motor by a motor control line  37 . The controller  35  generates motor control signals at an output connected to the line  37  which signals can be in the form of electrical power to operate the motor  35  if the controller includes a power switching circuit. Alternatively, the motor  35  can include a power switching circuit such that the controller  36  generates the motor control signals required to operate the switching circuit. 
     The controller  36  also generates a valve control signal at an output connected to the valve  26  by a valve control line  38  to open and close the valve. A torque sensor  39  has an input connected to the motor  35  by a torque sensing line  40  to sense the torque generated by the motor. The sensor  39  has an output connected to an input of the controller  36  by a torque signal line  41  to generate a torque value signal to the controller. During a paint filling operation, the controller  36  compares the torque value signal with a predetermined set point value and closes the valve  26  when the predetermined value is exceeded. Typically, the torque sensor  39  senses the value of the motor armature current, which current is proportional to the torque being generated by the motor  35 . 
     The control means, the controller  36 , typically can be a programmable logic controller or a programmed computer. The control means includes a software program for implementing a method of performing a paint canister filling operation according to the present invention as shown in FIG.  4 . Prior to the start of the filling operation, the controller  36  had closed the valve  26  at the end of the previous filling operation. The method begins at a circle  50  START and enters an instruction set  51  wherein the controller  36  sends a signal on the line  37  to actuate the motor  35  to extend the piston  27  to the extended position  29  thereby expelling the paint remaining in the interior  23  of the canister  22 . The method then performs a step in an instruction set  52  wherein the controller  36  determines the total amount of paint required for the next painting job. In an instruction set  53 , the controller  36  calculates the position of the piston  27  in the canister  22  required to provide the correct volume of the canister interior  23  to hold the total paint required as determined in the step  52 . Now the method enters an instruction set  54  to generate the motor control signal on the line  38  to actuate the motor  35  to retract the piston  27  from the extended position  29  to the required position. 
     The canister  22  is ready to receive the total paint required. According to an instruction set  55 , the controller  36  sends a signal on the line  38  to open the valve  26  allowing pressured paint to flow from the paint supply  24  to the canister interior  23  through the paint supply conduit  25 . In an instruction set  56 , the controller  36  monitors the torque signal from the torque sensor  39 . As paint flows into the canister interior  23 , the paint will exert a force on the piston  27  tending to push the piston away from the required piston position toward the retracted position  28 . In order to maintain the piston  27  at the required piston position, the motor  35  must apply increasing amounts of torque to counteract the increasing volume of paint. In a decision point  57 , the controller compares the sensed torque value “T” with a set point value “X” corresponding to the torque required to maintain the piston  27  at the required position when the canister  22  is filled with the total paint required. 
     If the set point has not been reached, the method branches from the decision point  57  at “N” and returns to the instruction set  56  to continue to monitor the motor torque. When the monitored torque value exceeds the set point value, the method branches from the decision point  57  at “Y” to an instruction set  58  wherein the controller  36  sends a signal on the line  38  to close the supply valve  26  and stop the paint from entering the canister interior  23 . The canister filling operation is complete at a circle  59  END. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.