Learning programmable limit switch and method providing...

Details Learning programmable limit switch and method providing... Learning programmable limit switch and method providing...

1998-06-24

2001-06-12

Lee, Thomas (Department: 2182)

Data processing: generic control systems or specific application

Generic control system, apparatus or process

Sequential or selective

C700S045000, C700S061000

Reexamination Certificate

active

06246916

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a learning programmable limit switch and, more particularly, a motion system which utilizes a learning programmable limit switch to provide real-time passive feedback for a controller to control the operation of a drive system in accordance with a user-programmed motion profile.
2. Brief Description of the Related Art
In the field of motion control systems, pressure transducers can be applied to control the ON/OFF state of external circuits. Such a system monitors position and sets or resets one or more outputs at predefined positions or “setpoints.” The mechanisms controlled by this system usually have a constant delay from actuation to the desired mechanical reaction. An offset can be added to the transducer position in order to activate the output circuit before its actual setpoint has been reached to compensate for this mechanical delay. As the velocity of the mechanical system increases, the time added by such a positional offset decreases. Therefore, a larger proportional offset is needed to offset for the same mechanical delay at higher speeds. In many mechanical systems the delay for mechanism activation is different from the delay for the deactivation of the mechanism. This requires two different positional offsets for delay compensation. When the circuit is OFF an ON offset is used, and when the circuit is ON an OFF offset if needed.
Usually such a system is implemented in software by a microcontroller. The microcontroller reads the transducers at fixed time intervals and calculates velocity. The microcontroller computes the offsets based on the calculated velocity and adds these offsets to the position. The microcontroller compares this offset position to a table of ON/OFF positions. The table entry specifies the desired output state. The microcontroller can control many circuits from one position reading. This position typically corresponded only to a stop position. Typically, the microcontrollers could not operate at high speeds or with many simultaneous controlled circuits.
Motion control system of the past oftentimes had difficulty handling environmental changes. For example, in a pallet shuttle system which shuttles a pallet between a first station and a second station, the shuttle system drives the pallet, for example, from a first station at a high speed until it reaches the second station, whereupon the pallet system slows to a slower speed until finally it is brought to a complete stop at the second station.
During a manufacturing process, it is desirable to minimize the overall cycle time it takes to shuttle the pallet between theses stations. When the motion system was “cold”, the time associated with the transition between the high to low speeds and the low speed to stop was very short. However, as the motion system warmed up, the duration of this time period would tend to increase, thereby causing the pallet to “overshoot” the desired stopping point and/or point at which the slow speed was suppose to occur. Consequently, it was required to make manual adjustments to the system so that the pallet would be shuttled as desired.
Similarly, as the system cooled down, the time required to transition between speeds was less which would result in the pallet “undershooting” the points corresponding to when the user desired the pallet to begin to be driven at the slow speed or hit the stop position.
U.S. Pat. No. 5,227,965 issued to Klaes et al. and assigned to Magnetek Controls of Clawson, Mich. illustrates the use of a programmable limit switch which employs high speed special purpose hardware and a general purpose microcontroller to achieve high speed limit switch operation. A state sequencer controls the high speed special purpose hardware in a fixed sequence. The states sequencer latches position data, and recalls position offsets from a memory. A hardware digital adder sums the position and offset with this summed offset position used as an address in output tables for selecting an output status word. The microcontroller operates asynchronously with respect to the state sequencer. The microcontroller initializes and updates the memories. The microcontroller calculates the velocity from repetitive position signals and writes the corresponding window offsets to memory. A selection circuit picks out the status bit of the particular output circuit. The outputs are shifted together and latched to the output drivers.
Unfortunately, this system and other motion systems of the past could only accommodate for a stop position. They did not automatically adjust during real time operation of the motion control system utilizing sensed feedback corresponding to the motion of, for example, an output shaft of the drive system to learn and adjust to provide real time passive feedback to the controller based upon position information.
Another problem with systems of the past is that they were utilized to detect velocity only and typically provide a velocity offset so that only a stop position would be properly achieved.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide a real-time, learning programmable limit switch for providing passive feedback for accurately controlling movement in accordance with an user-programmed motion profile.
Another object of the invention is to provide a passive feedback system which will accurately monitor, control and “learn” not only a stop position, but transition positions between, for example, a first speed or velocity and a second speed or velocity.
Still another object of the invention is to provide a learning programmable limit switch which accounts for more than a single speed or position application which only accommodates for a stop position and which “learns” the offsets for transition points associated with transitions between speeds, as well as any desired stop positions.
In one aspect, this invention comprises a method of providing passive feedback for controlling a drive system comprising the steps of providing a programmable switch for facilitating controlling the operation of the drive system, programming the programmable switch with a first programmed setpoint corresponding to a first position of the drive system, a second programmed setpoint corresponding to a second position of the drive system, a first initial trigger point corresponding to when the drive system performs a first change from a first speed to a second speed, and a second initial trigger point corresponding to when the drive system performs a second change from the second speed to a third speed, energizing the drive system, using said first and second initial trigger points to effect the first and second changes, respectively, determining a first actual setpoint corresponding to a first actual position and a second actual setpoint corresponding to a second actual position, comparing the first programmed setpoint to the first actual setpoint and the second programmed setpoint to the second actual setpoint and adjusting at least one of the first initial trigger point or the second initial trigger point to provide an adjusted first trigger point or an adjusted second trigger point, respectively, in response to the comparison, and repeating the energizing, triggering, determining and comparing steps and if the first and second actual setpoints are not within a predetermined tolerance range of the first and second programmed setpoints, respectively, then repeating the adjusting step.
In another aspect, this invention comprises a process for providing passive feedback to facilitate maintaining a cycle time in a motion control system comprising the steps of providing a programmable switch capable of receiving a motion profile associated with a driver, determining an actual motion profile associated with the driver, comparing the actual motion profile to the motion profile, and adjusting outputs of the programmable switch in response to the comparing step such that the cycle time is achieved and maintained.
In still another aspect, this invention comprises, a

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