In last month’s discussion “Automatic Size Change on a Budget – Part I”, we talked about the designer’s dilemma: accomplish size change at low-cost with an unsophisticated manual size change mechanism, or achieve high-performance automatic size change using costly servo drives. This month we will look at some alternatives that nicely bridge the gap between affordability and automatic (or, semi-automatic) operation.
There are four essentials to consider when implementing automatic size change on a budget:
- Actuator – motive power for the size change mechanism can be selected from several possibilities:
- Low-cost AC or DC motor driving a ball screw, worm gear, or belt drive
- Linear or rotary pneumatic actuator with position locking capability
- Human operator driving a hand crank
- Position Feedback Device – typical choices for position feedback include:
- Actuator Controller – the actuator controller is selected based on the type of prime mover being controlled:
- Reversing AC or DC motor starter
- Variable speed, reversing AC or DC motor drive
- Reversing electro-pneumatic valve
- Visible “Clockwise”, “Counter-Clockwise”, and “Stop” indicator lights (for human interface)
- Control System – the Control System accepts the commanded position, reads the actual position from the Position Feedback Device, and issues signals to the Actuator Controller to drive the mechanism until the actual position matches the commanded position.
Fully automatic size change using can be accomplished at relatively low total installed cost by careful selection and implementation of an electric motor drive or pneumatic actuator, an economical position feedback device, and an associated actuator controller. The control system activates the actuator controller to drive the mechanism in the required direction. The control system monitors the position signals coming from the position feedback device. When the actual position matches the required position, the control system shuts down the actuator.
Semi-automatic operation can be accomplished by having a human operator take direction from the control system via indicator lights. The human operator drives a traditional hand crank clockwise or counter-clockwise as directed by the indicator lights, depending on the move required to accomplish the size change. When the correct position is reached, the controller activates a “Stop” indicator light. The advantage of this approach over the fully manual approach is that the operator does not have to know the correct position for the particular size change being undertaken. All of the position information is stored in the controller, and the operator simply follows the directions of the controller to accomplish the correct machine movement.
Whether fully automatic or semi-automatic operation is required, it’s possible to implement fast, efficient size changing without busting the budget.
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