Support-focused enterprise controls: machine operating modes

Control applications developed for conveyors often adopt operating modes specific to each area. Area modes are usually a local segment, zone, sector, or multistation conveyor process. Operating modes are specific to enabling the machine, process, or mechanism to cycle, while support modes are operational characteristics that enhance user interactions and enable overall station or process control modes.

03/03/2016


Figure 1 shows two typical automatic mode activation circuits. The top seal circuit uses a momentary pushbutton to enable an automatic mode. To disable the mode signal, the design uses a cancel button to break the seal. Courtesy: Daniel B. CardinalMost manufacturers specify a set of mutually exclusive operating modes that enable support personnel to have operational interactions with machine and conveyor processes. Control applications designed for machines adopt operating modes that are local to individual station processes. Control applications developed for conveyors often adopt operating modes specific to each area. Area modes are usually a local segment, zone, sector, or multistation conveyor process. Most manufacturers define operating modes along industry standards.

It is important that system strategists understand the differences between operating modes and other support modes. Operating modes are specific to enabling the machine, process, or mechanism to cycle, while support modes are operational characteristics that enhance user interactions and enable overall station or process control modes. Common support modes include:

  • Guided manual mode is a design feature that requires a control application to use illuminated pushbuttons to prompt manually initiated actions.
  • Maintenance mode allows maintenance personnel to move objects or mechanisms out of position and then back into position without affecting the control application. 
  • Run-out mode allows operations personnel to purge objects from a station, conveyor, or process line.
  • Dry-cycle mode allows operations personnel to cycle a machine or process without having a part.
  • Repeat mode allows operation personnel to repeat cycle the station process on the same part.
  • Operating mode is one of four mutually exclusive modes that allow a person or application to enable the operation of a process, conveyor, machine, or mechanism.

It is important that system strategists recognize the purpose of each support mode. Guided mode is an enhancement to various operating modes that requires someone to press buttons. Specifically, it equates to having an illuminated pushbutton for: 1) activating a process, 2) cycling a mechanism, or 3) moving an object. Strategists often interchangeably use the "maintenance mode" term to mean a flexible manual control mode. In this case, maintenance mode is a prerequisite to enabling one or more hand-operated modes.

When this top-level mode is active, it prevents all control applications from moving or resetting data variables and latching or unlatching application signals. This typically includes those signals associated with movement detection circuits and control-based, one-shot signals. Run-out mode is a mode that prevents new parts from entering a machine or process. This mode enables machines to purge all objects from a process station, conveyor lane, or process line. Dry cycle mode is a special mode that allows operations personnel to run a machine or process without a part.

Designers preprogram this mode to prevent some sub process activities from functioning while the machine or process is running without a part. For example, this mode would prevent a robot from picking up and subsequently attempting to load a subpart onto the missing part. Repeat mode is similar to the dry cycle mode. However, this mode allows the machine or process to cycle multiple times on the same part. The following definitions describe the common operating modes:

  • Automatic mode is a restricted operating mode that allows a control application to control the movements of objects and mechanisms.
  • Semi-automatic mode is a restricted operating mode that allows a person to use one or more pushbuttons to enable automatic movements of objects and mechanisms.
  • Manual mode is a restricted operating mode that enables a person to move an object or mechanism by continually pressing a pushbutton.
  • Unrestricted mode is a free operating mode that enables a person to move an object or mechanism by continuously pressing a pushbutton.
  • Mode activation circuit is a setup, seal, or latch/unlatch circuit used to enable an operational mode signal.
  • Steady-state signal is an examinable condition that must stay in an enabled or disabled state to activate and keep a circuit activated.
  • Variable-state signal is an examinable condition that changes states after its original state activated the circuit.

Most discrete part manufacturers understand that automatic mode means the safe operation without human interaction. Semi-automatic mode implies the same operation but requires someone to initiate object and mechanism movements. There are several forms of manual modes that individually affect the need for support personnel to have restricted or unrestricted interaction with objects and mechanisms.

Restricted interaction means control circuits verify the clear position of other mechanisms and other objects. Unrestricted interaction means control circuits do not verify the positions of other mechanisms and other objects. In most cases, unrestricted mode means support personnel must visually verify the position of mechanisms and objects while physically pressing and maintaining pressure on a pushbutton. The term "pushbutton" will henceforth refer to a hard-wired pushbutton or defined membrane area of a touch panel.

Each of the above operating modes requires a mode activation circuit to enable a unique mode signal. These activation circuits are comprised of many steady and variable-state signals. Steady-state signals allow a circuit to transition to a mode when on and must stay on to keep the mode enabled. Variable-state signals must be on to enable the mode. The following definitions generically describe these examinable conditions:

  • Personnel safeties ready is a steady-state signal that ensures nobody is in an unsafe position before something moves.
  • Position ready is a variable-state signal that ensures a machine is in the proper position to enable movements.
  • Machine ready is a steady-state signal that ensures electrical, pneumatic, and hydraulic circuits are ready to enable movements.
  • No minor faults present is a variable-state signal that ensures no preselected faults are present before an activation circuit can enable an operating mode.
  • No major faults present signals that no preselected faults are present before an activation circuit can enable and keep an operating mode enabled.
  • Interlocks to achieve is a variable-state signal that ensures other process-related machines are ready before an activation circuit can enable an operating mode.
  • Interlocks to maintain signals that other process-related machines are ready before an activation circuit can enable and keep an operating mode enabled.

The various conditions programmed into a mode activation circuit depend mostly on the specifics of a machine. Since an automatic operating mode enables the unattended operation of a machine, the applied mode activation circuit usually has all the steady and variable-state conditions. Designers normally provide a special set up circuit to allow for the grouping of conditions by category.

A "personnel safeties ready" signal means all safety-related floor mats are unoccupied, machine perimeter doors are not open, light screens guarding openings are unobstructed, and motion detectors are not sensing movement. Many of these conditions are steady-state signals while others are variable-state. All variable-state signals must have a parallel condition to ensure the enabled ready signal remains on. For example, some light screens detect someone inadvertently accessing a machine through the normal part entrance and exit areas. Designers expect some screen signals to change state when a part enters or exits the machine. To prevent the activation circuit from disabling the specific mode when parts move, designers parallel the deactivated light screen signal with an activated muting switch signal.

The "machine position ready" signal is typically used to prevent a control application from achieving automatic mode. This is important when machine designers or manufacturers believe that a machine must first be in a known mechanical position before automatic movements can begin. Designers decide what physical state(s) or mechanism-specific positions must be present before allowing the transition to an operating mode. Since movement will start after the mode is enabled, designers expect position-ready signals to change state after mode activation occurs.

The "machine ready" signal indicates electric power is on, pneumatic pressure is normal, hydraulic pumps are running, and special equipment is on. These types of signals must stay on to enable a mode activation circuit. Some designers recognize that some conditions must be individually supported by a de-bounce circuit. This special timer circuit prevents momentary signal interruptions from disabling the mode activation signal.

The two "no faults present" signals identify what faults must be cleared to enable an operating mode and which must be off to keep the mode enabled. Designers typically review the set of identifiable faults and categorize them to be a minor or major fault relative to achieving and then maintaining an operating mode. How controller applications enable individual fault signals is a reserved topic for a future article.

The "interlocks to achieve and maintain" signals identify the signals that come from other machine controllers. Designers who program the other controllers combine all their pertinent variable-state signals to enable the interlock to achieve signal, while all steady-state signals enable the maintained interlock.

Figure 1 shows two typical automatic mode activation circuits. The top seal circuit uses a momentary pushbutton to enable an automatic mode. To disable the mode signal, the design uses a cancel button to break the seal. Designers place the seal signal around all variable-state conditions while programing all steady-state signals outside the seal. The bottom circuit uses a mode selector with maintained switch positions to continuously enable the automatic mode signal. Since the design does not include a sealed or latched signal, programmers cannot add variable-state signals to the circuit.

After support personnel select the desired operating mode, a mode activation circuit energizes to enable a discrete operating mode signal. Control applications use these mutually exclusive mode signals to differentiate the logic paths necessary to move objects and mechanisms.

Daniel B. Cardinal works as an engineering consultant for Insyte Inc., implementing integrated scheduling and part identification applications in the automotive industry. Edited by Chris Vavra, production editor, Control Engineering, cvavra@cfemedia.com.

MORE ADVICE

Key Concepts

  • Most manufacturers define operating modes along industry standards.
  • A mode activation circuit's conditions that are programmed depend mostly on the specifics of a machine.
  • Control applications use these mutually exclusive mode signals to differentiate the logic paths necessary to move objects and mechanisms.

Consider this

What operating mode do you most commonly use, and which is the most effective in your particular operation?

ONLINE extra

See prior stories in this series by Daniel Cardinal linked below.



Anonymous , 03/18/16 07:27 AM:

A very good technical work
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