Automatic Operations Start and Resume

Some automatic operations are permitted to begin or resume without additional operator input once Auto Mode is established. The PLC Pro must work directly with the customer in defining what installation activities do not require additional operator input to start or continue. Where this is the case, conveyors can start, machines can operate, and other defined automatic operations can proceed as normal.

Other automatic operations require additional deliberate operator action before they are permitted to start or continue. The PLC Pro must identify those requirements with the customer and implement them so the required operator intent is established before any affected automatic operation proceeds.

Regardless of whether an automatic operation may proceed immediately or requires additional operator action, it remains subject to the conditions required to start and continue without conflict. PLC Pro Sequencing provides the structure used to evaluate those requirements consistently for every automatic operation.

The PLC Pro must make no assumptions about what operator action is required before automatic operations may start or continue. He or she must become familiar with the regulations and standards governing the installation and work directly with the customer to identify any plant, corporate, regional, or machine-specific requirements that also apply.

International, national, machine-specific, and installation-level requirements do not always establish the same relationship between selecting an automatic operating mode and authorizing machine activity. General machinery standards establish broad safety principles, while requirements applicable to a particular machine, material-handling system, production line, or installation may impose more specific controls. The PLC Pro must work in concert with the customer to determine which requirements actually govern each element of the installation before deciding how automatic operations will be initiated.

Even where the governing requirements permit automatic operations to start or resume without additional operator action, their application to a particular installation may still require interpretation. The PLC Pro works directly with the customer's engineers and operating personnel to understand how those requirements are interpreted locally and to identify any additional customer standards that apply.

European machinery requirements illustrate one interpretation. They generally require deliberate actuation of a control device to start or restart machinery, but also recognize that machinery functioning in an automatic mode may start or restart without further operator intervention when doing so does not create a hazardous situation. Under an appropriate controls design, the deliberate request for Auto Mode can therefore establish the operator intent that allows defined automatic operations to proceed without a separate “CYCLE START” or similar command.

Different elements of the same installation may be subject to different requirements. A requirement governing the operation of a particular machine does not necessarily govern the conveyors, transfer devices, or other material-handling equipment that delivers work to and from that machine. The PLC Pro must evaluate the applicable requirements at the level where they actually apply rather than assume that one start or restart convention governs the entire installation.

An automatic operation that has already been legitimately requested does not necessarily require a new operator command each time its progress is interrupted. The original request for the work may remain valid through the interruption, allowing the operation to resume without additional operator action when the governing requirements permit automatic restart and no new operator intent is specifically required.

Some automatic operations require additional deliberate operator input after Auto Mode has been established. This requirement may apply to a particular machine, operation, or other element of the installation even while other automatic activity is permitted to proceed without additional operator input.

In these cases, establishing Auto Mode places installation actions under the direction of the control program but does not provide all of the operator intent required for the affected automatic operation. An additional deliberate operator action is required before that operation may start or continue.

Some installation specifications require a separate “CYCLE START” or similar operator action after Auto Mode has been established. In these installations, establishing Auto Mode places installation actions under the direction of the control program, but the affected automatic operation remains idle until the operator provides the additional start request. The PLC Pro must preserve that distinction when implementing the automatic operation.

Initial start and resumption of an interrupted automatic operation are not the same programming problem. Once a sequence is activated, its established step states should often be preserved through an interruption rather than forcing the sequence to reset and begin again from its initial step. PLC Pro Sequencing is structured to support both initial start and controlled resumption from the established sequence step states.

Resumption may require renewed operator intent after an automatic operation has been interrupted. Where governing requirements or customer interpretations require that additional intent, restoring Auto Mode and the other conditions required for continued operation is not by itself sufficient. The required operator action must also be provided before automatic activity resumes.

A single installation may therefore contain more than one start or restart requirement, depending on its operating elements. Conveyors, transfer devices, machines, and other installation elements may each be governed by different requirements or customer interpretations. A key distinction is the hazard associated with operation of each element. The PLC Pro must identify the applicable requirement for each affected element and implement the required operator intent accordingly.

For example, a single workstation may contain multiple forms of automatic operation. Conveyors and part-handling equipment may operate immediately when Auto Mode is established and their other required conditions are satisfied, while the machine they serve remains ready but idle until the operator deliberately requests its cycle with a “CYCLE START” or other defined control.

The PLC Pro works directly with the customer's engineers and operating personnel to identify where these additional requests are required and what each request authorizes. Controls such as “CYCLE START”, “START”, or “RESUME” should have a clear and consistent meaning so the operator understands what automatic activity may result from each action.

Regardless of the operator input required to authorize an automatic operation, the operation may start or continue only when its required conditions are satisfied. PLC Pro Sequencing provides the consistent structure used to evaluate those requirements before a sequence begins, resumes, or advances.

Auto Mode status is always placed in series with OK to Continue. When Auto Mode is established, OK to Continue is allowed to become TRUE when its other required conditions are satisfied. If the remaining pre-start conditions are satisfied and the sequence Trigger goes TRUE, the sequence may begin.

The Trigger logic for any particular PLC Pro Sequence can be selected based on how the automatic operation is required to be initiated. Where operator input is not required, a programmed condition or handshake may provide the Trigger. Where additional operator intent is required, a “CYCLE START” pushbutton or similar deliberate operator action may be included in the Trigger logic.

Where renewed operator intent is required before resumption, loss of OK to Continue resets OkToAdvance. Restoring OK to Continue does not then by itself authorize the sequence to advance. The required operator action must again be provided before OkToAdvance is restored.

Where no renewed operator intent is required, loss of OK to Continue prevents the sequence from advancing but does not reset OkToAdvance. When OK to Continue becomes TRUE again, the sequence may continue from its established point of progress without another operator action.

The difference between the two resumption methods above is therefore whether loss of OK to Continue also resets OkToAdvance. In either case, the established sequence step states are retained so the sequence can continue from its established point of progress when advancement is again permitted.

How an automatic operation is started or resumed does not change its other required operating conditions. If a sequence requires shop air, a particular equipment state, an available downstream operation, or another permissive, that requirement must be satisfied regardless of whether the sequence starts immediately upon establishing Auto Mode, requires a separate operator request, or is resuming from an interrupted state.

• Make no assumptions about the operator action required to start or resume automatic operations.
• Identify the regulations, standards, customer requirements, and interpretations applicable to each affected element of the installation.
• Use PLC Pro Sequencing to control the start, resumption, and advancement of automatic operations.
• Define the operator intent required to start and resume each automatic operation.
• Evaluate start and restart requirements independently for each affected installation element.
• Always place Auto Mode status in series with OK to Continue.
• Reset the sequence when an interrupted operation should not resume from its established step states.
• Existing operator intent may permit immediate activity when Auto Mode is established.
• Provide additional deliberate operator input where required.
• Reset OkToAdvance when renewed operator intent is required before resumption.
• Require all applicable operating conditions before an automatic operation starts or continues.