Define the Customer Need

The common application used throughout these tutorials is a fictional automotive seat-assembly workstation. The PLC Pro has been brought in to perform the PLC work for the new station while the equipment design is still being developed. The main elements of the customer’s initial description are listed below.

Whenever practical, the PLC Pro should participate from the beginning of the equipment design process. Early involvement allows controls-related requirements, control architecture, component selection, and operating considerations to influence the design before those decisions become difficult or expensive to change.

Whether or not the PLC Pro participates in the design, a clear understanding of the customer’s requirements, design constraints, production expectations, safety requirements, existing equipment, and established control conventions is essential to developing the control program for the application.

That understanding also establishes the requirements, guidelines, and measurable criteria that can be used throughout development and commissioning to judge how well the completed workstation satisfies the customer’s needs.

The tutorials use a new operator workstation being added to an existing automotive seat-assembly line as a common example application. Seat assemblies travel between workstations on individual carriers around an oval transfer conveyor, with each workstation performing part of the assembly process. Each workstation has a maximum cycle time of 48 seconds.

At the new workstation, an operator installs and secures a seat-back support bracket on each side of the seat assembly. The required operation includes accepting a carrier from the existing transfer system, providing the operator access to both sides of the seat, verifying that the required number of fasteners have been installed and that each fastening operation meets the required torque and angle, and returning the carrier to the production line when the work is complete.

The mechanical design team is proposing a feed stop immediately ahead of the workstation to hold the next carrier until the workstation is ready to receive it. They see this as a simple metal plate attached to a short, non-rotating cylinder that raises and lowers the plate to stop and position the carrier by its leading edge.

A separate and similar station stop will stop and locate the incoming carrier at its working position.

With the carrier located, a pneumatic lift with an attached turntable will raise it from the transfer conveyor to its working height, providing the operator access to the first work site on the delivered side of the seat.

The mechanical design team plans to provide a production torque gun for the operator to install the four screws needed for each bracket. An automatic screw presenter will supply the screws to the operator during the fastening operation.

After all four screws for the first bracket have been installed and their required fastening results confirmed, the turntable is unlocked, rotated 180 degrees to present the opposite side of the seat to the operator, and mechanically locked in the new working position.

At the second work site, the operator installs the second seat-back support bracket and uses the production torque gun to install its four screws. The fastening results for all four screws must again be confirmed before the workstation can continue.

After the four screws for the second bracket have been installed and confirmed, the turntable is unlocked, rotated back to its original position, and mechanically locked. The carrier is then lowered to the transfer conveyor, and the station stop is lowered to allow the carrier to exit the workstation.

Seat-back support brackets and other manually installed parts will be supplied to the operator in parts baskets at the workstation. The operator will install the required components and fasteners, with the workstation providing the carrier positioning and process verification needed to complete the operation within the required cycle time.

Early involvement allows the PLC Pro to contribute the controls perspective to the equipment design alongside the mechanical, electrical, safety, and other design disciplines. Controls considerations can then be coordinated with the developing design rather than discovered after the hardware has been built.

For example, the control system must confirm the rotational position of the carrier. Because the rotary drive provides full acceleration and deceleration control, the PLC Pro may recommend simple, repeatable position detection at the required operating positions rather than an external encoder. The position sensors cost substantially less, use standard PLC inputs instead of a dedicated encoder module, and provide the position confirmation needed before the shot pin locks the table in position.

The PLC Pro also brings practical knowledge of how the completed equipment will be programmed, operated, maintained, and recovered. That perspective can identify design choices that perform the required physical function but do not provide the control capabilities or information needed to satisfy the customer’s operating, verification, diagnostic, or recovery requirements.

The proposed production torque gun provides a specific example. The mechanical design team has selected a Desoutter SLBN screwdriver with an ESP2A controller. During review, the PLC Pro determines that the proposed system cannot provide confirmation that the required fastening angle has been achieved. The team upgrades to a Desoutter CVIR II tightening system, which does provide the torque, angle, and fastening-count information required by the application.

Making that "upgrade" during design is far less expensive than discovering the missing capability during startup or after the workstation has been placed into service. Including the PLC Pro early in the review can help prevent avoidable rework, field modifications, and, worst of all, a disappointed customer.

The lift-and-rotate equipment also illustrates the need to coordinate controls decisions with the safety analysis.

The safety analysis treats raising and lowering the carrier separately. Raising the carrier does not create an identified operator hazard. However, lowering it creates potential pinch points beneath the carrier. The design therefore requires safety-related protection against lowering while the operator is in the work area, and the team adds a light curtain across the front of the workstation.

The same analysis must consider what happens when electrical power is lost from the lift, whether caused by the assertion of an E-Stop or light curtain, or from loss of control power. Removing power from the raise and lower functions must not itself cause the carrier to lower, and it is desirable that it not unexpectedly raise either, since the station is part of a conveyor system when not in use. The PLC Pro therefore helps the team evaluate and approve the use of detented valves so the pneumatic controls retain their existing condition when electrical control is removed.

The customer uses a CODESYS-based control system with EtherCAT for distributed I/O and supported field devices. Where drives are indicated, Mitsubishi drives are already supported at the facility.

As the PLC Pro reviews the new workstation requirements against the existing assembly-line controls, several important boundaries become apparent. Emergency Stop reset and the controls used to start or stop automatic operation of the transfer conveyor are located at a separate line-level control console. The workstation will add its own E-Stop pushbutton to the existing area E-Stop circuit, while its Manual Mode and Auto Mode states will be developed locally using PLC Pro Methods.

Reset of the E-Stop safety system remains a line-level function performed from the line control console. The workstation control program monitors the resulting E-Stop status and provides the workstation's required programmatic response to that state.

Also supplied by the PLC Pro team is a drive and I/O cabinet supporting the new workstation. The line-level E-Stop safety system is linked to a safety relay inside this cabinet, which removes power from the workstation outputs when E-Stop is asserted. The status of this relay is the actual E-Stop condition monitored by the workstation control program.

The operator controls for the workstation include an E-Stop pushbutton, a lighted "AUTO" pushbutton, and an HMI screen. The E-Stop pushbutton is part of the assembly line's existing safety system rather than a separate workstation safety system. Its safety wiring originates remotely, passes through the workstation E-Stop device, and returns to the line-level safety controls.

The controls used to start or stop automatic operation of the transfer conveyor or surrounding production area are separate from the new workstation controls. Their operating state does not establish the workstation's Manual Mode or Auto Mode; those states will be developed locally using PLC Pro Methods.

The existing production line also provides a bypass capability at individual workstations so production can continue when a station is unavailable or its process is not required. The customer requires the new workstation to support the same operating convention.

Rotation presents a separate hazard because the raised carrier and seat can present several hazards to the operator while rotating. The light curtain provides operator protection against these hazards while its protective function is active. When the operator needs to enter the protected area to install and secure parts, the muting of the light curtain must also prohibit rotary motion through a safety-rated means independent of the ordinary PLC program.

The team therefore selects an EtherCAT servo drive and compatible servo motor that meet the customer's existing controls standards and provide the required safety-rated torque-off function. The workstation safety system uses safety-rated outputs to remove the drive's two STO channels and electrical power from the rotary-table lock-valve solenoid whenever rotary motion must be prohibited. Removing drive torque and power from the valve causes the spring-return valve to apply the mechanical table lock. Because coast-to-stop is not acceptable for this application, the locking mechanism is designed with sufficient capacity to stop and hold the rotating table even when it has not reached a recognized working position.

The completed workstation must accept a seat assembly on its existing carrier. It must provide the operator with access to both sides of the seat while the assembly remains associated with its workpiece carrier. At completion of the required work, it must return the carrier and seat assembly to the transfer system.

Each of the eight required fasteners must be accounted for, and every fastening operation must satisfy the customer's torque and angle requirements.

The workstation must also provide the customer's required bypass operation, allowing carriers to pass through the station without performing the workstation process.

The complete workstation cycle must not exceed 48 seconds. Cycle time is measured from arrival of the carrier in the station through confirmed exit after all required work has been completed.