Asking the right questions during an automation project can be very helpful. Here below a short list with some of the key points to be clarified when involved in a project with PLC programming:
> What is the program architecture (IO elements interface, Sequences and Sub-Sequences, HMI interface, Safety, Network topology) ?
> How to add/remove/edit an IO ?
> Are the IOs configurable (e.g. Alarm/Warning, Filter, Min/Max) ?
> How to edit a sequence (e.g. add/remove/edit a Step or a Transition) ?
> How is the flow chart of the sequences ?
> Do the sequences interact between each other (e.g. take-over/hand-over of product/data) ?
> Are there libraries in the project ?
> Are the variables Retained after PLC stops, is FirstScan bit used in the program ?
> What are the Start values of the variables, like Set-Points ?
> Is there any communication with external systems, including safety signals, (e.g. PN-PN Coupler, PUT/GET, Hard-wired with dry-contact relays, TCP/IP) ?
> How to simulate the manufacturing process (step-by-step, start/stop/pause/continue/end/safety, PLC stop, timeouts, faults) ?
> Are there Process Control Loops (Control strategy, PID algorithm, cycle rate, filter, Set-Point variability, unities) ? E.g.: Self-regulating loops: 1) First-order: Flow 2) First-order plus dead-time: Consistency 3) Second-order: Temperature
Non-Self-regulating loops: 4) Integrating: Level
> Are the firmware and PLC project versions up to date and defined ?
> How the Safety Program is divided by safety-zones ?
> The Safety resets should reset which parts of the plant ?
> What are the Safety Functions involved (e.g. STO, SS1, Safety dump of pneumatic air supply) ?
> Which are the Safety Inputs (ESTOP, SFDOOR, Safety Speed monitoring, Safety Laser barrier)
> What are the Motion Control applications involved (e.g. Speed Control, Position Control, Inverters, Servo Drives, Incremental or Absolute Encoders, Homing required, Control Word / Status Word, Breaking resistors, Hoist application, Master/Follower motion control) ?
> Are the alarms reliable and properly filtered/grouped (e.g. when power supply is disconnected, Profinet nodes diagnostics, avalanche of alarms to be avoided, transition alarms should be filtered TON/TOF ) ?
> What are top ten risks of the project (including is it the first time) ?
> Is the application a continuous process (e.g. start/maintain set-point/stop) or a discrete manufacturing process (e.g. cyclic processes) ?
> How documentation is managed (e.g. functional description, milestones, electro-mechanical and pneumatic drawings, IO lists) ?
> Are PLCs time sync'd with a NTP server? Is the HMI sync'd with PLC ?
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