The Crouzet Millenium Slim is a small PLC featuring a Bluetooth interface. This allows remote control via a cell, tablet, or PC using the Crouzet Virtual Display. With this Bluetooth link, you can monitor and control your small industrial system.
This engineering brief presents a modification of the classic start-stop circuit. It features local controls using the red stop and the green start pushbuttons as shown in Figure 1. The remote/local selector switch is a permissive that enables Bluetooth-based remote control. Finally, the remote control (cell-based user interface as shown in Video 1) presents the machine status and allows the user to remotely start and stop the mechanism.
Note: A recommended list of components (sold individually) is presented here.
Download the code here: RemoteLocalCell.gprs (69.9 KB)
Figure 1: Image of the test platform for remote/local control of the Crouzet PLC including cell phone display.
Video 1: Demonstration of local and cell phone based remote control.
Remote control safety
Remote machinery control presents a range of safety issues. The fact that the operator is not physically present and attentive to the process is one of the greatest hazards. There are questions about the integrity of the remote link as things can and will go wrong with any computer and radio-controlled system. There are also a host of cybersecurity considerations that are beyond the scope of this article.
With that said, consult a professional industrial control engineer who will ensure your system is compliant with all local, state, and federal regulations.
Local control safety
Figure 2 presents the Function Block (FB) program for the system. It includes several subtle fail-safe mechanisms.
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Power on reset: The center of the diagram includes a STATUS block. The “Cold init” and “Hot init” signals are ORed together and then sent to the set/reset block in the upper right-hand corner. This prevents unexpected power-on starts. Note that reset overrides set for this essential block.
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Remote selector switch: The system is programmed for a fail-safe return to local operation. For example, a broken wire will disable the remote control.
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Normally closed stop pushbutton: The red stop pushbutton features a normally closed switch mechanism. The broken-wire fail-safe is to disable the system.
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Stop from either position: The remote and local pushbuttons are always enabled. This is a human factors consideration that accommodates the user’s natural tendency to use muscle memory to stop the machine. Stated another way, the stop mechanism will always work the same.
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Yellow safety equipment: This article serves as a demo showcasing the capabilities of the PLC. It features low voltage 24 VDC provided by an external laboratory bench power supply. It does not include the necessary emergency stop(s), interlocks, or associated safety equipment. You must conduct a site- and equipment-specific safety analysis to determine the safety hazards. Use this to keep your personnel safe and abide by all local, state, and federal regulations.
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Other: Please leave your safety suggestions in the comments section. What have I missed or overlooked?
Figure 2: Screen capture of the Function Block (FB) program for the Millenium Slim PLC. The heart of the machine is the set/reset block placed immediately above the red highlighted display section.
Tech Tip: The Crouzet PLC is programmed using Function Block (FB) and Sequential Function Chart (SFC). The Figure 2 example is pure FB. See this article from additional information about the Crouzet SFC.
Local control
The local control consists of the green start and the red stop pushbuttons. In Figure 2 we see the associated signals identified as LocalStartPB and LocalStopPB respectively. Note that an inverter is used to convert the stop pushbutton’s normally closed contact into a positive logic signal. Also note that an inverter is used to create a positive logic Local signal, e.g., if not Remote then Local.
The AND gate in the upper middle portion of Figure 2 provides an enable signal for the set/reset block. IF Local AND LocalStartPB THEN set the machine to run.
The stop mechanism is outlined in this tech tip.
Tech Tip: The term positive logic implies that a signal is TRUE when active. For example, consider the input from the normally closed stop pushbutton. This signal is negative logic as the signal is TRUE when the pushbutton is not pressed. Inverting the signal makes it FALSE when the button is NOT pressed and TRUE when it is pressed. We can then make a clean, easy to understand statement such as, “IF LocalStopPB OR RemoteStopPB OR SysReset THEN stop the machine vis the reset line of the set/reset block.
Remote control
The remote control mechanism is similar to local. It begins with the block in the lower-left corner of Figure 2. Here we see the RemoteStartPB and RemoteStopPB derived respectively from the HMI’s A and B function buttons. The RemoteStartPB is ANDed with the Remote signal before being sent to the set input of the set/reset block. The RemoteStopPB operation was explored in the previous tech tip.
Recall that the stop functionality is preserved across both local and remote control. Therefore, the user can always stop the machine using either method. However, the start functionality is determined by the position of the local/remote selector switch.
Display
The highlighted display section contains 4 independent sections. The logic will allow one and only one display block to be active at any given time:
- local and idle
- local and running
- remote and idle
- remote and running
Each block presents a unique status message to the user along with instructions on what to do next. An example is included in Figure 3. The system is in the remote plus idle state awaiting the user to press the A button to start the machine.
Figure 3: Representative user display for a system in the remote plus idle state. This image is derives from the Crouzet-Soft simulation.
Parting thoughts
The Crouzet is an easy to program PLC with advanced features allowing cell-based remote control. The FB language allows quick and easy program development. However, the complexity of safety remains. You must carefully analyze your industrial control system. You must understand the process with respect to the user’s safety including all corner cases.
What have we missed in this introduction? Please leave your comments and suggestions in the space below.
Best wishes,
APDahlen
Related Information
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.