Suppose one of your clients asks for a ladder diagram for a motor, a fast contactor, and a slow contactor with auto control for a certain amount of time, say six minutes. Timed out, the motor is allowed to run fast for, say, ten minutes while in manual control. The motor can also be started in either slow or fast mode and latches in until the timer times out.
The first and foremost step is determining what specific PLC the client is using. This is because not every PLC shares the exact same instructions and addressing in their programs, and programming with the wrong type of instructions can cause problems.
There are three main types of timer codes in ladder logic: On-Delay Timers, Off-Delay Timers, and Retentive or Accumulating timer. On-delay timers have a set waiting period before initializing the output. Off-delay timers start the output immediately and end it after a set amount of time. The last type of timer listed, the retentive timer, requires two inputs for timing and resetting the event.
One of the first steps to programming timers is defining the outputs. This isn’t the only approach to programming timers, but it works well in specific situations such as discrete machinery. When defining the outputs, all states of the motor control are well-defined in a chart, even though faults and control modes are defined in other routines. If the slow output is somehow turned on, the fast output is disabled for safety reasons. If the outputs have been manually forced, neither will be disabled.
From there, the timer code is run to test the outputs. Being aware of the types of code you or the programmer has developed is important. For example, will the code prevent someone from ending the cycle early by taking the system out of the auto/manual mode? Are there are any instructions or commands which are unfamiliar or unclear to the person analyzing the results? It’s important to be aware of the names for different commands so as not to confuse which commands are which in the code.
There are certain ways of coding timers which are not legal to use due to safety concerns, unless under specific conditions. Safety measures such as light curtains or other barriers would need to be in place to run these types of timer codes. Using them without these measures increases risks to personnel and equipment.
All in all, it’s extremely important to be aware of the coding for timers in PLCs. Some codes take almost an hour to develop, while the accompanying documentation takes two more hours, as the tags, and I/O structure needs to be explained for other developers as well. Whether you’re using one of the delay timers or the retentive timer, being aware of what’s in the code itself and the specific instructions it’s been given can greatly affect the applications of the motor testing. And this post only goes into the basics of timer codes for PLCs.
Our PLC technicians at PanelShop.com have an understanding of PLC systems and ladder logic that goes far beyond the scale of these articles. If you have questions about ladder logic or timer codes, contact us today!