Another type of loop is the WHILE loop. Instead of executing a fixed number of times like a FOR loop, it can execute as long as some condition is true. For instance, let’s say we want to find the first index in an array where the value is greater than some value:
The purpose of this function is to search an array of 1000 values and return the first index where the value is greater than some Threshold. If it doesn’t find any values greater than the Threshold then it returns 0, which is an invalid index. Line 1 initializes a boolean flag, Found, to FALSE. Since this is a function, it’s not really necessary because the value would be initialized to false every time you call the function, but if this was a function block, then you’d want to include that line because the value would be retained from call to call.
Line 2 initializes the Index variable to the first array index (1). Lines 3 and 9 define theWHILE loop. Lines 4 through 8 will be executed repeatedly as long as the expression in line 3 returns true. As you can see, we loop until either we find it, or the Index passes the upper bound of the array.
Lines 4 through 8 comprises an IF/THEN/ELSE block. If the expression in line 4 is true, then it executes line 5. If the expression on line 4 is false, then it executes line 7 instead.
To demonstrate how this works, assume the values in the array are 25, 50, 75, 100, 125, etc. Also assume Threshold is 80. We would expect the function to return a value of 4. Here’s how the function executes:
•
Line 2 sets Index to 1•
Line 3 evaluates to true because Found is false and Index is 1•
Line 4 evaluates to false (25 is not greater than 80)•
Line 7 sets Index to 2•
Line 3 evaluates to true because Found is false and Index is 2•
Line 4 evaluates to false (50 is not greater than 80)•
Line 7 sets Index to 3•
Line 3 evaluates to true because Found is false and Index is 3•
Line 4 evaluates to false (75 is not greater than 80)•
Line 7 sets Index to 4•
Line 3 evaluates to true because Found is false and Index is 4•
Line 4 evaluates to true (100 is greater than 80)•
Line 5 sets Found to true•
Line 3 evaluates to false because Found is true•
Line 11 evaluates to true•
Line 12 sets the return value of the function to 4 (because Index has the value 4) While this is a perfectly reasonable function, there are also some problems with it.First of all, the scan time is quite variable. The worst case scan time is when the value isn’t found, and it returns 0. In that case it iterates through the entire array. In the best case it returns 1. Variable scan times can lead to problems if the worst case is never tested, or if you have a lot of functions like this and there’s some diabolical case where all of them have to execute the worst case on the same scan, and you exceed your allowable scan time.
Secondly, the logic is complex. Some of you might be laughing at me for saying that. If you’re a PC programmer writing code in C or BASIC then the function above is actually quite simple, yet in PLC programming we have an abnormal emphasis on simplicity. We want logic that is obviously correct when we look at it, and the above function isn’t obviously correct unless you give it a significant amount of analysis. To analyse it you really have to “play computer” and walk through at least 2 different scenarios: one where the value is found, and one where it’s not found.
Earlier in this section I talked about expecting electricians to go online with our programs and do troubleshooting. An electrician can understand Ladder Diagram, and with a little bit of work they can probably understand the FOR loop example above, but there are going to be a lot of people who won’t be able to understand this example of a WHILEloop with IF/THEN/ELSE blocks. If you believe these people don’t have any business going online with a PLC, then suggest you should change your attitude. Automation is a team sport and we have no room on the team for big egos.
Use the simplest logic you possibly can (not the shortest). If the machine you’re programming has 10 motors, don’t try to write the motor start/stop logic in Structured Text with a FOR loop. Don’t even make a function block and re-use it 10 times. Just write 10 different programs in Ladder Diagram and copy the logic. Sure they might share some common logic, like an OkToRunMotors coil that gets set in another program. Remember that these are 10 physically different motors and the conditions for starting and stopping them are likely to change over time. Recognize that and keep the logic separate. On the other hand, Structured Text is the right tool for the event-logging and recipe-handling logic of a program. An electrician logging into the PLC to understand why a motor isn’t starting isn’t going to be concerned with the event-logging module. Structured Text is also the right tool for manipulating data, such as a scan received from a barcode scanner or an RFID reader. Complex math is also more easily expressed in Structured Text.
Using the right tool for the job means taking more than the problem itself into account. Make sure you take the capabilities of your team and the customer’s capabilities into account too.