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Showing results for tags 'formula'.
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Formula block - unexpected result
Guest posted a topic in Vision & Samba PLC + HMI Controllers & VisiLogic Software
Hi, on Samba SM43-J-T20 I am using formula block to calculate simple equation "OUT = A / B * 1000000", where OUT is ML, operand A is MI (range 0 - 10000) and operand B is MI (constant 15747). Is it possible to get exact rounded integer result calculated with floating point registers without using any MF operand? In Help it says: In the following cases, controller will process the formula using floating registers: If the formula contains one or more floating operands. If a constant value in the formula is not a whole number If an operator, such as trigonometric operators, requires that the PLC use a floating register to complete its operation. If I use constant 1000000,0 or 1000,0 * 1000,0 in the formula it does not use floating point registers. For testing I have tried the following formulas (A = 5000, B = 15747): (1000,1 * ( 1000,1 * A ) ) / B result was 44772 (999,9 * ( 1000,1 * A ) ) / B result was 317521 ( ( 1000,1 * A ) / B ) * 1000,1 result was 317000 ( ( 1000,1 * A ) / B ) * 999,9 result was 317521 Can somebody explain these results and give some guidelines on how correctly use formula block. Regards, Matic -
I've noticed that I need to be careful when writing any formulas with MI operands. Depending on the order the equation is written in, the STL token list is different. If there is any division that is happening that could result in a result for that particular term to be LESS THAN 0, the 0 propagates to the result even though regular order of operations would be otherwise. For example: (A - 3277.0) * (150.0 - 0.0) / (16383.0 - 3277.0) + 0.0 If I start by typing in the divisible term first: (150.0 - 0.0) / (16383.0 - 3277.0), the result of this will be 0, because in the STL token list the division is evaluated first then get rounded down to 0 as an MI. However when I change the order I am typing, so that the first thing I write is: (A - 3277.0) * (150.0 - 0.0), Then the STL list executes a multiplication first and the scaling is correct. It took me forever to figure this out, and there is no mention of this in the manual. Is there any way to give us the ability to edit the STL tokens rather than rewrite equations? Better yet, are there any plans to just fix the way the STL is generated so it doesn't matter in what order the equation is formed? Thanks. -
