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Module09
Title

A blending station in SCL

Synthetic Course: written to test the Site template, from no Professor's material

Worked examples

TopicA blending station in SCL
Title

Weighing the hopper in one scan

ExampleW09.1
Step1 / 3
Mode
Given
Given

An S7-300 runs the Blender function block below once a scan. Its three load cells read 00 to 2764827648 for 00 to 500500 kg, and the recipe wants 120120, 8080 and 120120 kg. For each set of raw values, fill in the weight in the hopper, how full the batch is, and how many 2525 kg bags it fills, rounded to the nearest bag.

  • load cells: raw 00 to 2764827648 is 00 to 500500 kg
  • recipe: 120120, 8080 and 120120 kg
  • bags of 2525 kg
One scan, three sets of raw values
raw 1raw 2raw 3totalkgfill%bags
345625924320187.5
648043206480312.5
734445366912339.8

Weigh the silos

The FOR loop scales each load cell with ToKg, kg=raw27648⋅500\text{kg} = \frac{\text{raw}}{27648} \cdot 500, and adds it up: 117.1875+78.125+117.1875=312.5117.1875 + 78.125 + 117.1875 = 312.5 kg.
Sim

The blending station, running

Set the load cells and the buttons, then step a statement or run a whole scan. Each scan calls the Blender block once; the trace shows what every statement decided and wrote.

  1. // A blending station: three silos dose into one hopper, then a mixer runs the batch.
  2. TYPE "Silo"
  3. STRUCT
  4. target_kg : REAL; // the recipe's weight from this silo
  5. raw : INT; // load cell: 0..27648 = 0..500 kg
  6. kg : REAL; // the weight it reads
  7. short : BOOL; // still under its target
  8. END_STRUCT
  9. END_TYPE
  11. DATA_BLOCK "Recipe"
  12. VAR
  13. silo : ARRAY[1..3] OF "Silo";
  14. batches : INT; // batches mixed so far
  15. END_VAR
  16. BEGIN
  17. silo[1].target_kg := 120.0;
  18. silo[2].target_kg := 80.0;
  19. silo[3].target_kg := 120.0;
  20. END_DATA_BLOCK
  22. FUNCTION ToKg : REAL
  23. VAR_INPUT
  24. raw : INT;
  25. END_VAR
  26. BEGIN
  27. ToKg := INT_TO_REAL(raw) / 27648.0 * 500.0;
  28. END_FUNCTION
  30. FUNCTION_BLOCK "Blender"
  31. VAR_INPUT
  32. start : BOOL; // the operator's start button
  33. raw1 : INT; // the three load cells
  34. raw2 : INT;
  35. raw3 : INT;
  36. mixed : BOOL; // the mixer's timer has run out
  37. END_VAR
  38. VAR_OUTPUT
  39. valve : INT; // the silo dosing now, 0 = none
  40. total_kg : REAL; // in the hopper
  41. fill_pct : REAL; // of the batch's weight
  42. bags : INT; // 25 kg bags the hopper fills
  43. mixer : BOOL;
  44. END_VAR
  45. VAR
  46. step : INT; // 0 IDLE, 1 DOSE, 2 MIX
  47. last_start : BOOL; // start, one scan ago
  48. END_VAR
  49. VAR_TEMP
  50. i : INT;
  51. target : REAL;
  52. END_VAR
  53. BEGIN
  54. "Recipe".silo[1].raw := raw1;
  55. "Recipe".silo[2].raw := raw2;
  56. "Recipe".silo[3].raw := raw3;
  57. total_kg := 0.0;
  58. target := 0.0;
  59. FOR i := 1 TO 3 DO
  60. "Recipe".silo[i].kg := ToKg(raw := "Recipe".silo[i].raw);
  61. "Recipe".silo[i].short := "Recipe".silo[i].kg < "Recipe".silo[i].target_kg;
  62. total_kg := total_kg + "Recipe".silo[i].kg;
  63. target := target + "Recipe".silo[i].target_kg;
  64. END_FOR;
  65. fill_pct := total_kg / target * 100.0;
  66. bags := REAL_TO_INT(total_kg / 25.0); // 312.5 kg is 12.5 bags (a Divergence: the exam's answer differs from a real S7)
  67. valve := 0;
  68. CASE step OF
  69. 0: // IDLE: wait for start to rise
  70. IF start AND NOT last_start THEN
  71. step := 1;
  72. END_IF;
  73. 1: // DOSE: open the first silo still short
  74. FOR i := 1 TO 3 DO
  75. IF "Recipe".silo[i].short THEN
  76. valve := i;
  77. EXIT;
  78. END_IF;
  79. END_FOR;
  80. IF valve = 0 THEN
  81. step := 2;
  82. END_IF;
  83. 2: // MIX: run the mixer until its timer runs out
  84. mixer := NOT mixed;
  85. IF mixed THEN
  86. "Recipe".batches := "Recipe".batches + 1;
  87. step := 0;
  88. END_IF;
  89. ELSE
  90. step := 0;
  91. END_CASE;
  92. last_start := start;
  93. END_FUNCTION_BLOCK

Scan 1 · statement 27 of 27

Statement92 last_start := start;
Writtenlast_start := FALSE
VariableValue
step0
valve0
total_kg312.5
fill_pct97.65625
bags12
mixerFALSE
Recipe.silo[1].kg117.1875
Recipe.silo[2].kg78.125
Recipe.silo[3].kg117.1875
Recipe.batches0
LineStatementResult
54"Recipe".silo[1].raw := raw1;Recipe.silo[1].raw := 6480
55"Recipe".silo[2].raw := raw2;Recipe.silo[2].raw := 4320
56"Recipe".silo[3].raw := raw3;Recipe.silo[3].raw := 6480
57total_kg := 0.0;total_kg := 0.0
58target := 0.0;target := 0.0
59FOR i := 1 TO 3 DOinto the loop
60"Recipe".silo[i].kg := ToKg(raw := "Recipe".silo[i].raw);Recipe.silo[1].kg := 117.1875
61"Recipe".silo[i].short := "Recipe".silo[i].kg < "Recipe".silo[i].target_kg;Recipe.silo[1].short := TRUE
62total_kg := total_kg + "Recipe".silo[i].kg;total_kg := 117.1875
63target := target + "Recipe".silo[i].target_kg;target := 120.0
64END_FOR;again
60"Recipe".silo[i].kg := ToKg(raw := "Recipe".silo[i].raw);Recipe.silo[2].kg := 78.125
61"Recipe".silo[i].short := "Recipe".silo[i].kg < "Recipe".silo[i].target_kg;Recipe.silo[2].short := TRUE
62total_kg := total_kg + "Recipe".silo[i].kg;total_kg := 195.3125
63target := target + "Recipe".silo[i].target_kg;target := 200.0
64END_FOR;again
60"Recipe".silo[i].kg := ToKg(raw := "Recipe".silo[i].raw);Recipe.silo[3].kg := 117.1875
61"Recipe".silo[i].short := "Recipe".silo[i].kg < "Recipe".silo[i].target_kg;Recipe.silo[3].short := TRUE
62total_kg := total_kg + "Recipe".silo[i].kg;total_kg := 312.5
63target := target + "Recipe".silo[i].target_kg;target := 320.0
64END_FOR;done
65fill_pct := total_kg / target * 100.0;fill_pct := 97.65625
66bags := REAL_TO_INT(total_kg / 25.0);bags := 12
67valve := 0;valve := 0
68CASE step OF0: line 69
70IF start AND NOT last_start THENFALSE
92last_start := start;last_start := FALSE
Derived
worked out here, no official key: 12.512.5 bags, held as 1212
Divergence
The exam answer is 1313 bags, the Professor's. REAL_TO_INT rounds 12.512.5 to the even number on a real S7, so the CPU holds 1212.