Precision Requirements for Circular Move Input Parameters

The input parameters to MC_MoveCircAbs and MC_MoveCircRel are validated when constructing a circle.

  • The distance of the center point from the start and end points is checked.
  • Ideally, the two distances are the same but calculation errors or input data precision can cause the two distances to be slightly different.
  • If the validation fails, PLCopen error 50 (Cannot construct a circle with specified parameters) is returned from the function block.

Parameter Validation

Input parameters are validated using these methods:

See PLCopen Function Block ErrorIDs.

Here is a circle we want to create.

The distances from the center to the start and end point are measured as r1 and r2 respectively.

  1. Center point.
  2. Start point.
  3. End point.

The methods for validating the circle parameters are:


  • If MC_GRP_PARAM_CIRCLE_TOLERANCE is set to zero (0), the default validation method will be used.

Default Validation Method

  • The difference between the two measurements must not be larger than one part in 100,000.
  • If all positions are specified to six significant digits, this requirement is met.
  • Assuming that r2 is greater than r1, we can write r2 in terms of r1 and a small deviation value named ε:
r2= r1(1+ε)
  • If ε exceeds a value of 10-5, the validation fails.
  • A PLCopen error 50 is returned from the function block.

Examples of Default Validation

If the desired circle has a radius of:

  • 50 user units, the center must be specified with a precision of 0.0005 user units.
  • 2,000 user units, the center must be specified with a precision of 0.02 user units.
  • 500,000 user units, the center must be specified with a precision of 5 user units.

Validation with Custom Tolerance

A custom tolerance can be specified via the group parameter MC_GRP_PARAM_CIRCLE_TOLERANCE.

  • See Axes Group Parameters.
  • This parameter represents the amount of error allowed, measured in user units.
  • The parameter can be set by using MC_GrpWriteParam and can be read via MC_GrpReadParam.
  • Assuming that r2 is greater than r1, we can write r2 in terms of r1 and a deviation value named ∆:
r2= r1 + ∆
  • The validation fails if ∆ exceeds the value of the group parameter MC_GRP_PARAM_CIRCLE_TOLERANCE.
  • A PLCopen error 50 is returned from the function block.

Examples of Validation using a Custom Tolerance

  • If r1 has a value of 50 user units and the circle tolerance parameter is .1 user units, then r2 must be in the range of 49.9 to 50.1 user units.
  • If r1 has a value of 2000 user units and the circle tolerance parameter is 1 user units, then r2 must be in the range of 1999 to 2001 user units.

  • Use LREAL variables and LREAL versions of math functions when calculating the desired circle parameters inside a KAS application.
    The LREAL versions of functions usually have an ‘l’ at the end of their name.
    Example: The LREAL version of cos is cosl.
    This helps avoid errors.