Fail-safe speed monitoring of a drive

A method for fail-safe monitoring of the speed of a drive to indirectly monitor web speed of a material web which is moved by rollers, said drive having at least a converter, a motor and a rotational speed sensor, the method comprising:

• specifying a target rotational speed to the drive and capturing an actual rotational speed by the rotational speed sensor;
• determining a substitute rotational speed and performing a plausibility check in a safety program of a safety CPU using in each case two of the following three values:
• (i) target rotational speed, (ii) actual rotational speed and (iii) substitute rotational speed, the substitute rotational speed being determined from one of (i) an output frequency of the converter in cases of three-phase drives and (ii) from a quotient of terminal voltage and magnetic flux in cases of direct-current drives, a plausibility check occurring in each case (i) between the target rotational speed and the actual rotational speed, (ii) between the target rotational speed and the substitute rotational speed, and (iii) between the actual rotational speed and the substitute rotational speed;
• and
• verifying during the plausibility checks whether a difference between two of the three values changes continuously;
• wherein at least one of (i) the actual rotational speed and (ii) the substitute rotational speed are checked in the safety program to determine whether one of (i) the at least one of the actual rotational speed and the substitute rotational speed exceed at least one specified positive limit value and (ii) the at least one of the actual rotational speed and the substitute rotational speed do not reach at least one specified negative limit value.