Description
Mechanical backlash
The play between the drive and a moving machine part or between an encoder and a moving machine part is referred to as mechanical backlash. For a moving machine part, mechanical backlash results in a deviation between the commanded position and the actual position. This especially has an effect when the direction of motion is reversed.
A distinction is made between the following kinds of mechanical backlash:
Positive backlash
Positive backlash occurs in systems in which the measuring system is connected directly to the drive and the backlash occurs between the drive and the moving machine part. When the direction of movement reverses, the measurement system will detect a position change although the machine part is not yet moving due to the backlash. This leads to a situation in which the machine part does not reach the commanded position, but travels too short by the amount of the backlash because the encoder that indirectly measures the position of the machine part is ahead of the machine part's actual position.
Negative backlash
Negative backlash is encountered in systems in which the backlash occurs between the moving machine part and the measurement system. When the direction is reversed, the machine part directly moves in the new direction without the measuring system detecting a position change. In this case, the machine part moves further by the backlash than has been commanded because the encoder that directly measures the position on the machine part lags behind the machine part's actual position.
Backlash compensation
The effect of backlash can be compensated in all kinds of drives.
In the case of positional control, the amount of the backlash (P-AXIS-00103) is taken into consideration depending on the nature of the occurring mechanical backlash (P-AXIS-00021) and has an effect on calculation of the command variables.
Notice | |
The output of the moving machine part's actual position does not include the compensation values and therefore represents the position of an ideal machine. |
Effect
When backlash compensation is selected, it is active directly after startup of the control system, regardless of whether reference point travel has taken place.
The algorithm compensates the backlash in the first cycle of the travel movement. For large backlash this can cause a strong machine excitation. To avoid this, the compensation of the backlash can be distributed on multiple positions control cycles (P-AXIS-00243).