All A6 series servos currently sold by STEPPERONLINE have built-in soft limit functionality. Before introducing it, let's first understand what a soft limit actually is.
1. Purpose
Two values can be set inside the servo drive (for example, a maximum positive position of 10000 and a maximum negative position of -10000 mm). When the motor's actual coordinate is about to exceed these two values, the system will forcibly decelerate the motor to a stop and raise an alarm, thereby protecting the equipment.
Its main functions are as follows:
● **Collision prevention:** Mechanical equipment (such as ball screws, guide rails, and robotic arms) has a limited stroke. If a programming error, human misoperation, or excessive speed causes the motor to keep driving forward, a "crash" will occur. Soft limits brake in advance through data calculation before any visible physical collision happens, preventing expensive machinery from being destroyed.
● **Complementing hard limits and reducing mechanical wear:** Normally, equipment is fitted with an origin switch and hard limit switches. However, hard limits require physical contact or proximity (for example, a dog pressing against a switch), and making the machine hit the hard limit switches over and over tends to damage the switches and subjects the mechanics to vibration.
**What is the reference for a soft limit?**
Put simply, it is the zero point of the absolute mechanical coordinate system established after homing calibration. Physically, when the servo motor homes, a specific position on the mechanism (for example, the position where it hits the origin switch, or the encoder's Z-phase zero position) is defined as the absolute zero point, and the soft limit uses this as its reference coordinate.
2. Parameter settings
As shown in the figure below, enabling the soft limit function only requires attention to three parameters: C06.07, C06.08, and C06.0A.
● C06.07 = 1/2, enables the soft limit function;
● C06.08, C06.0A = xxxx, sets the positive and negative limit positions.
The two settings of C06.07: enable directly vs. enable after homing is complete
2.1. Enable directly
When set to direct enable, the servo system compares the currently fed-back coordinate value against the "soft limit positive/negative maximum values" you have set as soon as it is powered on.
For example, the A6 servo's default encoder state is incremental (without using an absolute encoder cable), so the servo's internal coordinate is cleared after every power-off. Suppose you have set the soft limits to +10000 and -10000, and the mechanism actually stops very close to the mechanical limit position. After power-up, the servo considers the current position to be 0. As soon as you jog in the direction away from the origin, the motor may hit the mechanical limit position directly before the soft limit is even triggered.
2.2. Enable after homing is complete
After the servo motor is powered on, the coordinate system is regarded as "uninitialized." The soft limit function is off at this point, and no over-travel alarm will be triggered no matter how the motor is run. Only after homing is complete and the drive issues the homing-complete signal is the internal soft limit function formally activated. This is the absolutely reliable logic, because the position coordinate has already established a true mechanical zero point via the physical switch or the encoder Z phase.
In summary, C06.07 = 1 is best suited for enabling in absolute mode, where no second homing is needed after a power failure; C06.07 = 2 is best suited for enabling in the default incremental encoder mode, where homing is performed first after each power-off and restart.
3. Example settings
Here we use the software for reference. As shown in the figure below, the servo motor is at the zero position, all three parameters have been set, and the soft limit function has been successfully enabled.
When the motor's travel exceeds the set upper limit — for example, when the current position > 10000 (positive limit):
or < -10000 (negative limit),
the drive will issue the relevant alarm and the motor will stop working.
Note that faults such as positive over-travel and negative over-travel cannot be reset. They are cleared only when the motor position is back within the specified range. Therefore, the motor can only be moved manually to clear the alarm.