This application explains how to use the Home to Torque-level sequence in Studio 5000 software, and the considerations required when using this homing method. This document provides an example for a typical homing program routine.
Home to torque-level homing is a process that references a known position by monitoring torque while driving an axis into a mechanical hard-stop. Once the actual torque level reaches or exceeds a specified torque level for a set time of 500 ms, a status flag is set in the controller.
General Features
Advantages:
This sequence facilitates the implementation and quick configuration for Homing an Axis when is required monitor the torque.
Limitations/Disadvantages:
This sequence is available for CompactLogix / ControlLogix Controllers.
This code is defined for applications to monitoring torque while driving an axis into a mechanical hard-stop.
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Installation Guide
Step 01 - Configure the Axis Homing Tab
Because the process of home to torque-level requires axis motion, the axis homing mode must be configured as Active.
1- Open the Axis Properties tab. And complete the next configuration. The Torque level option sets the Home Position after the output torque reaches the Torque Level value, reverses direction, and moves until the Homing Torque Above Threshold bit is low and the status bit sets. The Torque Level – Marker sets the Home Position after the output torque reaches the Torque Level value, reverses direction, and encounters an encoder marker.
When either the Torque Level or Torque Level - Marker homing option is selected on the Homing tab, the Torque Level field is activated in the Active Home Sequence Group. The units for this field are a percentage of the continuous torque of the motor (% Continuous Torque) limited by the drive-rated current/motor-rated current ratio. This number is interpreted in the drive as an absolute value and the range is 0...TorqueLimitPositive value.
Forward Bi-directional and Reverse Bi-directional are the only options for the Direction field when Torque Level or Torque Level - Marker homing sequence is selected. Uni-directional homing is not possible, because the Home to Torque-level sequence relies on a mechanical hard-stop.
Step 02 - Drive Bipolar Torque Limit Adjustment
When homing an axis to a mechanical hard-stop, set the Home Torque-level value above the torque value required to move the system, but low enough not to cause problems with the system mechanics.
As part of the process of homing to a torque limit, limit the Peak Torque value to a level 10% above the Home Torque value to reduce the stresses on the mechanics and to eliminate the chance of an over-current fault.
The 10% value is an estimated starting point. This value may need adjustment based on the application requirements. Limit the Peak Torque value before issuing the homing instruction (MAH) and reset the Peak Torque field to the original value after homing completes.
Step 03 - Disable Soft Overtravel Limit
If the application requires the use of soft-overtravel limits (Limits tab) to safeguard the system mechanics, the Soft Travel Limits must be disabled for the axis to home.
The Soft Travel Limits must be disabled through programming to prevent a fault from occurring during the homing operation, but re-enabled after homing completes.
Step 04 - Ladder Code Sequence
This sequence shows how to adjust the peak torque limit and disable the soft-overtravel limit checking when homing to a torque limit. The code uses a state model methodology in which each rung of code needs to complete successfully before moving to the next rung.
Rung 11 of the ladder code checks to make sure that the axis position, after homing, is within the Soft Travel Limits before re-enabling soft-overtravel limit checking. In this code a 1.15 cm Offset move is configured in the Homing tab and that is the final home position. The 0.5 cm is within the Soft Travel Limits set on the Limits tab.
List of Tags Used
Axis Properties - Homing Tab
Axis Properties - Limits Tab
Ladder Code Example
Potential for Position Error Fault
When executing a torque limit homing procedure there is potential for a Position Error fault. As mentioned earlier, for the home to torque limit to complete, the output torque to the motor must reach (or exceed) the specified torque level for a set time of 500 ms. During this time the axis is against the mechanical hard-stop, and following error is increasing in the position loop. If the Position Error Tolerance value is exceeded before completion of the homing instruction, a Position Error fault (E19) will result.
There are two ways to limit the occurrence of an E19.
- Set the Position Error Tolerance value on the Limits tab of the Axis Properties to a value high enough to eliminate a fault from occurring.
- Modify the Position Error Tolerance value via an SSV instruction similar to the method used in code above to change the Bipolar Torque Limit.
Rung 6 of the ladder code opens the Position Error window. This allows the Homing to complete without causing a Position Error fault. The original value is reset after Homing completes.
Home to Torque Limit Sequence in Studio 5000
Version 1.0 - August 2024