Gravity Compensation
Overview
Gravity compensation is a function to reduce the impact of the gravity on the Force Sensor.
The Force Sensor measures the difference from "0" that indicates the state of force at the moment the force sensor is reset. Therefore, if the posture of the robot is changed to another posture after resetting the Force Sensor, the Force Sensor also measures the weight of the hand and workpiece impacted by the gravity as the force. As a result, the force control function sometimes cannot perform the intended operation due to this effect. Gravity compensation reduces the impact of gravity from the measured force to retrieve only the force from an external object that is applied during the intended operation.
Even if the gravity compensation is used, Force Sensor is influenced by gravity. When the measured has exceeded the rated value, even if the displayed value is within the rated value of Force Sensor, a 5548 error occurs.
CAUTION
If the setting of the mass property or gravity direction is incorrect or if the mass property number to be used is incorrect, the force control function may perform an unintended operation. Configure the settings carefully, and first verify the operation and then perform the force control function.
Manually define mass properties
A mass property object is an object to handle the mass properties for gravity compensation.
The mass property object is an object that has the weight (Mass property) and the center of gravity (GravityCenter property) of all objects (e.g. hand and workpiece) mounted to the area closer to the tip than the Force Sensor. For the weight, set the value including the weight of all objects such as the hand and workpiece, and for the center of gravity, set the gravity center position in the tool 0 coordinate system.
Up to 15 mass property object values can be set for each robot simultaneously. They can be set in the [Mass/Gravity] panel in Robot Manager or by the MPSet statement.
The weight and gravity center position can be set directly in the [Mass/Gravity] panel. They can also be set automatically in Mass / Gravity Wizard for 6-axis robots.
For the details, refer to the following section.
Software [Robot Manager] (Tools Menu)
- [Tools]- [Robot Manager]-[Mass/Gravity] Panel
The value of each property can be set directly in the MPSet statement.
For details, refer to the following manual.
"Epson RC+ 8.0 Option Force Guide 8.0 SPEL+ Language Reference"
Gravity Direction
Gravity direction is the direction of gravity for the robot necessary for gravity compensation.
The gravity direction is specified by the gravity vector in the base coordinate system of each robot. The robot coordinate system is a coordinate system in which "+z" indicates the upward vertical direction and "+y" indicates the front direction of the robot, and by default, the base coordinate system is also aligned with the robot coordinate system. The gravity works in the downward vertical direction so the gravity direction is represented by the vector (0, 0, -1). This applies to both cases where the robot is mounted to the table and to the ceiling. However, if the base coordinate system was changed using the Base statement, or if the robot is mounted in an inclined state, you need to calculate and set the gravity direction vector in the base coordinate system.
For the gravity direction, set one value for each robot. The gravity direction can be set in the [Mass/Gravity] panel in Robot Manager or in the F_GravityDirection statement.
The values of the gravity direction can be set in the [Mass/Gravity] panel. Furthermore, the gravity direction can be set automatically in Mass / Gravity Wizard for 6-axis robots.
For the details, refer to the following section.
Software [Robot Manager] (Tools Menu)
- [Tools]- [Robot Manager]-[Mass/Gravity] Panel
The values of the gravity direction can be set in the F_GravityDirection statement.
For details, refer to the following manual.
"Epson RC+ 8.0 Option Force Guide 8.0 SPEL+ Language Reference"
Executing Gravity Compensation
Gravity compensation is always performed for the Force Sensor that is linked with the robot. Gravity compensation cannot be performed for a Force Sensor that is not linked with the robot. Furthermore, selecting the object to be used from the stored mass property objects allows you to select a parameter according to the operation state at any time. Object selection is performed in the MP statement. After executing the MP statement, reset the Force Sensor in the Reset property of the Force Sensor object.
Example: When performing gravity compensation using Mass Property 1
MP 1
For details on the MP statement, refer to the following manual.
"Epson RC+ 8.0 Option Force Guide 8.0 SPEL+ Language Reference"
When performing the force control in each of the states where there is a workpiece and where there is not a workpiece, set the mass property in each of the states since a workpiece has also a weight, and perform the force control while switching between both states.
Selecting number 0 (MP0) or selecting the mass property object with a weight of "0" stops the gravity compensation function. If the gravity compensation is not required for operations where, for example, the posture change is small, select "MP0" to stop the gravity compensation. Selecting a mass property object other than "MP0" after stopping it restarts the gravity compensation.
The selected mass property number and set mass property are retained after the Robot Controller is turned OFF until they are changed. Turning ON the Robot Controller also automatically starts the gravity compensation.