Robot and machine dynamic joint limits

JointLimits

Dynamic joint limits

Many robot and machine controllers allow the definition of dynamic joint limits, where the minimum and/or maximum value of a limit depend on the value of another axis. Reasons for such limits can be obstacles within the work envelop of the robot or machine or to avoid unwanted wind up of dress packs, cables or hoses.

FASTSUITE Edition 2 provides a Python API to create such required limit rule(s).

How to define the dynamic joint limits

This page demonstrates the creation of a set of rules for an OTC-DAIHEN robot.

1	Acquire the limits
First we need to acquire the so called Link Soft Limits of the OTC controller:

1.1	Raise the user protection level to at least Specialist. (R->314 Enter Password FD-ST: 12345)
1.2	Navigate to: (1) Constant Settings (2) Machine Constants (3) Link Soft Limit.
1.3	Each page represents the limits of one driving and one dependent axis. Only the pages that are enabled need to be considered. Joint 2 moves in between -65° and +180°. Joint 1 can normally move in between -170° and +170°. Only when Joint 2 is within the specified ranges as shown below, the limits of Joint 1 are restricted. E. g. Limit2 defines that the lower limit of Joint 1 ramps down from -125° to -154° while Joint 2 moves from -65° to -47° and so on. All applicable limit conditions need to be considered in the Python script specific to this particular robot.
1.4	On page 2, Joint 3 is also dependent on Joint 2. Limit1 defines that the lower limit of Joint 3 ramps down from -15° to -80° while Joint 2 moves from -65° to 35°. The maximum limit of Joint 3 remains unchanged.

2	Create the Python script

2.1	Next we need to create a Python file and store it in the E2Plugin folder you use for your customization.
2.2	Open the robot resource in your Edition 2 session. Open its properties dashboard and add a string attribute by the name of DynamicLimitsScriptPath in the User defined attributes container. The Python file name and its path can be provided relative to the E2Plugins folder or as an absolute path including drive letter.
2.3	The following Python code demonstrates how multi-range limits for multiple joints can be composed. Add these lines to the Python file of step 2.1. # Import libraries from centypes import * from cenpylib import DynamicLimits as DL def CheckJointsDynamicLimits(joints): j1 = joints[0] j2 = joints[1] j3 = joints[2] j5 = joints[4] j6 = joints[5] # Check J1 dependent on J2 j1min = InRange(j2, -65, -47)DL.Ramp(j2, -65, -125, -47, -154) + InRange(j2, -47, 180)-170 j1max = InRange(j2, -65, -41)DL.Ramp(j2, -65, 130, -41, 151) + InRange(j2, -41, 180)170 if not (j1 >= j1min and j1 <= j1max): return UNREACHABLE # Check J3 dependent on J2 j3min = InRange(j2, -65, 35)DL.Ramp(j2, -65, -80, -35, -15) + InRange(j2, -35, 180)-80 j3max = InRange(j2, -65, 180)100 if not (j3 >= j3min and j3 <= j3max): return UNREACHABLE # Check J6 dependent on J5 j6min = InRange(j5,-140,-70)DL.Ramp(j5,-140,-110,-70,-165) + InRange(j5,-70,-35)DL.Ramp(j5,-70,-165,-35,-200) + InRange(j5,-35,-20)DL.Ramp(j5,-35,-200,-20,-300) j6max = InRange(j5,-140,-70)DL.Ramp(j5,-140,110,-70,165) + InRange(j5,-70,-35)DL.Ramp(j5,-70,165,-35,200) + InRange(j5,-35,-20)*DL.Ramp(j5,-35,200,-20,300) if not (j6 >= j6min and j6 <= j6max): return UNREACHABLE return REACHABLE def InRange(joint, x1, x2): """Dynamic limits utility functions. Returns: integer: 0 or 1. """ if joint < x1 or joint > x2: return 0 else: return 1
	More information about Python customization and be found in the documentation Technology customization with Python API.
2.4	There are two limitations with respect to dynamic joint limits: a.The joint limits that are shown in the Simulation monitor are not updated when a dynamic limit kicks in. b.The Dynamic Limits function is not called when an axis is modified in the Teach panel. Instead, use the manipulator or grab the robot joint directly in 3D to invoke the inverse kinematics.

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