Clearance Shapes: Creation and Use

Clearance Shapes are a key component of Autonomous bin picking.  Clearance shapes indicate where to maintain clearance between parts of the system during motion planning.

Clearances during Bin Picking

ActiNav automatically calculates robot paths between parts in the bin and the placement target. To avoid collisions along these paths, the programmer must specify what clearances to maintain between the robot and various parts of the system. ActiNav sometimes finds unusual paths between points, so these clearances must be 100% inclusive of all possibilities. Clearances are specified by surrounding parts of the system with Clearance Shapes.

Clearance Shapes

Clearance Shapes are simple shapes that specify keep-out zones around parts of the system.   ActiNav automatically loads clearance shapes for the robot, and automatically creates them for the bin.  The programmer must define clearances around new tools, the part, and the environment (including the placement target, the machine being tended, support structures, walls, and the 3D sensor).  Some tools have pre-programmed clearances.

Clearance shapes may also be used to block off parts of the workcell that the robot should avoid, even if there is nothing physically in that space.

Exceptions

There are two cases where ActiNav temporarily allows “collisions”:

1. From Pre-Pick to Pick, the tool clearance shapes are allowed to overlap with the part clearance shapes.  This is to allow the part to be picked.  After pick, the part becomes attached to the tool and thus moves with the robot.
2. From Pre-Place to Place, the part clearance shapes are allowed to overlap with the environment clearance shapes. This is to allow the part to be placed in a fixture.   After place, the part is disconnected from the robot. 

Other than these two cases, the robot will never collide with a clearance shape. If your robot collides with anything within your environment, you did not draw in your clearance shapes correctly. The goal is to get your clearance shapes to tightly surround your environment, with shapes to be as close to reality as possible. Please remember that path planning calculations are made on ONLY the clearance shapes, not any imported CAD data. 

Safety

ActiNav clearance shapes are not safety rated. If you need safety planes, you need to define those in the UR Safety panel. Every UR Safety Plane must also have a corresponding ActiNav clearance shape, so ActiNav will never cause a Protective Stop by attempting to cross the safety plane.

Types of Clearance Shapes

There are 3 types of clearance shapes. More complex shapes are built up from these three types.

• Sphere. Spheres are used for very general keep-out zones.  You will rarely use spheres, except for very particular part sections. Spheres have an origin (●) in the center, and a diameter.

 

• Capsule. A capsule is used to define clearances around long, thin items, like posts. Capsules have an origin (●) in the center, a length, and a diameter.

 

• Lozenge. A lozenge is used to define clearances around planar objects, or at the boundaries of volumetric objects. Lozenges have an origin (●) at one corner; and a length, a width, and a thickness.

 

 

Keep Clearance Shapes Simple

ActiNav uses clearance shapes, rather than CAD files or scans, to define clearance zones. For optimum plath planning speed, keep your uses of clearance shapes as simple as possible, using as few shapes as possible. For example, the 3D Sensor can be encapsulated in a single capsule. In many cases, it’s OK to make clearance shapes larger than the object. It’s always OK to overlap clearance shapes of the same function, e.g. for the environment or the tool.

The tighter clearance shapes are around an object, the more flexibility ActinNav has for motion planning. This is generally most important for parts, and for the sections of the environment around the placement fixture.

 

Defining Clearance Shapes

Clearance shapes are defined by Specifying their coordinates and size.  For environment shapes, they can also be Taught by moving the TCP. 

For Spheres, you teach the origin (Point 1) and enter the diameter.

For Capsules, you teach one end (Point 1), the other end (Point 2), and enter the diameter.  The origin will be centered between the two Points.

For Lozenges, you teach the origin in the corner (Point 1), the length by teaching the next corner (Point 2), the width by teaching the third corner (Point 3), and enter the thickness.

You define clearance shapes in the Installation tab of ActiNav for the Tool, the Environment, and the Part pages.  The method is the same in all three pages, but the reference frame for locating the shape relative to other parts of the system is different for each one.

 

For the Tool, the reference is the center of the tool flange (the default Tool Feature)

For the Environment, the reference is the center of the robot base (the Base Feature)

For the Part, the reference is the part origin.

When teaching clearance shapes on the Environment page, make sure your TCP is very accurate, including orientation, or your Clearance Shapes may be incorrect.

While all clearance shapes look the same on the ActiNav Viewer, clearance shapes differ by function, i.e. whether it surrounds a part, a tool, the robot, the bin, or the environment.  Clearance shapes with the same function may overlap: this is how complex shapes are built out of simple ones. Clearance shapes of different functions may not overlap (except for the Exceptions above), because this is a collision.

Tool Clearance Shapes

After importing a tool CAD file, the tool is automatically connected to the robot tool flange.

When specifying the clearance shapes around the tool, shape coordinates are offset relative to the tool flange. You will need to manually enter each shape around your tool. It should look similar to the image below. The goal is to create a shape that surrounds your tool as tightly as possible to accurately represent the real tool, especially near the TCP. From the right side drop down menu select “Add New Shape” and add the minimum number of shapes to completely surround your tool. 

Note that edges of the tool base are still slightly exposed. This should be ok as most path planning is a very close approximation.  If you are seeing the tool slightly collide when the system is running, just expand your shapes to accommodate. 

 

Part Clearance Shapes

The steps are exactly the same as specifying the tool, except the reference frame is the part origin. Part clearance shapes can generally be less precise than tool clearance shapes. Clearance shapes only need to be specified for one instance of the part: the AMM will automatically put clearance shapes around any part it finds.

 

Environment Clearance Shapes

The environment includes anything the robot might touch, including the placement location. This is the most important step at any point within ActiNav, and you will need to spend the most amount of time in this section. The most common error in ActiNav programming is an inaccurate environment, resulting either in collisions with the robot (underspecified environment), or poor performance / failure to pick parts at all (overspecified environment or too many clearance shapes). Fortunately, specifying the environment only needs to be done once, and usually takes no more than an hour. Environment clearance shapes are defined in Robot Base coordinates (Base Feature).

Unlike tool and part clearance shapes, Environment clearance shapes can be taught as well as specified. Both methods have drop down menus to select one of the 3 shapes. For teaching, you will need a very accurate TCP with the correct orientation. 

When using the Teach drop down menu you will teach your shapes based on the points Inside the shapes. Move your robot's TCP about your environment and touch our TCP to EVERYTHING your robot can touch. After drawing a shape you will need to offset your shape away from your robot base ½ the diameter of your shape to accurately create your environment. This is extremely important!

We teach Inside the shapes so you need to offset your shape ½ the diameter away from the robot base to compensate.  You can offset your numbers by adjusting the X,Y,Z,RX,RY,RZ values under Shape Coordinates. Feel free to “square your values up” by adjusting the values if they are off by a few degrees. For example if your RX value is -88.73 degrees you can adjust the value to -90 degrees.

A best practice is to double check all of your shapes after you offset them by moving your TCP and observing where it touches the clearance shapes on the ActiNav Viewer. Your TCP should just barely graze the exterior of the shape when it is touching an obstacle in the real world.

When using the Specify drop down menu you will need to manually type in the location of your TCP. This is useful if your robot can only touch part of a bar in your environment and you can not move your robot tcp to both points. You can get your TCP values by alternating from the “Move” screen and this Environment page. On the “Move” Screen you can copy your TCP values when you're with respect to the base. 

Using these values you can move your shapes to the specified TCP. You will need to give your shapes their lengths and diameters. Remember that you are teaching Inside a shape so you will need to offset the shape by ½ the diameter away from the robot base. This is a very tedious process so we recommend using teach by demonstration as much as possible. 

Once you are done teaching your Environment your work cell should look similar to the images below. Understand that each environment will be unique to your work cell so some will have different place fixtures, walls, ceilings etc. Your bin will not be in your environment until you follow the bin wizard under the Bin Page. 

Clearance Shapes in the ActiNav Viewer

The ActiNav Viewer shows clearance shapes in a translucent gray (for the environment) or green (for the robot, tool, part, and bin). If there is a red outline around one of the clearance shapes, this indicates that this is the current clearance shape selected and you are currently editing this shape.  The red outline is only seen in the Installation tabs, when clearance shapes are defined and edited.

If the clearance shape is red, this means that somewhere, a robot, tool, or part clearance shape is in collision with an environment or bin clearance shape. For example, in the image below, the tool is colliding with a placement fixture, which is part of the environment. All clearance shapes turn red at the same time: you have to examine the robot position to determine where the collision is.

Collisions of this sort should only happen when you manually move the robot. ActiNav will not move the robot so as to cause collisions like this.  The red color is a useful indication of a virtual collision when you may not be expecting one, for example if a shape is misplaced or too large.