Grasping methods

A grasping method describes how the robot picks one object: the waypoints it moves through, how it moves between them, and which robot procedures it calls on the way. Each vision system uses one grasping method. This page applies to Bin Picking Studio (BPS) solutions only, the CAD Picking and AI Picking solution types.

The five path stages that every grasping method has. The robot waits in a scanning pose out of the scanner’s view, the Start pose is above the center of the bin, and the End pose is above the bin wall nearest the robot. The blue line is the linear part of the path.

A few terms used on this page:

• Waypoint – a pose that the tool point of the gripper moves through.

• Path stage (picking path stage) – one waypoint, the move that leads to it, and an optional robot procedure. It is one row of the path stage table.

• Bin picking routine – all path stages of a grasping method in order. The robot runs it in each picking cycle. It begins in the Start pose and ends in the End pose, with the picked object in the gripper.

Grasping method list

A new solution has one grasping method, the Basic Grasping Method. You can add more, for example when different vision systems need different paths. The page title shows how many methods the solution has and how many it can have, for example Grasping method (2/13).

The Grasping method page of the CAD Picking example solution. Red frames: Add new grasping method, the default method with the vision systems that use it, and Set default of another method.

• To add a method, type a Name and click Add new grasping method. The new method has only the five mandatory path stages, with their default values. To start from an existing method instead, click Duplicate on it.

• The Vision system column shows which vision systems use each method.

• One method is the default. It has a default tag, and it has no Set default and Delete buttons. To make another method the default, click Set default on it.

Choosing the method of a vision system

The grasping method of a vision system is set in the Grasping method field of the vision system.

The Grasping method field of a vision system, with all options shown

Option	What the vision system uses
Current default ( name )	Always the method that is the default. When you make another method the default, the vision system switches to it.
A named method	Always this method, even when another method becomes the default.

Path stages

Click Edit on a grasping method to open its Picking Path Stages table. Each row is one path stage, in the order the robot goes through them.

A grasping method with the five mandatory path stages. Red frames: the Picking Path Stages table and Add a new picking path stage.

The table shows the Trajectory Type of the move to each waypoint (Joint or Linear) and its Position Tolerance (Precise or Approximative). The info icon next to each stage shows the help texts quoted below.

#	Stage	What happens (help text of the Studio)	Pose comes from
1	Start	“The Start path stage is used to prepare the robot and the gripper for a new pick. The pose is defined on the robotic controller.”	The robot program
2	Approach	“The gripper is moved to a pose defined by a relative offset with respect to the found gripping point on the part. Usually the offset is set to several centimeters in the opposite direction of the gripping point’s Z-axis. This, along with linear path planning, should help to make the grasping movement easier and more reliable to perform. The assigned procedure could, for example, initiate suction in case of a suction gripper.”	An offset from the found gripping point or the robot base
3	Grasp	“The gripper is moved to the grasping pose given by the found gripping point. The assigned procedure should execute the grasp action, e.g. by closing a two-finger gripper.”	The found gripping point and the tool point
4	Deapproach	“The gripper is moved to a pose defined by a relative offset with respect to the found gripping point on the part. Usually the offset is set to several tens of centimeters in the direction parallel with the robot’s Z-axis. This, along with linear path planning, should help to better avoid collisions with other parts and/or the container.”	An offset (see Origin of the relative offset)
5	End	“The gripper is moved to the end pose defined on the robotic controller. The assigned procedure could contain some post-picking operations.”	The robot program

The Start and End poses are the ones the robot program sends in the initialization request.

Mandatory and custom path stages

• The five stages above are mandatory. Their order is fixed and they cannot be deleted.

• You can add up to six custom path stages, 11 in total. Click Add a new picking path stage. After saving, drag the double arrow at the start of a custom row to move it between the mandatory stages.

• Some settings are fixed for the mandatory stages:

  • Start has no move leading to it, so it has only a procedure.

  • Start and End have no position settings, because their poses come from the robot program.

  • Approach and Deapproach are always relative to the found gripping point or another origin.

  • The moves to Grasp, to Deapproach and to every stage between Approach and Deapproach are always linear.

Editing a path stage

Click Edit on a path stage to open its form below the table. Which fields are shown depends on the stage and on its type.

The form of the Deapproach stage of the example solution. Red frames: Type, Origin of relative offset and Relative offset Z.

Field	Description	Shown for
Type	Relative to an origin, or a Fixed position in robot space. Mandatory stages show their own name here.	All stages (editable for custom stages)
Name	A name for the path stage.	Custom stages
Origin of relative offset	“Choose one of origins to determine offset orientation.” See below.	Relative stages
Fixed plane	“Plane towards which the motion will be executed. The motion is executed along the axis perpendicular to the plane.”	Origin Fixed plane
Relative offset X/Y/Z [mm]	The offset from the origin. For Z: “in case of found gripping point’s origin, the Z-axis points towards the picked object, so a negative value should be used. In case of robot’s origin, the Z-axis points up in the global coordinate frame, so a positive value should be used.”	Relative stages
Relative offset value [mm]	The distance used with the Fixed plane, Start and End origins.	Relative stages
Absolute position X/Y/Z [mm]	“Absolute position relative to the robot base origin.”	Fixed stages
Plan linear trajectory	“With linear path enabled, robot makes a straight movement along linear path. With disabled linear path, starting and end point of path remains the same. However, effector of robotic arm can make a rather curved movement based on current alignment of joints.”	All stages except Start
Position tolerance	“How close should the gripper be moved to the defined point in space. In case of approximative option, the absolute tolerance can be configured on the robotic controller.”	All stages except Start
Procedure name	“Name of the procedure to be called when the robot gets to this path stage, for example shutting pneumatic valves, etc. The methods are defined on robotic controller.” Options: Attach, Detach, User 1, User 2, User 3.	All stages

Click Save to store the path stage.

Type and name

• Relative – the waypoint is computed from an origin, see the next section.

• Fixed – the waypoint is a fixed point in robot space, see Absolute position.

Custom path stages also have a Name, shown in the path stage table.

Origin of the relative offset

The origin decides where the offset is measured from and in which coordinate system.

Origin	The waypoint is	Available for
Found gripping point	offset from the gripping point found in the current cycle, in the coordinate system of that gripping point.	Approach, Deapproach, custom
Robot	offset from the previous waypoint, in the coordinate system of the robot base.	Approach, Deapproach, custom
Fixed plane	on a plane parallel to a plane of the robot base. The robot moves to it along the normal of the plane.	Deapproach, custom
Start / End	on the way towards the Start or End pose, or towards a point offset from it.	Deapproach, custom

Found gripping point – the offset follows the orientation of the gripping point. Its Z-axis points into the object, so a negative Z moves the gripper away from it.

Robot – the offset is measured in the robot base coordinate system, whatever the orientation of the gripping point. Its Z-axis points up, so a positive Z moves the gripper up.

Fixed plane – choose a plane parallel to the XY, XZ or YZ plane of the robot base. Relative offset value is the distance of the plane from the robot base. In each cycle, the robot moves from the previous waypoint along the normal of the plane until it reaches the plane, so two of the three coordinates stay the same.

Start / End – the robot moves towards the Start or End pose from the robot program, for example towards the middle of the bin and away from its walls. To aim at a point next to that pose, enter a Relative offset X/Y/Z. This creates a virtual helper point. Relative offset value is the length of the move. If it is not set, the waypoint is the Start or End pose (or the helper point) itself.

The move is always measured from the waypoint on the Grasp side: for a stage between Start and Approach, from Approach; for a stage between Grasp and Deapproach, from Grasp.

A custom waypoint on the way from the preceding waypoint (for example Grasp) towards a helper point offset from the Start or End pose. The length of the move is the Relative offset value.

Absolute position

A Fixed path stage goes to the point set in Absolute position X/Y/Z, relative to the robot base origin. The gripper keeps the orientation it had in the previous waypoint.

A Fixed path stage: the robot moves to the same point in robot space in every cycle

Plan linear trajectory

When it is selected, the robot moves to the waypoint in a straight line. Otherwise the path planner may plan a joint move, where the gripper can follow a curve. The moves to Grasp, to Deapproach and to every stage between Approach and Deapproach are always linear.

Position tolerance

Option	Behaviour
Precise	The robot reaches the waypoint exactly. The procedure (if any) is called at that position.
Approximative	The robot may pass the waypoint within a tolerance and blend into the next move. The tolerance is set on the robot controller. The procedure may be called while the robot is still moving, so do not use this option with a procedure that must run at the exact position. See the manual of your robot.

Procedure name

The robot procedure called when the robot reaches the waypoint: Attach, Detach or User 1 to User 3. The procedures are defined in the robot program, see the BPS Robot integration guide of your robot brand. With Approximative position tolerance, the procedure may be called during the move.

For the placing data that the robot receives with the trajectory, see Oriented placing.