calibrate_hand_eye — Perform a hand-eye calibration.
calibrate_hand_eye( : : CalibDataID : Errors)
The operator calibrate_hand_eye determines the 3D pose of a robot
(“hand”) relative to a camera or 3D sensor (“eye”) based on the
calibration data model CalibDataID. With the determined 3D poses,
the poses of the calibration object in the camera coordinate system can be
transformed into the coordinate system of the robot which can then, e.g.,
grasp an inspected part. There are two possible configurations of
robot-camera (hand-eye) systems:
The camera can be mounted on the robot or be stationary and observe the
robot. Note that the term robot is used in place of a mechanism that moves
objects. Thus, you can use calibrate_hand_eye to calibrate many
different systems, from pan-tilt heads to multi-axis manipulators.
In essence, systems suitable for hand-eye calibration are described by a closed chain of four Euclidean transformations. In this chain two non-consecutive transformations are either known from the robot controller or computed from camera data, e.g., calibration object poses observed by a camera. The two unknown constant transformations are computed by the hand-eye calibration procedure.
A hand-eye calibration is performed similarly to the calibration of the
external camera parameters (see Calibration): You
acquire a set of poses of a calibration object in the camera coordinate
system, and a corresponding set of poses of the tool in robot base
coordinates and set them in the calibration data model CalibDataID.
In contrast to the camera calibration, the calibration object is not moved
manually. This task is delegated to the robot. Basically, two hand-eye
calibration scenarios can be distinguished. A robot either moves the camera
(moving camera) or it moves the calibration object (stationary camera).
The robot's movements are assumed to be known. They are used
as an input for the hand-eye calibration and are set in the calibration
data model CalibDataID using set_calib_data.
The results of a hand-eye calibration are two poses: For the moving camera scenario, the 3D pose of the tool in the camera coordinate system ('tool_in_cam_pose') and the 3D pose of the calibration object in the robot base coordinate system ('obj_in_base_pose') are computed. For the stationary camera scenario, the 3D pose of the robot base in the camera coordinate system ('base_in_cam_pose') and the 3D pose of the calibration object in the tool coordinate system ('obj_in_tool_pose') are computed. Their pose type is identical to the pose type of the input poses. If the input poses have different pose types, poses of type 0 are returned.
The two hand-eye calibration scenarios are discussed in more detail below, followed by general information about the data for and the preparation of the calibration data model.
In this configuration, the calibration object remains stationary. The camera is mounted on the robot and is moved to different positions by the robot. The main idea behind the hand-eye calibration is that the information extracted from an observation of the calibration object, i.e., the pose of the calibration object relative to the camera, can be seen as a chain of poses or homogeneous transformation matrices from the calibration object via the base of the robot to its tool (end-effector) and finally to the camera:
Moving camera: camera_H_cal = camera_H_tool * (base_H_tool)^(-1) * base_H_cal | | | | 'obj_in_cam_pose' 'tool_in_cam_pose' 'tool_in_base_pose' 'obj_in_base_pose'
From the set of calibration object poses ('obj_in_cam_pose') and
the poses of the tool in the robot base coordinate system
('tool_in_base_pose'), the operator calibrate_hand_eye
determines the two missing transformations at the ends of the chain, i.e.,
the pose of the robot tool in the camera coordinate system
(,
'tool_in_cam_pose') and the pose of the calibration object in
the robot base coordinate system
(, 'obj_in_base_pose').
These two poses are constant.
In contrast, the transformation in the middle of the chain, , is known but changes for each observation of the calibration object, because it describes the pose of the tool with respect to the robot base coordinate system. In the equation the inverted transformation matrix is used. The inversion is performed internally.
Note that when calibrating SCARA robots, it is not possible to determine the Z translation of 'obj_in_base_pose'. To eliminate this ambiguity the Z translation 'obj_in_base_pose' is internally set to 0.0 and the 'tool_in_cam_pose' is calculated accordingly. It is necessary to determine the true translation in Z after the calibration by moving the robot to a pose of known height in the camera coordinate system. For this, the following approach can be applied: The calibration plate is placed at an arbitrary position. The robot is then moved such that the camera can observe the calibration plate. Now, an image of the calibration plate is acquired and the current robot pose is queried (ToolInBasePose1). From the image, the pose of the calibration plate in the camera coordinate system can be determined (ObjInCamPose1). Afterwards, the tool of the robot is manually moved to the origin of the calibration plate and the robot pose is queried again (ToolInBasePose2). These three poses and the result of the calibration (ToolInCamPose) can be used to fix the Z ambiguity by using the following lines of code:In this configuration, the robot grasps the calibration object and moves it in front of the camera. Again, the information extracted from an observation of the calibration object, i.e., the pose of the calibration object in the camera coordinate system (e.g., the external camera parameters), are equal to a chain of poses or homogeneous transformation matrices, this time from the calibration object via the robot's tool to its base and finally to the camera:
Stationary camera: camera_H_cal = camera_H_base * base_H_tool * tool_H_cal | | | | 'obj_in_cam_pose' 'base_in_cam_pose' 'tool_in_base_pose' 'obj_in_tool_pose'
Analogously to the configuration with a moving camera, the operator
calibrate_hand_eye determines the two transformations at the ends
of the chain, here the pose of the robot base coordinate system in camera
coordinates (,
'base_in_cam_pose') and the pose of the calibration object
relative to the robot tool (,
'obj_in_tool_pose').
The transformation in the middle of the chain, , describes the pose of the tool relative to the robot base coordinate system. The transformation describes the pose of the calibration object relative to the camera coordinate system.
Note that when calibrating SCARA robots, it is not possible to determine the Z translation of 'obj_in_tool_pose'. To eliminate this ambiguity the Z translation of 'obj_in_tool_pose' is internally set to 0.0 and the 'base_in_cam_pose' is calculated accordingly. It is necessary to determine the true translation in Z after the calibration by moving the robot to a pose of known height in the camera coordinate system. For this, the following approach can be applied: A calibration plate (that is not attached to the robot) is placed at an arbitrary position such that it can be observed by the camera. The pose of the calibration plate must then be determined in the camera coordinate system (ObjInCamPose). Afterwards the tool of the robot is manually moved to the origin of the calibration plate and the robot pose is queried (ToolInBasePose). The two poses and the result of the calibration (BaseInCamPose) can be used to fix the Z ambiguity by using the following lines of code:
pose_invert(BaseInCamPose, CamInBasePose)
|
pose_compose(CamInBasePose, ObjInCamPose, ObjInBasePose)
|
ZCorrection := ObjInBasePose[2]-ToolInBasePose[2]
|
set_origin_pose(BaseInCamPose, 0, 0, ZCorrection, BaseInCamPoseFinal)
|
Before calling calibrate_hand_eye, you must create and fill the
calibration data model with the following steps:
Create a calibration data model with the operator
create_calib_data, specifying the number of cameras in the setup
and the number of used calibration objects. Depending on your scenario,
CalibSetup has to be set to 'hand_eye_moving_camera',
'hand_eye_stationary_camera',
'hand_eye_scara_moving_camera', or
'hand_eye_scara_stationary_camera'. These four scenarios on the
one hand distinguish whether the camera or the calibration object is
moved by the robot and on the other hand distinguish whether an
articulated robot or a SCARA robot is calibrated. The arm of an
articulated robot has three rotary joints typically covering 6 degrees
of freedom (3 translations and 3 rotations). SCARA robots have two
parallel rotary joints and one parallel prismatic joint covering only
4 degrees of freedom (3 translations and 1 rotation). Loosely speaking,
an articulated robot is able to tilt its end effector while a SCARA robot
is not.
Specify the optimization method with the operator
set_calib_data. For the parameter
DataName='optimization_method', two options for
DataValue are available, DataValue='nonlinear'
and DataValue='linear'
(see paragraph 'Performing the actual hand-eye calibration').
Specify the poses of the calibration object
For each observation of the calibration object,
the 3D pose can be set directly using the operator
set_calib_data_observ_pose. This operator is
intended to be used with generic 3D sensors that observe the
calibration object.
The pose of the calibration object can also be estimated using
camera images. The calibration object has to be set in the
calibration data model CalibDataID with the operator
set_calib_data_calib_object. Initial camera parameters have
to be set with the operator set_calib_data_cam_param.
If a standard HALCON calibration plate is used, the operator
find_calib_object determines the pose of the
calibration plate relative to the camera and saves it in the
calibration data model CalibDataID.
The operator calibrate_hand_eye for articulated (i.e.,
non-SCARA) robots in this case calibrates the camera
before performing the hand-eye calibration. If the provided
camera parameters are already calibrated, the camera calibration
can be switched off by calling
set_calib_data(CalibDataID,'camera','general','excluded_settings','params').
In contrast, for SCARA robots calibrate_hand_eye always
assumes that the provided camera parameters are already calibrated.
Therefore, in this case the internal camera calibration
is never performed automatically prior to the hand-eye calibration.
This is because the internal camera parameters cannot
be calibrated reliably without significantly tilting the calibration
plate with respect to the camera. For hand-eye calibration,
the calibration plate is often approximately parallel to the
image plane. Therefore, for SCARA robots all camera poses are
approximately parallel. Therefore, the camera must be calibrated
beforehand by using a different set of calibration images.
Specify the poses of the tool in robot base coordinates.
For each pose of the calibration object in the camera coordinate system, the
corresponding pose of the tool in the robot base coordinate system has to be
set with the operator
set_calib_data(CalibDataID,'tool', PoseNumber, 'tool_in_base_pose', ToolInBasePose).
The operator calibrate_hand_eye can perform the calibration in two
different ways. In both cases, all provided calibration object poses in
camera coordinates and the corresponding poses of the tool in robot base
coordinates are used for the calibration. The method to be used is
specified with set_calib_data.
For the parameter combination
DataName='optimization_method' and
DataValue='linear', the calibration is performed using a
linear algorithm which is fast but in many practical situations not accurate
enough.
For the parameter
DataName='optimization_method' and
DataValue='nonlinear', the calibration is performed using
a non-linear algorithm, which results in the most accurately calibrated poses
and thus is the method of choice.
The operator calibrate_hand_eye returns the pose error of the
complete chain of transformations in Errors.
To be more precise, a tuple with four elements is returned, where the first
element is the root-mean-square error of the translational part, the second
element is the root-mean-square error of the rotational part, the third element
is the maximum translational error and the fourth element is the maximum
rotational error. Using these error measures, it can be determined,
whether the calibration was successful.
The Errors are returned in the same units in which the input
poses were given, i.e., the translational errors are typically given
in meters and the rotational errors are always given in degrees.
The poses that are computed with the operator calibrate_hand_eye
can be queried with get_calib_data.
For the moving camera scenario, the 3D pose of the tool in the camera
coordinate system ('tool_in_cam_pose') and the 3D pose of the
calibration object in the robot base coordinate system
('obj_in_base_pose') can be obtained. For the stationary camera
scenario, the 3D pose of the robot base in the camera coordinate system
('base_in_cam_pose') and the 3D pose of the calibration object
in the coordinate system of the tool ('obj_in_tool_pose') can
be obtained.
If the poses of the calibration object relative to a camera were computed
with find_calib_object, then for articulated (i.e., non-SCARA) robots
they are used in an internal camera
calibration step preceding the hand-eye calibration and are calibrated
as well. The calibrated 3D poses can be queried using get_calib_data
with the parameter ItemType='calib_obj_pose'. If the poses
of the calibration object were observed with a generic 3D sensor, they cannot
be calibrated and are set by set_calib_data_observ_pose. These raw
3D poses can be queried using get_calib_data_observ_pose. The
corresponding 3D poses of the tool in the coordinate system of the robot
base can be queried using get_calib_data.
The following conditions, especially if using a standard calibration plate, should be considered:
The position of the calibration object (moving camera: relative to the robot's base; stationary camera: relative to the robot's tool) and the position of the camera (moving camera: relative to the robot's tool; stationary camera: relative to the robot's base) must not be changed between the calibration poses.
Even though a lower limit of three calibration object poses is theoretically possible, it is recommended to acquire 10 or more poses, in which the pose of the camera or the robot hand are sufficiently different.
For articulated (i.e., non-SCARA) robots the amount of rotation between the calibration object poses is essential and should be at least 30 degrees or better 60 degrees. The rotations between the poses must exhibit at least two different axes of rotation. Very different orientations lead to more precise results of the hand-eye calibration. For SCARA robots there is only one axis of rotation. The amount of rotation between the images should also be large.
For cameras, the internal camera parameters must be constant during and after the calibration. Note that changes of the image size, the focal length, the aperture, or the focus cause a change of the internal camera parameters.
As mentioned, the camera must not be modified between the acquisition of the individual images. Please make sure that the focus is sufficient for the expected changes of the camera to calibration plate distance. Therefore, bright lighting conditions for the calibration plate are important, because then you can use smaller apertures, which result in a larger depth of focus.
We recommend to create the robot poses in a separate program and save them
in files using write_pose. In the calibration program you can then
import them and set them in the calibration data model CalibDataID.
Via the Cartesian interface of the robot, you can typically obtain the pose
of the tool in robot base coordinates in a notation that corresponds to the
pose representations with the codes 0 or 2 (OrderOfRotation =
'gba' or 'abg', see create_pose). In this case,
you can directly use the pose values obtained from the robot as input for
create_pose.
If the Cartesian interface of your robot describes the orientation in a
different way, e.g., with the representation ZYZ (), you can create the
corresponding homogeneous transformation matrix step by step using the
operators hom_mat3d_rotate and hom_mat3d_translate and then
convert the resulting matrix into a pose using hom_mat3d_to_pose.
The following example code creates a pose from the ZYZ representation
described above:
Please note that the hand-eye calibration only works if the poses of the tool in robot base coordinates are specified with high accuracy!
This operator modifies the state of the following input parameter:
During execution of this operator, access to the value of this parameter must be synchronized if it is used across multiple threads.
CalibDataID (input_control, state is modified) calib_data → (handle)
Handle of a calibration data model.
Errors (output_control) number-array → (real)
Average residual error of the optimization.
create_calib_data,
set_calib_data_cam_param,
set_calib_data_calib_object,
set_calib_data_observ_pose,
find_calib_object,
set_calib_data,
remove_calib_data,
remove_calib_data_observ
K. Daniilidis: “Hand-Eye Calibration Using Dual Quaternions”;
International Journal of Robotics Research, Vol. 18, No. 3,
pp. 286-298; 1999.
M. Ulrich, C. Steger: “Hand-Eye Calibration of SCARA Robots Using Dual
Quaternions”; Pattern Recognition and Image Analysis, Vol. 26, No. 1,
pp. 231-239; January 2016.
Calibration