Baxter has seven rotary joints as shown in the following figure. Each arm is often referred to as a 7-DOF arm, since motion of the arm is controlled by seven actuators (motors) that are capable of independent rotation.
Baxter's 7-DOF arms are described on the Rethink Robotics site at http://sdk.rethinkrobotics.com/wiki/Arms.
Baxter's arm joints
The arm joints are named in the following manner:
- S0: Shoulder Roll
- S1: Shoulder Pitch
- E0: Elbow Roll
- E1: Elbow Pitch
- W0: Wrist Roll
- W1: Wrist Pitch
- W2: Wrist Roll
The designation of the joints as S0, S1, E0, E1, W0, W1, and W2 enables us to define and even monitor each of the angles for these joints with respect to the coordinates of the joints. The angles are measured in ROS in radians. As there are 2π radians in a complete circle, one radian is 360/(2π) or about 57.3 degrees. A 90-degree angle is π/4 or about 0.7854 radians. These conversion values are given because it is often required to move the joints to a 90-degree position or other angle defined in radians and it is usual for us to think in terms of degrees of rotation.
The joints of the arms are connected by links of various lengths. Although all the joints are rotary joints, there is a distinction between bend joints and twist joints. The bend joints, also called pitch joints, are S1, E1, and W1. They pitch up and down on the arm and rotate about their axis perpendicular to the joint. The twist or roll joints S0, E0, W0, and W2 rotate about an axis that extends along their centerline.
Information on maximum joint speeds, joint flexure stiffness, peak torque, and other detailed arm specifications can be found at: http://sdk.rethinkrobotics.com/wiki/Hardware_Specifications.
Three of the arm joints, S1, E1, and W1, are defined as pitch joints and are labeled in the following figure for positive and negative direction:
Baxter's pitch joints
The following table shows Baxter's pitch joint limits and range of motion measured in degrees and radians:
|
Joint |
Min limit |
Max limit |
Range |
Min limit |
Max limit |
Range |
|---|---|---|---|---|---|---|
|
in degrees |
in radians | |||||
|
S1 |
-123 |
+60 |
183 |
-2.147 |
+1.047 |
3.194 |
|
E1 |
-3 |
+150 |
153 |
-0.052 |
+2.618 |
2.67 |
|
W1 |
-90 |
+120 |
210 |
-1.571 |
+2.094 |
3.665 |
Four of Baxter's arm joints, S0, E0, W0, and W2, are defined as roll joints and are labeled in the following figure for positive and negative direction.
Baxter's roll joints
The following table shows Baxter's roll joint limits and range of motion measured in degrees and radians:
|
Joint |
Min limit |
Max limit |
Range |
Min limit |
Max limit |
Range |
|---|---|---|---|---|---|---|
|
in degrees |
in radians | |||||
|
E0 |
-173.5 |
+173.5 |
347 |
-3.028 |
+3.028 |
6.056 |
|
S0 |
-141 |
+51 |
192 |
-2.461 |
+0.890 |
3.351 |
|
W0 |
-175.25 |
+175.25 |
350.5 |
-3.059 |
+3.059 |
6.118 |
|
W2 |
-175.25 |
+175.25 |
350.5 |
-3.059 |
+3.059 |
6.118 |
Before we can discuss the details of the arm positions and orientations, it is necessary to define a base coordinate system from which other positions are measured. The following figure shows Baxter's reference coordinate system:
Baxter's base coordinate system
Standing behind Baxter, the positive x axis is forward along the centerline of Baxter, the positive y axis is to the left from the center line, and the positive z axis is up vertically. The z axis center of Baxter's base coordinate system is at the base of Baxter's torso. This is the z=0 position. The position x=0, y=0 is back from Baxter's front plate, along the center line along the vertical axis.
An important use of this base coordinate system is to define the position of the grippers at the end of Baxter's arms in terms of the distance in x, y, and z from the base origin considered (0, 0, 0). This is useful since Baxter on the pedestal has a base coordinate system that does not move during operations. The coordinates of each joint and the gripper will change as Baxter performs various tasks.
Cartesian positions are defined in meters in ROS as defined in REP-103, titled Standard Units of Measure and Coordinate Conventions (http://www.ros.org/reps/rep-0103.html).
Another measure for any three-dimensional object is its orientation or angles with respect to a given coordinate system, usually a coordinate system centered at the object itself. We will discuss the orientation of Baxter's grippers when the transformation of coordinate systems, tf, is introduced.
Baxter's outstretched arms in the previous figure represent the joint angles of zero degrees for all the joints. The various conventions for measuring the distance and rotation of Baxter's grippers will be presented later.
There are four modes of controlling Baxter's arms: joint position, joint velocity, joint torque, and (the most recent mode) raw joint position control. Please note the descriptions and the important differences between these four joint control modes:
- Joint position control: This mode is the most fundamental control mode and is the primary method for controlling Baxter's arms. The angle of each of Baxter's joints is specified in the message to the motor controllers. This message contains seven values, one value for each of the seven joints. The motor controller processes the message, checking for collisions in the URDF model between the arms and also with the torso. If a potential collision is detected, the collision-avoidance model plans offsets to the commanded path to avoid the impact.
- Joint velocity control: This mode is for the advanced control of Baxter's arms. Joint velocities are specified for the joints to simultaneously achieve. The joint command message will contain seven velocity values for the controllers to achieve. Collision avoidance and detection is applied. If the commanded velocity takes the joint to a position outside its limits, no joints will move. This mode is dangerous and its use is not advised.
- Joint torque control: This mode is also for the advanced control of Baxter's arms. Joint torques are specified for the joints to simultaneously achieve. The joint command message will contain seven torque values for the controllers to achieve. Collision avoidance and detection is not applied. This mode is dangerous and its use is not advised.
- Raw joint position control: This mode provides a more direct position control, leaving the execution of the joint commands primarily for the controllers. Collision avoidance and detection is not implemented and motor velocity limits are not monitored. This mode is dangerous and its use is not advised.
For an in-depth description of the joint control modes, refer to Rethink's wiki at http://sdk.rethinkrobotics.com/wiki/Arm_Control_Modes.
Rethink provides two options of grippers for Baxter: electric gripper and suction gripper. The electric gripper, as shown on the left in the following figure, has two fingers with removable inserts to allow different configurations of the gripping surface. Force control of the grippers allows them to pick up rigid and semi-rigid objects. The electric grippers can grasp an object from the inside or the outside. The gripper can open fully to 144 mm (approximately 5.6 inches) to grasp an object, though the fingers have various configurations within this grasping range.
The suction gripper supports the attachment of a single vacuum cup or a multi-cup vacuum manifold. The figure shows a single suction gripper (on the right). The gripper is powered by an external air supply line. This gripper works well on smooth, non-porous, or flat objects:
Baxter's electric and suction grippers
Each of Baxter's arms has a number of sensors on the cuff at the end of the arm. An integrated camera is mounted on the cuff and pointed towards objects that the gripper could potentially pick up. The camera has a frame rate of 30 frames per second and a maximum resolution of 1280 x 800 pixels. An Infrared sensor pointed in the same direction can detect distances from 4 to 40 cm (1.5–15 inches). The following figure shows the position of the cuff camera and the Infrared sensor. Each cuff also contains a 3-axis accelerometer. Additional information on these sensors can be found at the following websites:
- http://sdk.rethinkrobotics.com/wiki/Hardware_Specifications
- https://github.com/RethinkRobotics/sdk-docs/wiki/API-Reference
Baxter's cuff camera and Infrared sensor