Calibration and Configuration of Cobra Robots

Calibration and Configuration of AdeptCobra Robots (600/800)

Calibration Basics

In order for the system to know where the robot is positioned, the robot must be calibrated when the system power is turned on. The encoders used in the AdeptCobra robots are incremental encoders. They report changes in position rather than absolute position. Calibration involves locating an absolute reference point, a hard stop or calibration sensor, and a zero-index signal on the encoder, from which all subsequent motion can be referenced.

Motor Commutation

Prior to moving the robot, the position of the rotor must be determined. This is done through the process of commutation. Voltage is applied to one phase (A, B, or C) of the motor. When voltage is applied to the stator, the rotor aligns with that particular phase. During this process, the change in encoder position is read to determine the direction of travel. Voltage is then applied to a different phase and the rotor moves, the encoder position is again read to verify direction of travel. The process of commutating the motor sometimes causes a “jerking” motion. This motion occurs under normal conditions and it is more noticeable on Joint 1 and 2.

Calibration for Joints 1 and 2

The status of the home (calibration) sensor is examined to determine search direction. If the home sensor is off, the motor drives the target toward the sensor (joint 1) and it drives the sensor toward the target (joint 2). If the sensor is on, the motor drives the target in the opposite direction. The target is located near the hard stop, because the joint can only be driven in one direction to find the home sensor target.
Next, the joint backs off the home sensor target to search for the zero-index. The zero-index is a mark on the encoder from which the motor’s position, and ultimately the robot’s position, is defined. The zero-index is used to eliminate any positional uncertainty the calibration sensor may cause due to hysteresis.
The joint is then moved to a final park position. Steps 1 - 3 are repeated for the next joint to be calibrated.

Joint Calibration Order

AdeptCobra robots are designed to commutate and calibrate in a specific sequence. When the CALIBRATE command is issued with High Power enabled, the following tasks are performed:

Joints 1, 2 and 4 commutate.
Joint 3 commutates and calibrates.
Joints 2 and 4 calibrate.
Joint 3 calibrates. (the second calibration is required after Joint 4 calibrattion).
Joint 1 calibrates

Restricting the Robot Working Envelope

Often times, it is necessary to restrict the space within which a robot can operate (staying within the constraints of cell space, for instance). This area is referred to as the robot's working envelope. To define the working envelope, you must set joint motion to fall within the boundaries of the desired working envelope. This is particularly important for joints 1 (shoulder) and 2 (elbow) of the AdeptCobra Robots.

The joint 1 and joint 2 calibration targets are 180° steel sectors that mount to the inner link joint 1 and joint 2 axes. The calibration sensors are inductive proximity sensors mounted inside the joint 1 harness support bracket for joint 1 and externally on the outer link for joint 2. The targets are oriented so that only one edge is detected by the sensor throughout the entire range of joint motion. The factory default orientation places the target edge detection position approximately at joint 0° location.

The targets can be repositioned or removed from the robot and modified (target area can be increased or decreased). The sector angle can be reduced from 180° to as small as 30°. When the target is reinstalled, the non-detected target edge must be oriented in the same location, but the detected target edge can be oriented on either side of the joint 0° position. For example, if the target is modified to a 150° sector, the target can be reinstalled on the robot with a target edge position of +30° or -30°. Targets can also be repositioned to change joint motion limits. In order to change the position of the joint 1 sensor target, you must be able to access the calibration target screws under the Upper J1 Cover (see left figure below). In order to change the position of the joint 2 sensor target, you must have access to the target mounting screws underneath the robot's inner link (see right figure below).

Joint motion limits are initially established in the robot's specifications setup using the SPEC.V2 utility. Locations (park positions) can be modified using the CFG_COBRA utility. This utility provides you with the following options:

Change the J1 sensor target location
Change the J2 sensor target location
Change park position after calibration
Change the J1, J2 calibration order

Changing the J1 Sensor Target Location

Step 1 - On the Adept Cobra Configuration Main Menu, select option 1 => Change J1 sensor target location

Do you want to move to the + hardstop (Y/N)? Y

Do you want to abort (Y/N)? N

Step 2 - Modify the Joint 1 calibration target:

Use a 4mm hex wrench to remove the four large socket head screws holding the J1 harness support bracket in place. Use a No. 1 Phillips (cross slot) screw driver to remove the two small screws.

Step 3 - Lift the J1 harness support bracket to expose the J1 sensor area.

Note - Do not remove the sheet metal bracket to which the J1 sensor is mounted

Step 4 - Rotate the inner link so that the screws in the J1 calibration target align with the access holes in the J1 sensor mount bracket. Remove the four 2.5mm cap screws holding the J1 calibration target to the inner link.

Step 5 - Modify the target as desired and replace all screws.

Step 6 - Press ENTER to continue.

Step 7 - Verify that the motion of J1 is unobstructed. Press ENTER to continue. At this point, you receive and indication that the Home sensor is ON. The robot returns to start position and the utility searches for target edge.

Step 8 - When prompted, Is Joint 1 at the correct target edge (Y/N)? Enter Y.

Step 9 - When prompted, Search entire joint range for extraneous target edges (Y/N) ? Enter Y. At this point, the utility checks for joint interference by searching for extraneous target edges.

Step 10 - When you receive the message, Found 1 sensor edge as expected. Press ENTER to continue. The unit returns to the start location.

Step 11 - When prompted, Preparing to update the RSC. Do you want to abort (Y/N)? Enter N.

Step 12 - When prompted, Recalibrate robot to verify change (Y/N)? Enter Y.

Step 13 - When prompted, Move arm straight out (Y/N)? Enter Y.

Changing the J2 Sensor Target Location

Step 1 - On the Adept Cobra Configuration Main Menu, select option 2 => Change J2 sensor target location

Do you want to move to the + hardstop (Y/N)? Y

Do you want to abort (Y/N)? N

Step 2 - Modify the Joint 1 calibration target:

The J2 sensor and J2 sensor target are located at the intersection of the robot's inner link and outer link.

Step 3 - From underneath the inner link, remove the four small socket-head screws that hold the J2 calibration target in place. Modify the target as desired and replace screws.

Step 4 - Press ENTER to continue.

Step 5 - Verify that the motion of J2 is unobstructed. Press ENTER to continue. At this point, you receive and indication that the Home sensor is OFF. The robot returns to start position and the utility searches for target edge.

Step 6 - When prompted, Is Joint 2 at the correct target edge (Y/N)? Enter Y.

Step 7 - When prompted, Search entire joint range for extraneous target edges (Y/N) ? Enter Y. At this point, the utility checks for joint interference by searching for extraneous target edges.

Step 8 - When you receive the message, Found 1 sensor edge as expected. Press ENTER to continue. The unit returns to the start location.

Step 9 - When prompted, Preparing to update the RSC. Do you want to abort (Y/N)? Enter N.

Step 10 - When prompted, Recalibrate robot to verify change (Y/N)? Enter Y.

Step 11 - When prompted, Move arm straight out (Y/N)? Enter Y.