Gripper mode

A driver can run a gripper instead of a joint: the same board, with a state machine that moves a jaw to a position between open and closed, at a speed, with a current limit that becomes the grip force, and that reports when it holds something. The MSG gripper is built on STEPFOC this way.

Turning it on

#Gripper 1
#Save

#Default clears the flag again. A driver in gripper mode still answers every other command; the gripper commands only work with the flag set.

Calibrating the gripper

After every power-up the gripper has to find its end stops: #Gripcal or Send_gripper_calibrate. The jaws open until they stop, then close until they stop, slowly and at 500 mA; the positions become 0 (open) and 255 (closed). The driver prints nothing when it is done. If it has not found both end stops within about 24 s it gives up, and the gripper stays uncalibrated. Check the result with #Gripinfo (Gripper calibrated is: 1) or the gripper_calibrated bit of Respond_Gripper_data_pack: a move is carried out only once the gripper is calibrated. The motor itself must be calibrated (#Cal) and saved before that, once.

While the gripper calibrates, send only empty status polls. A 5-byte Send_gripper_data_pack, Send_Clear_Error or #Clear stops the calibration without a message, and the next one then starts part way through, where the stopped one left off: switch the driver off and on before you calibrate again (known issue KI-7).

Moving

Over UART:

#Gripvel 100       speed, 0 to 255
#Gripcur 400       current limit of the move in mA, the grip force
#Grippos 200       go to position 200 (0 open, 255 closed); also activates the gripper

Over CAN, Send_gripper_data_pack carries position, speed and current in one frame with the activate and action bits; the driver answers with Respond_Gripper_data_pack: the position, the measured current, whether it is moving, and the object detection:

Object detection	Meaning
0	moving
1	an object was detected while closing: the jaws stopped on it and hold it with the current limit
2	an object was detected while opening
3	the position was reached

An empty Send_gripper_data_pack (0 bytes) only asks for the state. The answer to a 5-byte command goes out before the driver acts on it, so its object detection and error bits are still those of the moment before (KI-6); the answers to the next frames show the new move. The action bit in the answer is a copy of the last command's action bit and stays 1 after the move ends (KI-8): the object detection tells when it has ended.

Send_gripper_data_pack's E-stop bit raises the gripper E-stop error; #Gripstop 1 does the same over UART. The error stays: while the E-stop bit is 1 the driver raises it again on every control cycle, and #Error prints only In error state!. To recover, clear the bit (#Gripstop 0, or a Send_gripper_data_pack with the bit 0), then #Clear or Send_Clear_Error.

Current

The firmware does not limit the current you give a gripper move. The safe range depends on the gripper's mechanics and motor; the gripper's manual gives it. The driver is rated for 2.8 A, but nothing enforces that here: a gripper move uses the current as sent.

Where the details are

The bytes of the two gripper frames are in the CAN reference, the UART commands in the UART reference.

Comments

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