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Over CAN

CAN is the gripper's interface: the one cable carries it next to the power, and a robot's control board, a PC with the CANvas adapter or any microcontroller with a CAN transceiver drives the gripper with it. The gripper is a STEPFOC driver on the bus, so the frames are the driver's; this page is the part a gripper needs.

The frame

CAN 2.0 data frames with 11-bit identifiers at 1 Mbit/s. The identifier carries the node, the command and the error state:

Bits 10 to 7 of the identifier are the node ID, bits 6 to 1 the command ID, bit 0 the error bit
Figure 6.1: Bits 10 to 7 of the identifier are the node ID, bits 6 to 1 the command ID, bit 0 the error bit.
  • Node ID, bits 10 to 7: which device. Every gripper ships as node 0; set another with Send_CAN_ID or #CANID and save it. Use 0 to 13: the driver's bootloader reserves 14 and 15, and a device on one of those cannot be updated over CAN.
  • Command ID, bits 6 to 1: which message. The gripper command is 61, its answer 60, the calibration 62.
  • Error bit, bit 0: in a frame from the gripper, 1 while any error is active. In a frame from the host it is ignored.

So the gripper command to node 0 travels on identifier (0 << 7) | (61 << 1) = 0x7A, and the answer comes back on (60 << 1) | error = 0x78 or 0x79; to node 3 the command is 0x1FA and the answer 0x1F8. Multi-byte values are big-endian; flags inside a byte are numbered from the most significant bit.

The messages a gripper needs

Message ID What for
Send_gripper_data_pack 61 the move command: position, speed, current, the activate, action and E-stop bits; 0 bytes to poll
Respond_Gripper_data_pack 60 the answer: position, current, the state and error bits
Send_gripper_calibrate 62 the gripper calibration, after every power-up
Send_Clear_Error 1 clears the errors
Send_ESTOP, Send_Idle 0, 12 stop
Send_Watchdog 15 the gripper stops when the host stops talking
Send_CAN_ID, Send_Save_config 11, 13 another node ID, saved
Send_Respond_State_of_Errors, Send_Respond_Temperature 26, 23 the driver's error flags, the motor temperature

The CAN reference has the bytes of each; the driver's other messages, which a gripper does not need, are on the STEPFOC site.

A control loop

  1. At power-up: Send_Clear_Error, then Send_gripper_calibrate; poll with empty Send_gripper_data_pack frames until the calibrated bit is 1 (up to about ten seconds; the routine gives up after 24 s). Send nothing else meanwhile.
  2. For each grip: one Send_gripper_data_pack with the position, speed and current and the activate and action bits set. Its answer still carries the state from before it; ignore it. Then poll with empty frames until the object-detection field reads 1, 2 or 3, or your own timeout passes.
  3. Keep polling at a steady rate (50 Hz is plenty) if you set the watchdog: every frame resets it, and the gripper stops by itself when your program stops.

The answer to every frame tells the whole state, so a host never has to ask twice.

Host checklist

  • Clear, then calibrate. With an error active, the driver drops the calibration request; a clear sent during the calibration ends it.
  • During the calibration, only empty frames. A full Send_gripper_data_pack ends the calibration as well, and the gripper stays uncalibrated. Wait for the calibrated bit; the routine gives up after 24 s.
  • The first answer is old. The answer to a full command is sent before the gripper acts on it. Ignore it, and give the move about 100 ms before you trust the object detection: at the start of a move the jaws are slow, which can briefly read as an object found.
  • The action status repeats your bit. It is the action bit you last sent and stays 1 while the gripper holds. Follow a move by its object detection.
  • An identical command does nothing. To repeat a move to the position the gripper holds, change the speed or the current by one.
  • E-stop recovery: first a command with the E-stop bit at 0 (and the action bit at 0), then Send_Clear_Error. While the last command carries the E-stop bit, a clear does not hold.
  • Watchdog: with Send_Watchdog set, keep the polls coming. The gripper commands and the status requests reset it; Send_Clear_Error and Send_gripper_calibrate do not. After a timeout, fix the host, clear, and resume.
  • Current: keep the commanded current within the stepper's maximum, 1.5, 2.0 or 2.1 A; the firmware does not clamp it (Force and speed).

From a PC

The CANvas adapter shows up as a serial port and speaks slcan; python-can opens it with bustype='slcan', and the Python library wraps the messages. Any CAN adapter that python-can supports works the same way.

On a bus with other devices

A gripper on a robot shares the bus with the robot's joints or other grippers. Give each device its own node ID (connect one at a time, Send_CAN_ID, Send_Save_config), terminate both ends of the bus (CAN termination), and keep in mind that a lower node ID wins the bus arbitration.

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