This ROS package provides robot-models and software for a versatile two-finger gripper using four digital-bus servos from Feetech Inc.
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Using four rotational joints, the gripper can execute parallel pinch grasps and tip-pinch grasps, but the fingers can also wrap around an object (e.g. a bottle) if needed.
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The fingertips can be easily exchanged. We provide URDF models for generic fixed fingers, finray-style flexible fingers, and adapters for mounting the camera-based high-res tactile DIGIT sensors.
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Both fingers are mounted on a pair of standard load-cells, providing measurements of grasping force and shearing force.
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Our Arduino firmware also supports multiple optional finger-mounted IMUs, which can be used to measure vibration, impact, and fingertip orientation.
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The SCS-20 servo already provides a lot of sensor inputs, including joint (finger) position, velocity, motor torque, motor voltage and current, and motor temperature.
The suggested electronics is based on an Arduino Nano 33 IOT microcontroller, which includes another IMU (STM LSM6DS3). The first serial interface (pins RX/TX) of the Nano 33 IOT is connected to a Feetech "TTLinker Mini" board, which in turn controls the SCS-20 servos.
Both GY-521 boards are connected to the first I2C-bus (pins A4/A5) and are powered from the 5V/VUSB pin of the Arduino, which requires to bridge the corresponding jumper on the underside of the Nano 33 IOT. The GY-521 includes its own 5V->3V3 regulator and data communication uses 3V3 levels, compatible with the Nano 33 IOT. Allocated I2C addresses are 0x68 and 0x69 for the GY-521 boards, and 0x6A for the on-board LSM6DS3.
The load-cells of both fingers are connected to a common HX-711 chip breakout board each. The HX-711 provides all required amplification and filtering circuits together with a 24-bit ADC running at roughly 80 Hz sample rate. The digital interface only requires two lines (CLK, Data) each, connected to the Arduino pins (D2,D3), (D4,D5), (D6,D7), and (D8,D9).
Communication with the host is via USB, and the whole electronics is bus-powered by the USB. Of course, the SCS-20 servos require its own power supply (e.g. 7.2V, 8A). For now, the 3V3 supply from the Arduino is used to power the load-cells, as the HX-711 is specified for 2.7..5.5V. Using a stabilitzed 5V supply might increase signal-to-noise ratio. but the 5V USB is too noisy for that, and another power supply would add to the hardware complexity.
The arduino transmits a (unique?) id that can be matched in udev to identify the serial device. doc/10-diana-gripper-udev.rules provides a rule file that can be copied to /etc/udev/rules.d/ to add a symlink /dev/diana_gripper pointing to the correct device.
Maybe try an active bus transceiver or at least a bus terminator, e.g. https://learn.adafruit.com/adafruit-ltc4311-i2c-extender-active-terminator
See ~/.arduino15/packages/arduino/hardware/samd/1.8.11/cores/arduino/wiring_private.c and below for the pin assignments and pin capabilities of your specific board. Then try to edit the source for the Wire library a bit: /homeL/hendrich/.arduino15/packages/arduino/hardware/samd/1.8.11/libraries/Wire
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void TwoWire::begin(void) {
//Master Mode
sercom->initMasterWIRE(TWI_CLOCK);
sercom->enableWIRE();
pinPeripheral(_uc_pinSDA, g_APinDescription[_uc_pinSDA].ulPinType);
pinPeripheral(_uc_pinSCL, g_APinDescription[_uc_pinSCL].ulPinType);
// FNH! I2C always dying on Arduino Nano 33 IOT... see cores/arduino/wiring_private.c
PORT->Group[g_APinDescription[_uc_pinSDA].ulPort].PINCFG[g_APinDescription[_uc_pinSDA].ulPin].reg |= (uint8_t) (PORT_PINCFG_DRVSTR);
PORT->Group[g_APinDescription[_uc_pinSCL].ulPort].PINCFG[g_APinDescription[_uc_pinSCL].ulPin].reg |= (uint8_t) (PORT_PINCFG_DRVSTR);
// FNH end
}
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The SCS commander utility scans the bus for devices and reports all detected servos, together with their protocol type. Once the initial scan is completed, the commander enters an interactive loop where the user can select the active servo and then execute read/write commands on the selected servo.
The SCS commander is not using ROS. The program is supposed to be used with a terminal application or the serial console from the Arduino IDE. All commands should be terminated by a \n character, either \f (linefeed) or \r (carriage-return) or both.
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S \n : select servo at address (decimal 3..253) for the next commands.
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i\n : print motor current (SCS) (may read -1: not implemented by servo)
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j\n : print motor current (SMS) as multiple of 0.01A
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l\n : print motor load (torque) (SCS), unknown units
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m\n : print motor load (torque) (SMS), unknown units
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p\n : print motor position (SCS), counts (0..1023)
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q\n : print motor position (SMS), counts (0..4095)
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t\n : print servo temperature (degrees celsius)
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v\n : print servo supply voltage (deci-volts)
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P \n: (SCS-only) move to given position (counts) using current speed and/or duration
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Q \n: (SMS-only) move to given position (counts) using current max-speed and max-acceleration
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S \n: set motion speed for next P/Q command
The hand microcontroller currently uses ASCII-based communication with the host, so that debug messages appear as plain text in the output stream, and motor values are also human-readable.
The following lists summarize the most common output messages and commands (input messages), but may be out-of-sync with the current release. Please check the source code for the actually implemented versions and their formatting.
<node_name>/joint_states (sensor_msgs/JointState) <node_name>/joint_errors (sensor_msgs/JointState: pos=pos vel=vel eff=pos-error) <node_name>/voltages (sensor_msgs/Joy: axes:raw values) <node_name>/temperatures (sensor_msgs/Joy: axes:raw values) <node_name>/currents <node_name>/forces_raw (sensor_msgs/Joy: axes: raw values) <node_name>/palm_imu_raw (sensor_msgs/Imu) <node_name>/left_finger_imu_raw (sensor_msgs/Imu) <node_name>/right_finger_imu_raw (sensor_msgs/Imu)
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E: servo position errors (current - goal)
E seq_number stamp pos err1 err2 err3 err4
published as joint_state message on topic ~joint_position_errors with = errors, = current servo positions, = current servo velocities
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F: "forces" current force measurementss in Ascii format
F seq_number stamp forces lc1 lc2 lc3 lc4
Latest raw values from the load-cells, as measured by the HX-711 A/D converters. No scaling or temperature compensation is applied to the values, which are published as joy messages on topic ~/forces_raw and zero-biased on ~/forces_cooked.
Exponential filtering with alpha=0.1 and 0.01 is also applied to the values to generate high-pass filtered versions on topics ~force_filtered_10 and ~forces_filtered_100
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I: "current" servo motor currents (in Feetech units)
I seq_number stamp amps i1 i2 i3 i4
published as joy messages on topic ~currents
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M: "moments" servo motor torques (in Feetech units)
M seq_number stamp trq m1 m2 m3 m4
Stored in internal variable and sent as part of next '''joint_state message'''. Also re-published as raw-values (servo counts) to the '''~torques''' topic.
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P: "positions" servo positions (in Feetech counts)
P seq_number stamp pos p1 p2 p3 p4
Published as joint_state on topic ~joint_states, where = incoming positions converted to radians, = joint velocity in radians/sec calculated from current and previous position message (numerical difference), or zero unless at least two messages have been received, = torques from pervious "M" message
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T: "temperatures" current servo temperaturs in degrees celsius
T seq_number stamp temp t1 t2 t3 t4 t5 t6 t7
published as joy messages on topic ~temperatures
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U: "voltages" current servo supply voltages (in deci-volts)
U seq_number stamp volts u1 u2 u3 u4 u5 u6 u7
published as joy messages on topic ~voltages
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V: "velocities" current servo velocities (in Feetech units)
V seq_number stamp vel v1 v2 v3 v4 v5 v6 v7
currently not used or re-published as ROS message