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F/T Sensors

The F/T sensor (6-dof) has also been specially designed to fit the iCub. However, the size of the sensor has been made compatible with an existing commercial product. On the other hand, the signal conditioning electronics has been made to fit the sensor itself, consequently reducing the space required. The F/T sensor is based on a classical Wheatstone bridge design employing 12 semiconductor strain gauges arranged in a 6 half-bridges configuration.


Mechanical specifications

Physical specifications


The physical specifications of the sensor are reported in Table 1:

Weight 0.122[kg]
Diameter 45[mm]
Height 18.4[mm]
Table 1: Physical specifications of the sensor

Measurement frame specifications



The F/T sensor reference frame

Please notice that the key elements to localize the reference frame on the sensor are two: 1. the hole where the CAN exits the sensor; 2. the thick VS the thin sensor cover.

The sensor is calibrated to measure the Force/Torque applied by the upper (blue) part of the sensor on the lower (red) part and express it on the F/T sensor reference frame.

Calibration specifications


The sensors are calibrated in order to obtain high resolution in typical operating regions. Typical values of the range and resolution for a sensor after the calibration procedure are reported in Table 2. The resolution is typical for most applications and can be improved with filtering. Resolutions quoted are the effective resolution after dropping three counts of noise.

Fx, Fy [N] Fz [N] Tx, Ty [Nm] Tz [Nm]
Range 1500 2000 35 25
Resolution 0.25 0.25 0.005 0.004
Table 2: typical values after sensor calibration

Creo CAD model


The CAD model of FTSens is available in the hardware repository of the icub-tech-iit organization on GitHub.

In particular, the FT sensor assembly name is ic_008_a_001.asm.1.

Additional info


For more info on how to flash the firmware and the calibration matrix on a FT sensor, check the documentation page on Firmware.

Electronics


There are three versions of the electronic board of the iCub's FT sensor: STRAIN (also known as STRAIN1), STRAIN1.5 or STRAIN2. All these electronic boards sample the analog strain gauges voltages, and expose the measurements over a CAN interface.

Low-level documentation and materials about the STRAIN2 board are available here.

Firmware source code:

FT45 and FT58 Assembly Variants (Mechanics + Electronics)


IITCODE ALIAS REV Description Note
12487 12487 (da cambiare) - FT45 M4 cod.12191 - IC_008_A_001 +STRAIN2 cod.11996.B + Wiring materials cod. 12478.0.0 Strain-gauge code missing
12491 FT45_M1_E2 - FT45 M1 cod. ? - STRAIN2 cod.11996.B + Wiring materials cod.12478.0.0 Mech. assembly (with strain-gauge code) missing
12031 FT45_M1_E1.5 - iCub, assembly - Force/Torque sensor (5x gain), with mechanical parts, strain-gauge, strain board and cabling
12621 FT58_M2_E2 - FT58 M2 cod.12176.A1 - IC_005_A_001 + STRAIN2 cod.11996.B + Wiring materials cod.12478.0.0

FT45 Mechanical variants


FTSENSOR45A IC_001_A_001_ftsensor45 IC_007_A_001 IC_008_A_001
informal name FT45 FT45 v.2 (a.ka. Diego's sensor) - -
variant id FT45_M1 FT45_M2 FT45_M3 FT45_M4
first mechanical structure YES NO YES NO
second mechanical structure * NO YES NO YES
central through hole ** NO NO YES YES
obsolete YES YES YES NO

* no countersunk screw, with pin assembly

** requires STRAIN2 board

FT58 variants


ICUB3_005_A_001_ftsensor58 IC_005_A_001
informal name FT58_M1 FT58_M2
variant id FT58_M1 FT58_M2
central through hole ** NO YES

** requires STRAIN2 board

FT58 assembly


The FT58 sensor is assembled as described hereafter.

The assembly of the sensor is a 6 step process:

  1. the top part, the sensor part and the bottom part are screwed together
  2. the pin holes are enalrged with the proper bore tool
  3. the sensor part is gaged
  4. the sensor is assembled and wired with the electronic board
  5. the three sensor parts are screwed together: the sensor is closed
  6. if needed pins are added with the proper strain relieving tool

For further details on the assembly refer to the component drawing in the repository folder. The drawing code is depends on the sensor to be assebled (refer to the table in the above section for the code).

The top interface is fixed with 6 M4x10 UNI 5931 counterbore hex. screws (cod. V4-10--_-_U5931_GC). The recommended tightening torque is 2.8Nm.


The bottom interface is fixed with 6 M4x10 UNI 5931 counterbore hex. screws (cod. V4-10--_-_U5931_GC). The recommended tightening torque is 2.8Nm.


FT58 assembly on iCub 3


The FT58 sensor is assembled on iCub 3 as represented in th following pictures. The top interface is fixed with 6 M4x12 UNI 5933 countersunk hex. screws (cod. V4-12--_-_U5933_C). The recommended tightening torque is 2Nm.


The bottom interface is fixed with 6 M4x12 UNI 5933 countersunk hex. screws (cod. V4-12--_-_U5933_C). The recommended tightening torque is 2Nm.


FT Sensor Electronics Variants


There are three cards for reading six-axis F/T Sensors:

Name IIT Code and .Revision Description Status Compatible with Mechanical variant 
STRAIN1  832.B STRAIN_01F, IIT - Electronic board, 6-axial strain gauges amplification card with enhanced noise capability end of life FT45_M1,FT45_M2,FT58_M1
STRAIN1.5 832.F STRAIN_01F, IIT - Electronic board, 6-axial strain gauges amplification card with enhanced noise capability, 5X gain end of life  FT45_M1,FT45_M2,FT58_M1
STRAIN2 11996.B STRAIN2, IIT - Electronic board, 6 channels strain gauges variable gain interface board with CAN/UART, Temperature sensor, IMU, STM32L4 in production

FT45_M1*,FT45_M2*,FT58_M1*

FT45_M3**,FT45_M4**,FT58_M2**

* if breaking the legs

** if removing the central ring


Cable connections


STRAIN


STRAIN2


Summarized usable codes


short description Asm. cod. Wingst Rev. Dwg. Micron Istrumnents code status
FTSensor 45 - Strain 1 (mech 1) FTSENSOR45A 3407 A2 150923 only maintenance
FTSensor 45 - Strain 1 (mech 2) IC_001_A_001_ftsensor45 not present 0 150923 OBSOLETE
FTSensor 45 - Strain 2 (mech 3) IC_007_A_001 12183 A2 151245 OBSOLETE
FTSensor 45 - Strain 2 (mech 4) IC_008_A_001 12191 A1 151245 ACTIVE
FTSensor 58 - Strain 1  ICUB3_005_A_001_ftsensor58 10308 A1 151351 OBSOLETE
FTSensor 58 - Strain 2  IC_005_A_001 12176 A1 151351 ACTIVE

More information