MODULE M8 · 10.0 HOURS

Sensor Integration and Feedback Control

0% complete

LEARNING OBJECTIVES

Module objectives

  1. Understand the configuration principle of a voltage divider circuit using an FSR 402 sensor and a 10 kΩ resistor
  2. Master the ADC function of the OpenCR controller and its input range (0-3.3 V) constraints
  3. Acquire sensor data filtering and calibration techniques
  4. Implement feedback-based grasping control algorithms and practice robot hand contact force control

Sensor Integration and Contact Force Feedback

Precise grasping control of a robot hand begins with accurately measuring the force applied to the fingertips. The FSR 402 sensor is a Force-Sensing Resistor whose resistance value decreases as the applied pressure increases [S12]. To convert this into a voltage signal that a microcontroller can read, a voltage divider circuit is required.

1. Voltage Divider Circuit

Connect the FSR sensor and a 10 kΩ divider resistor in series and supply 3.3 V sensor power [B4, B5, B2]. The ADC pin is connected to the junction of the sensor and the resistor, and the output voltage $V_{out}$ is calculated as follows: For a 10 kΩ pull-down voltage divider, $V_{out}=V_{ref}\frac{R_{fixed}}{R_{FSR}+R_{fixed}}$ is used.

  • The OpenCR controller’s ADC has a 12-bit resolution, and the input range is limited to 0~3.3 V [B2]. Inputs outside this range can damage circuit components, so you must use only the specified sensor power rail (3.3 V) [B2].

2. Control Loop and Feedback

Measured force data is used as an input for PID control algorithms or adaptive control strategies [S3]. When the robot hand grasps an object, the tendon-driven motor (DYNAMIXEL XM430-W350-T) refers to sensor values and fine-tunes the torque until the target contact force is reached [B1, B4].

WORKED EXAMPLES

Worked examples

  1. Example 1: FSR output calculation. When $R_{FSR}$ is 5 kΩ and $R_{fixed}$ is 10 kΩ, given a 3.3 V input, $V_{out} = 3.3 \times (5 / (5 + 10)) = 1.1 V. This is normally within the ADC input range (0-3.3 V).
  2. Example 2: Grasping force calibration. If sensor values fluctuate due to noise, applying a simple moving average filter can reduce rapid variations in sensor readings and maintain stable grasping force.

LAB PROTOCOL

Fingertip FSR Sensor Circuit Configuration and Calibration

  1. 1

    Connect the OpenCR's 3.3 V sensor rail and GND to the breadboard.

  2. 2

    Configure a voltage divider circuit by connecting the FSR 402 and a 10 kΩ resistor in series [B4, B5].

  3. 3

    Connect the voltage divider junction to an OpenCR ADC pin [B2].

  4. 4

    Connect the PC to the OpenCR and run the test code to read sensor values.

  5. 5

    Record ADC values in no-load and target force-applied states to create a calibration table.

Safety check
  • Before applying power, you must verify with a multimeter that there is no short circuit between the 3.3 V rail and the 12 V actuator rail [B2].
  • Always wear safety glasses and do not place your hands within the robot hand's range of motion while it is powered.
  • If abnormal heat, odor, or smoke is detected, do not approach; evacuate after cutting the supply power to the 3 adapters at the pre-designated building distribution panel breaker or an authorized upstream master disconnect outside the hazardous zone. If there is no accessible upstream disconnection means outside the zone, do not energize the system. Torque release is not a substitute for power disconnection. Maintenance/access must only be performed after a planned shutdown, physical disconnection, and confirmation by de-energized measurement.
  • Before maintenance or sensor access, physically disconnect the 3 isolated power adapters and verify by measurement that the voltage in all branches is less than 1 V.

Lab deliverables

  • ADC sensor reading test result data
  • Sensor calibration table (ADC value vs. physical force)
  • Sensor data filtering implementation code

ASSIGNMENT

Grasping Force Feedback Control Algorithm Implementation

Deliverables

Rubric

  • Is the ADC data measured stably within the 0-3.3 V range?
  • Does the motor appropriately release or maintain torque when the sensor value reaches the target?
  • Does the torque release function work normally in software during an emergency?
  • Is the power disconnection verification procedure described in the report?

KNOWLEDGE CHECK

Knowledge check

1What must be strictly observed when inputting an FSR voltage divider signal to an OpenCR controller ADC pin?
2What is the relationship between the FSR sensor's resistance change and physical force?
3What is the safety state that must be verified after disconnecting power for maintenance or access while working on the robot hand prototype?

FIELD CHECK

Completion criteria

  • Passed ADC-based FSR value reading lab
  • Grasping force feedback control code reaches at least 90% of the target
  • Proof of compliance with all safety rules (physical power disconnection and voltage measurement)
  • Final results report submission

MODULE SOURCES

Module sources

  1. Intelligent soft robotic gripper for non-destructive grasping ...nature.com · paper
  2. FSR Model 402interlinkelectronics.com · datasheet
  3. 10K Ohm 5 Percent 1/4W Through-Hole Resistorsadafruit.com · technical_documentation