Introduction

Advancements in technology over recent years have made tactile sensing one of the key features enabling more complex operations and tasks in industrial and commercial robotic systems. These tasks often involve manipulating and grasping objects within various applications. Tactile sensing technology has also been introduced in robot grippers, allowing them to estimate the properties of objects being gripped. It is important for a robot to have a strong sense of interaction forces and contact points to efficiently cooperate with tasks, without damaging the objects or causing accidents. This crucial information can be gathered through direct measurements, mimicking human touch senses. This is achieved by using tactile sensors in combination with other independent sensors. In this article, we will explore the concept of tactile sensors, their types, working principles, circuit diagrams, applications in robotics, and the benefits they offer.

The Tactile Sensor Definition

A tactile sensor is a measurement and sensing device designed to collect and process information as it interacts with its surroundings through touch. These sensors aim to replicate the human sense of touch.

Figure 1: Robot Gripper with Tactile Sensors (Courtesy of Pixabay)

The tactile sensor is designed to simulate the kinesthetic and cutaneous touch senses in humans. The cutaneous sense detects stimuli such as pain, temperature, and mechanical pressure, while the kinesthetic sense receives input from joints, tendons, and muscle receptors.

Tactile Sensor Working and Circuit Diagram

The development and implementation of tactile sensors aim to provide touch sensing capabilities for intelligent robots and teleoperational manipulators. These sensors operate based on the normal force applied to tactile pixels, which measure the control force and generate tactile images to recognize objects. Generating accurate data to detect and identify objects is a complex process, requiring three-dimensional techniques. Below is the circuit diagram for a three-dimensional tactile sensor.

Figure 2: Tactile Sensor Circuit Diagram (Courtesy of Simon Mugo)

 

 

Tactile Sensor Types

Below is an overview of some common types of tactile sensors:

Torque/ Force Sensor

Torque sensors, also known as force sensors, are often combined with tactile arrays to control force. These sensors detect the load in various applications, such as on the manipulator’s distal link or skin sensors. Skin sensors, in particular, provide accurate force measurements when operating at higher bandwidths. A force sensor, placed on the manipulator’s link, can provide vital information about the moments and locations of force during operations, a process known as intrinsic tactile sensing.

Figure 3: Touch Sensor Calibration Connection (Courtesy of MDPI)

Piezoelectric Tactile Sensor

The piezoelectric effect occurs when electricity is generated across a sensing element when subjected to pressure. The voltage generated is proportional to the applied pressure. Piezoelectric tactile sensors are durable and operate effectively within a wide dynamic range, making them ideal for pressure measurement applications.

Figure 4: Piezoelectric Tactile Sensor (Courtesy of Simon Mugo)

Magnetic Tactile Sensor

Magnetic tactile sensors operate in two main ways:

• By measuring changes in flux using magneto-resistive materials.
• By detecting changes in magneto-coupling resulting from the deformation of magnetoelastic core windings.

This type of sensor is known for its high sensitivity and absence of mechanical hysteresis.

Resistive Tactile Sensor

Resistive tactile sensors function by detecting changes in electrical resistance between electrodes and a polymer conductor. These sensors are widely used and are known for their durability and high tolerance to overload. When pressure is applied, the resistance of the conductive material changes, which is then measured by the sensor.

Figure 5: Circuit of a Resistive Tactile Sensor (Courtesy of Simon Mugo)

Optical Tactile Sensor

Optical tactile sensors are classified into extrinsic and intrinsic types. These sensors rely on the principle of light intensity modulation, where an obstacle either blocks or unblocks the light path. They offer high resolution and immunity to electromagnetic interference, with the added benefit of requiring minimal wiring. Some optical tactile sensors can also be operated remotely.

Figure 6: Optical Tactile Sensor Incorporated with Pneumatic Actuation (Courtesy of Simon Mugo)

Capacitive Tactile Sensor

Capacitive tactile sensors work on the principle of measuring changes in capacitance between two capacitor electrodes. The capacitance varies depending on the pressure applied to the capacitor, which in turn affects the load. These sensors offer a wider dynamic range and a more linear response.

Figure 7: Capacitive Tactile Sensor Simple Diagram (Courtesy of Simon Mugo)

Tactile Sensors in Robotics

Tactile sensors play a significant role in robotics. One notable example is their use in NASA’s robotics for planetary exploration. These sensors are crucial in NASA’s International Space Station to assist astronauts with repairs in the challenging conditions of space.

Applications of Tactile Sensors

Tactile sensors are widely used in:

• Computer hardware
• Robotics
• Touchscreen devices such as mobile phones, tablets, and more

Advantages of Tactile Sensors

• Provide efficient tactile solutions
• Compact design
• Do not require external wiring or components, making them cost-effective and compact

Simulating a Tactile Sensor Using the Proteus EDA Tool

To test a simple tactile sensor, we will use the Proteus EDA tool to simulate its behavior.

Components Required

Touchpad

BC547 Transistor

Resistors: 10k, 100k, and 330Ω

555 Timer

Capacitors: 10nF and 10µF

LED

The Simple Circuit