Robotic Arm Sensors

We will sort out what kind of perception is made by the robot arm and what kind of sensors are used for that purpose. The perception of the robotic arm can be divided into three main types. There are three main types of perception by the robot arm: perception of the object to be manipulated, perception of the objects around the robot arm, and perception of the situation inside the robot arm. The sensor used to perceive the object and its surroundings is called the “external sensor”, and the sensor used to perceive the internal situation is called the “internal sensor”.

Sensors to perceive the object of operation

Typical sensors used to perceive objects for manipulation are cameras, proximity sensors, and force sensors. Cameras are used to check the approximate position of parts and other objects on a conveyor belt. For example, if the object on the conveyor belt is relatively flat, such as an electronic board or nut, an ordinary camera (2D camera) can be used to determine the position of the object. On the other hand, if the object has a three-dimensional shape like a car frame, a three-dimensional camera (3D camera) that can perceive the depth of the object can be used.

When a camera alone cannot measure the position of an object with sufficient accuracy, a proximity sensor is used. A proximity sensor is a sensor used to measure the distance of an object within a few millimeters to a few centimeters. The end-effector is moved closer to the object based on the positional information obtained by the camera, and once it is within the range of the proximity sensor, the robot arm is controlled with higher precision based on the distance information from the proximity sensor. There are different types of proximity sensors, such as eddy current and capacitive.

When the end-effector comes into contact with the object, the force sensor measures how much force is applied to the object. By using this information, it is possible to absorb the errors of the situation and individual objects, such as positioning and material of the object, and to perform welding, cutting, grasping, and other operations with high accuracy. Force sensors are often made using a structure called a “strain gauge”.

Perception of objects around the robotic arm

If a human or an unintended object enters around the robot arm, it may cause a collision with the robot arm. In order to avoid accidents and operate the robot safely, sensors are installed to detect surrounding objects. Typical sensors are ultrasonic sensors (sonar) and infrared TOF sensors.

Ultrasonic sensors detect the presence of an object by emitting ultrasonic waves and detecting the ultrasonic waves that hit the object and bounce back. The distance to the object can be determined by measuring the time between the emission and the bounce back. The formula is “distance = time x speed of sound”. The same principle applies to the infrared TOF sensor, where TOF stands for Time of Flight. Since infrared is a type of light (electromagnetic wave), the formula is “distance = time x speed of light”. Both sensors are widely used because they are relatively inexpensive and can be manufactured compactly.

Sensor to perceive the inside (Internal sensors)

The sensor used to perceive the internal state of the robot arm itself depends on the type of actuator that drives the joint. For example, a stepping motor can directly indicate the angle of the joint, so there is no need to know the internal angle information. However, the disadvantage is that the angle that can be specified is in increments of 1°, making it difficult to control the position with high precision.

DC and AC motors are further subdivided according to their structure and control method, but they all have one thing in common: the input information is not angular position information. The only information that can be input is voltage and current, and it is not possible to directly input angle position information. Therefore, a sensor is used to detect the position of the angle, and a feedback control mechanism is used to determine the voltage and current input to the motor to control the position of the motor.

A sensor often used to detect angle position information in motors for robot arms is a “rotary encoder. A rotary encoder consists of three elements: an LED (light source), an optical sensor, and a disk with a hole in the periphery. The disk is attached to the motor’s rotating shaft so that it becomes one with it. An LED is installed so that light passes through the hole in the disk, and an optical sensor is installed across the disk to detect the light. When the motor rotates, the disk and the hole also rotate, causing light to pass through or be blocked by the disk. The flickering light generated in this way is captured by the optical sensor and analyzed by the processing circuit in the later stage to determine the angle of the motor. The finer the size and pitch of the holes in the disk, the higher the accuracy of the angle that can be detected. However, simply placing holes of the same size at the same pitch will give us the angular velocity, but not the absolute position information of the angle. In order to obtain absolute angular position information, the size of the holes can be changed depending on the position of the disk, or multiple holes of different sizes can be placed in parallel toward the center of the disk.

Other times, “acceleration sensors” are attached to the link and end-effector parts of the robot arm to monitor the status of the robot arm’s movement. In addition, current sensors are often used to measure the current flowing in the motor. However, since the current value is calculated from Ohm’s law using the potential difference between the two ends of a series of resistive elements, it is sometimes regarded as part of the electronic circuit and not called a sensor.

The mechanism that controls the angle (position), angular velocity, torque, etc. of the motor through feedback control by detecting the motor’s own information with some kind of sensor is called a “servo mechanism. A motor equipped with a servo mechanism is called a “servomotor. The word “servo” has nothing to do with the structure of the motor itself, but indicates the presence of a feedback control system (circuit) around the motor.

Since the servo mechanism of a motor is used only to control the motor, it is often commercialized as an integrated “module” that includes the motor, sensors, and feedback system. For this reason, the name “servomotor” is used to describe a type of motor, even if it is in principle different from the motor itself.

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