Unit 7: End Effectors and Sensors - Subjective Questions

An end effector is a device attached to the wrist or final joint of a robotic arm to enable it to interact with objects or perform a specific task.

Its role in the Orangewood Robotic Arm includes:

• Holding, gripping, lifting, or moving objects.
• Performing operations such as writing, drawing, picking, or placing.
• Providing task-specific functionality to the robot.
• Converting the motion of the robot into useful physical work.

The robotic arm itself provides position and movement, while the end effector performs the actual interaction with the workpiece.

The major types of end effectors are:

• Mechanical grippers: Use fingers or jaws to hold objects by applying mechanical force.
• Vacuum grippers: Use suction cups and a pressure difference to lift smooth, non-porous objects.
• Magnetic grippers: Use permanent magnets or electromagnets to handle ferromagnetic materials.
• Pneumatic grippers: Use compressed air to open and close the gripping fingers.
• Adhesive grippers: Use adhesive surfaces to pick up suitable objects.
• Tool-type end effectors: Include drills, welding tools, screwdrivers, markers, and cutting tools.

For an Orangewood Robotic Arm, a mechanical gripper is commonly used for educational demonstrations because it is simple to control and suitable for pick-and-place experiments.

A two-finger mechanical gripper generally consists of:

• Two movable gripping fingers.
• A linkage or gear mechanism.
• A servo motor or actuator.
• A mounting arrangement for attachment to the robot wrist.
• Mechanical stops to limit finger movement.

Working principle:

1. A control signal is sent to the servo motor.
2. The servo rotates to a specified angular position.
3. The internal linkage converts rotary motion into the opening or closing motion of the fingers.
4. The fingers contact the object and apply a gripping force.
5. The arm moves the object while the gripper maintains its position.

The gripper must apply sufficient force to prevent slipping, but excessive force should be avoided because it may deform or damage the object.

A mechanical gripper is generally more versatile for the Orangewood Robotic Arm because it can handle many common laboratory objects without requiring a special surface or material property.

Important selection factors include:

• Object size and shape: The gripper must accommodate the dimensions and geometry of the object.
• Object weight: The end effector must safely support the load without exceeding the robot's payload.
• Surface properties: Texture, porosity, temperature, and material affect the choice of gripper.
• Required gripping force: The force must prevent slipping while avoiding damage.
• Accuracy and repeatability: The gripper should position the object consistently.
• Operating speed: The end effector should open and close quickly enough for the task.
• Robot compatibility: Its mass, mounting pattern, and control interface must match the Orangewood Robotic Arm.
• Power requirements: Pneumatic, electrical, or servo-based operation must be available.
• Safety and maintenance: The gripper should operate reliably and be easy to inspect and replace.

A sensor is a device that detects a physical quantity or environmental condition and converts it into a usable electrical signal.

Sensors are important because they allow a robot to:

• Detect the presence or absence of objects.
• Measure position, speed, force, distance, or temperature.
• Determine whether an operation has been completed.
• Correct errors caused by external disturbances.
• Improve accuracy, safety, and repeatability.
• Operate using feedback instead of relying only on pre-programmed motion.

In the Orangewood Robotic Arm, sensors can provide information about joint position, object detection, or the interaction between the gripper and an object.

Robotic sensors can be classified in several ways:

1. According to the source of information

• Internal or proprioceptive sensors: Measure the robot's own state, such as joint angle, velocity, or motor current.
• External or exteroceptive sensors: Detect conditions outside the robot, such as objects, distance, light, or force.

2. According to the type of output

• Analog sensors: Produce a continuously varying output, such as a force sensor voltage.
• Digital sensors: Produce discrete states, such as an object-detected or object-not-detected signal.

3. According to the physical quantity measured

• Position sensors, such as encoders and potentiometers.
• Proximity sensors, such as infrared and ultrasonic sensors.
• Force and torque sensors.
• Touch or tactile sensors.
• Vision sensors and cameras.
• Temperature and light sensors.

An infrared proximity sensor contains an infrared transmitter and a receiver. The transmitter emits infrared radiation, and an object near the sensor reflects part of that radiation toward the receiver.

The sensor's electronic circuit analyzes the received light and produces an output when the reflected intensity crosses a specified threshold.

Applications include:

• Detecting whether an object is present in the gripper area.
• Identifying objects placed at a workstation.
• Preventing the arm from moving into an occupied region.
• Confirming that an object has reached a target location.

Its performance depends on object color, surface reflectivity, distance, ambient light, and sensor alignment. Dark or highly absorbing surfaces may reduce the reflected signal.

An ultrasonic sensor transmits a high-frequency sound pulse and measures the time taken for the echo to return after reflection from an object. The distance is calculated using:

where is the distance to the object, is the speed of sound, and is the round-trip travel time. The factor is used because the sound travels to the object and back.

Advantages:

• Works with many colors and lighting conditions.
• Can measure distance without physical contact.
• Suitable for detecting objects over a comparatively broad range.

Limitations:

• Soft or angled surfaces may absorb or deflect sound.
• Nearby objects can cause multiple echoes.
• Temperature and air conditions affect the speed of sound.
• It may have a relatively wide detection cone, reducing positional precision.

For the Orangewood Robotic Arm, a contact sensor may confirm that a gripper has reached an object, while a non-contact sensor may detect the object before the gripper closes.

A limit switch or tactile sensor detects physical contact or movement beyond a permitted position. It can be installed at a joint limit, gripper mechanism, or protected region.

When the sensor is activated:

• The controller receives a digital input.
• The robot can stop or reduce its motion.
• The gripper can detect contact with an object.
• The arm can avoid excessive travel or mechanical collision.
• A fault condition can be indicated to the operator.

The sensor should be connected through a reliable control circuit. The program should also define the response to activation, such as stopping the motor, reversing the motion, or entering a safe state. Hardware protection is preferable for critical safety functions because software alone may fail.

A joint-position sensor measures the angular position of a robot joint.

Potentiometer:

• Produces an analog voltage proportional to shaft angle.
• Is simple and inexpensive.
• Requires calibration to map voltage to joint angle.
• Has mechanical wear because of sliding contact.

Encoder:

• Generates digital pulses or coded signals as the shaft rotates.
• Provides better resolution and repeatability than a basic potentiometer.
• Can measure relative motion and, in some designs, absolute position.
• Requires suitable signal processing by the controller.

In the Orangewood Robotic Arm, joint-position feedback helps the controller determine whether each joint has reached its commanded position and supports accurate end-effector placement.

Calibration establishes the relationship between sensor readings or actuator commands and actual physical values.

Calibration is important because it:

• Corrects offsets and scale errors.
• Improves the accuracy of joint and end-effector positioning.
• Ensures that a proximity sensor detects objects at the intended distance.
• Allows the gripper opening to correspond to the commanded value.
• Compensates for mounting tolerances and mechanical variations.
• Improves repeatability during pick-and-place operations.

A typical calibration procedure includes setting a reference position, recording the sensor output, checking known positions or distances, and applying correction values in the controller. Calibration should be repeated after changing the gripper, sensor position, or mechanical configuration.

Accuracy is the closeness of a measured or achieved value to the true value.

Precision describes how closely repeated measurements agree with one another, even if they are offset from the true value.

Resolution is the smallest change that a sensor can detect or report.

Repeatability is the ability of the robotic arm or sensor to return to the same position or produce the same reading under the same conditions.

For example, a joint sensor may have high resolution but poor accuracy if it produces many detailed readings with a constant offset. In a pick-and-place task, repeatability is essential for returning to the same pickup location, while accuracy is essential for correctly locating the workpiece relative to the station.

A force or tactile sensor detects the interaction force between the gripper fingers and the object.

A typical gripping procedure is:

1. Move the gripper toward the object.
2. Close the fingers at a controlled speed.
3. Monitor the sensor output while the fingers move.
4. Detect a force increase or contact event.
5. Stop closing when the required gripping force is reached.
6. Lift the object slowly and verify that the force remains within the expected range.

If the force remains near zero, the gripper may have missed the object. If the force is too high, the object may be fragile or incorrectly positioned. Feedback from the sensor allows the arm to distinguish between an empty grip, a successful grip, and excessive contact.

A sensor-based pick-and-place sequence can be organized as follows:

• Initialization: Move the arm to a known home position and check sensor status.
• Object detection: Use an infrared, ultrasonic, or vision sensor to confirm the object's presence.
• Approach: Move the end effector toward the pickup coordinates at a safe speed.
• Alignment: Use sensor feedback or a programmed reference position to align the gripper with the object.
• Gripping: Close the mechanical gripper and monitor a tactile or force signal if available.
• Verification: Lift the object slightly and confirm that it remains held.
• Transport: Move along a collision-free path to the destination.
• Release: Open the gripper after confirming that the target position has been reached.
• Return: Move the arm to the home or next-task position.
• Fault handling: Stop or retry the sequence if the object is not detected or is dropped.

Sensor noise is an unwanted variation in a sensor signal that does not represent a real change in the measured quantity.

Noise can cause:

• False object detection.
• Unstable gripper operation.
• Incorrect distance or position readings.
• Repeated starting and stopping of motors.
• Reduced accuracy and poor repeatability.

Noise can be reduced by:

• Using proper electrical grounding and shielding.
• Separating sensor wires from high-current motor wires.
• Supplying stable power to the sensor.
• Applying software filtering, such as a moving average.
• Using hysteresis in threshold-based decisions.
• Mounting the sensor rigidly and avoiding vibration.
• Selecting a sensor suitable for the operating environment.

Filtering should be selected carefully because excessive filtering introduces delay into the control response.

In open-loop control, the controller sends a command to the end effector without measuring whether the expected result occurred. For example, the gripper may be commanded to close for a fixed time and the program assumes that the object has been held.

In closed-loop control, sensor feedback is used to compare the actual result with the desired result. For example, a force sensor can detect contact and allow the controller to stop closing when the correct gripping force is reached.

Open-loop control:

• Is simple and inexpensive.
• Works well in predictable conditions.
• Cannot automatically correct disturbances or missed grips.

Closed-loop control:

• Improves reliability and adaptability.
• Detects errors such as object absence or excessive force.
• Requires sensors, signal processing, and suitable control logic.

The controller must be configured with the correct input type and operating voltage for each sensor.

Object slipping can occur because of:

• Insufficient gripping force.
• Smooth, oily, dusty, or wet object surfaces.
• Incorrect finger shape or poor contact area.
• Object weight exceeding the gripper capacity.
• Excessive acceleration or sudden changes in direction.
• Misalignment between the gripper and the object.
• Vibration or backlash in the robot mechanism.

Preventive measures include:

• Selecting fingers with suitable shape and friction material.
• Increasing the gripping force within the safe limit.
• Centering the object before lifting it.
• Reducing acceleration and transport speed.
• Using tactile or force feedback to verify the grip.
• Checking the payload and center of gravity.
• Performing a short test lift before moving the object over a larger distance.