 Welcome to the video PID Control. A common application of an industrial robot is to move an object connected to a robotic arm from one position to another. Specific examples are a parts insertion machine and a robotic welder. If the robotic arm is to move to a specific position, it needs a command signal, typically from a computer. The analog command signal voltage produced by the D to A converter determines the position the arm is required to take. The analog command signal voltage produced by the D to A converter is the signal that indicates the position the arm is required to move. The actual position of the arm is indicated by the voltage produced by a sensing device called a potentiometer. This voltage is called the feedback signal. As the robotic arm moves, it causes the wiper arm of the potentiometer to slide up or down to produce a variable voltage representing the position. When the feedback signal voltage matches the command signal voltage, the arm is in the desired position. In this illustration, the arm is in the home position or starting position. The feedback signal and the command signal are both 0 volts. To move the arm from the home position to another position, a command signal is produced by the computer. The computer outputs binary numbers that increment until a value is reached that represents the desired position. The D to A converter produces an analog voltage, which increases in small increments that are proportional to the binary numbers. Since there is not an immediate response by the robot to the command signal, the robotic arm and the feedback signal lag behind, creating a positive error signal voltage at the output of the difference op amp. The error signal is inverted to a negative voltage by the proportional op amp. The output of the proportional op amp is further amplified by the power amp and inverted back to the positive voltage before being applied to the motor of the robotic arm. After the command signal stops changing, the robotic arm, which is lagging behind, continues to move because there is an error signal at the output of the difference op amp. When the arm reaches the desired position, the feedback signal voltage from the potentiometer equals the command signal voltage, causing the difference op amp voltage to reduce to 0 volts. Therefore, the voltage produced at the proportional op amp and at the power amp drops to 0, causing the robotic arm motor to stop the desired position. The desired operation of the robot is to respond as quickly as possible to the command signal. This response is achieved by increasing the gain of the proportional amplifier. However, the drawback to the high gain is that it causes the arm to move so fast that it overshoots the desired position. By moving too far, the feedback signal voltage becomes greater than the command signal voltage, causing the polarity of the difference op amp to switch from positive to negative. This causes the polarities of the proportional amplifier and the power amplifier to change and reverse the rotation of the motor. However, the high gain that caused the arm to overshoot the desired position now causes it to overshoot again in the opposite direction. The overshoots will occur several times before the arm stabilizes. A fast response without an overshoot can be achieved by setting the gain of the proportional amplifier back to a normal level and by adding a derivative amplifier. The reason the fast response occurs is that when the command signal starts to change, the robotic arm and the resulting feedback voltage do not immediately respond. The result is that the error signal builds, causing the derivative amplifier output to increase. The derivative amplifier only produces an output while the signal applied to the amplifier's input is changing. The faster the input changes, the greater the output. Therefore, as the robotic arm lags behind the command signal, the growing error signal causes the derivative amplifier to produce a voltage that adds to the output of the proportional amplifier. Adding both voltages to the power amp produces a boosting action that causes the robotic arm's motor to accelerate as fast as the command signal is changing. When the voltage from the potentiometer indicates the robotic arm's position is changing as fast as the command signal, the error signal becomes constant. This causes the derivative voltage to drop to zero. Only the proportional amplifier is producing voltage. When the command signal stops changing, the lagging arm keeps moving, which causes the error signal to begin to decrease. When the error signal decreases, it causes the output voltage of the derivative amplifier to change polarity and cancels the proportional voltage. If the error signal decreases quickly enough, the derivative voltage may even be greater than the proportional voltage, causing the polarity of the power amp to reverse along with the result in current through the motor. This creates a braking action and prevents the arm from overshooting. As the arm approaches the desired position, the feedback voltage from the potentiometer becomes closer to the command signal voltage. As the feedback voltage approaches the command signal, the error signal produced by the difference op amp becomes smaller, causing the arm to slow down. At some point, the weight of the arm with the object attached to it and friction cause the motor to stop, falling short of the arm's desired position. When the command signal voltage and the feedback signal voltage are not the same, a condition called a steady-state error exists. This causes the difference op amp to create a small steady voltage. The steady-state error can be overcome by adding an integral amplifier. The polarity of the integral amplifier is the same as the polarity of the proportional amplifier. Whenever there is no error signal, which is when the feedback signal and the command signal voltages are equal, the output voltage of the integral amplifier is zero. However, in a situation when friction or weight causes the robotic arm to stop just short of the desired position, the feedback signal will be slightly less than the command signal, causing a small error voltage to develop at the output of the difference op amp. With an integral op amp, the longer the error signal is present at the input, the larger the amplifier gain. This causes the output voltage to build. Eventually, the output of the integral amplifier increases, which is further amplified by the power amp. When this voltage becomes large enough, it causes the motor to overcome the weight of the robotic arm and its load, along with friction, and moves the arm the rest of the way to the desired position. When the feedback signal equals the command signal, the output voltages at all of the amplifiers drop to zero, causing the arm's motor to stop running. You have completed this video. PID control.