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Position sensor principle and application

Published time: 2019-10-07

Position sensor principle

A sensor used to measure the position of the robot itself. Position sensors can be divided into two types, linear displacement sensors and angular displacement sensors.

A position sensor is a sensor that senses the position of a measured object and converts it into an available output signal. It senses the position of the object being measured and converts it into a sensor that can use the output signal.

Position sensor

Position sensor classification

The position sensor can be used to detect the position and reflect the switch of a certain state. Unlike the displacement sensor, the position sensor has two types of contact and proximity.

Contact sensor

The contact of the contact sensor is actuated by the contact of two objects, and a common one is a travel switch, a two-dimensional matrix position sensor, and the like. The stroke switch has a simple structure, reliable operation and low price. When an object touches the travel switch during motion, its internal contacts will act to complete the control. For example, if the travel switch is installed at both ends of the X, Y and Z axes of the machining center, the movement can be controlled. range. A two-dimensional matrix position sensor is mounted on the inside of the palm of the hand to detect its own contact with an object.

A proximity switch is a switch that emits an "action" signal when an object approaches a set distance. It does not need to be in direct contact with the object. There are many types of proximity switches, mainly electromagnetic, photoelectric, differential transformer, eddy current, capacitive, reed switch, Hall type and so on. The application of proximity switches on CNC machine tools is mainly knife holder selection, table stroke control, cylinder and cylinder piston stroke control.

Hall sensor

The Hall sensor is a sensor made using the Hall phenomenon. When a semiconductor such as germanium is placed in a magnetic field and a current is applied in one direction, a potential difference occurs in the vertical direction, which is the Hall phenomenon. The small magnet is fixed on the moving part, and when the part is close to the Hall element, a Hall phenomenon is generated to determine whether the object is in place.

Hall sensor

Application of position sensor on DC motor

Conventional brushless DC motors mostly use Hall elements or other position detecting elements as position sensors, but the position sensor is difficult to maintain, and the temperature characteristics of the Hall elements are not good, resulting in poor reliability of the piecemeal. Therefore, the position sensor brushless DC motor is an ideal choice and has a broad prospect, but its control circuit is quite complicated.

The most fundamental difference between the control of the position sensor brushless DC motor and the position sensor brushless DC motor control is the application of the back EMF waveform to find the optimum commutation point. When the permanent magnet brushless DC motor is running, the back electromotive force (EMF) of each phase winding is closely related to the rotor position. Since each phase winding is an alternate conduction task, some special points of the back electromotive force waveform can replace the function of the rotor position sensor and lose the required information when a phase is not conducting.

The external speed control system is a typical DC motor PWM double closed-loop speed control system. Two conditioners are set in the system to distinguish the conditioning speed and current. The cascade connection is implemented between the two, that is, the input of the speed regulator is used as the current. The position sensor conditioner controls the switching device with the input of the current conditioner. The double closed-loop system composed in this way exhibits a constant current conditioning system in a given transition process, and in the steady state and near steady state operation, it also shows no static speed control system, that is, the speed and current are carried forward. The respective functions of the two position sensor conditioners prevent defects such as single-ring fragmentation, which are mutually restrained, thereby achieving good static and static quality.

The source current of the internal power component MOSFET flows through R. The voltage is inversely proportional to the motor winding current of the position sensor brushless DC motor control. The loop filter (which filters out the noise spike that triggers the one-shot circuit) The current, the normal sample is within 300ns of the current constant) to the positive terminal of the current comparator), and the negative terminal of the comparator has a diode with a clamping voltage of 0.5V, thus limiting the maximum peak current of the motor stator circuit. When the voltage of the circuit is higher than the negative terminal voltage of the comparator, the monostable circuit is triggered, the input MOSFET is turned off, and the current drops until the one-shot current is reset.

The position sensor controller chip simplifies the control of the brushless DC motor. It not only has good current limiting and maintenance functions, but also the double closed loop speed control function composed of ML4428 will also be improved. The position sensor controller is adopted. The chip handles the problem of applying the back-EMF detection to complete the commutation and the open-loop starting at low speed. The experiment proves that the control structure is complicated and the function is complete, and the reliability of the piecemeal is improved. This method has important practical significance for the universal use of DC brushless motors.

The commutation is completed by the back-EMF signal sampling and detection by the position sensor phase-locked loop. When the motor moves and runs at low speed, the back-EM potential is zero or very low, which cannot be detected. Therefore, it must be opened by other methods. Until a large enough back EMF occurs, normal commutation can be entered.


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