Types and Application of Position Sensors

Catalog

I Position Sensor Types

A position sensor is a sensor that can sense the position of the measured object and convert it into a usable output signal. There are mainly two types of position sensors: the contact type and proximity type.

1. Contact Position Sensor

The contact terminal of the contact position sensor is moved by two objects touching and squeezing. Common contact position sensors are travel switches and two-dimensional matrix position sensors.

The travel switch has a simple structure, reliable action, and low price. When an object touches the travel switch when it moves, its internal contacts will act to complete the control. If travel switches are applied to both ends of the X, Y, and Z axes of the machining center, you can control the moving range.

Contact Position Sensor

The two-dimensional matrix position sensor is installed inside the palm of the robot to detect the contact position between itself and the object.

2. Proximity Position Sensor

The proximity switch refers to a switch that can send out an actuating signal when an object approaches it within the set distance rather than direct contact with the object. There are many types of proximity switches, mainly electromagnetic, photoelectric, differential transformer, eddy current, capacitive, reed switch, Hall type, etc. Proximity switches are commonly used in tool selection control, table travel control, cylinder and cylinder piston travel control, etc. on computer numerical control(CNC) machine tools.

Wire Proximity Switches

II Application of Position Sensors

1. Brushless DC Motor

The position sensor is one of the three major parts of the brushless DC motor system, which is also the main symbol that distinguishes it from the brush DC motor. It's used to detect the position of the main rotor in the movement, and convert the position signal of the rotor magnetic pole into an electrical signal, providing correct reversing information for the logic switch circuit to control their conduction and cutoff. In this way, it can make the current in the winding reversed in order with the change of the rotor position, forming a stepping rotating magnetic field in the air gap, and driving the permanent magnet rotor to rotate continuously.

The brushless DC motor needs a position sensor to measure the position of the rotor. The motor controller receives the position sensor signal to synchronize the inverter with the rotor to drive the motor to continue running. Although the brushless DC motor can also detect the position of the rotor through the inductive electromotive force generated by the stator winding without a position sensor when the motor is started, the speed is too small and the electromotive force signal will be too small to detect.

Hall sensor chips that can be used as position sensors for DC brushless motors are divided into two types: switch type and lock type.

For electric bicycle motors, both Hall sensor chips can be used to accurately measure the position of the rotor magnet. The performance of the DC brushless motors made with these two Hall sensor chips, including the motor's output power, efficiency, and torque, does not have any difference and can be compatible with the same motor controller.

Position sensors can reduce the noise of the motor operation, improve the life and performance of the motor, and at the same time reduce energy consumption, which undoubtedly provides a strong driving force for the development of the motor market.

2. Crankshaft and Camshaft

What is a camshaft position sensor? And how about the crankshaft position sensor?

• Crankshaft Position Sensor(CPS), also known as engine speed and crankshaft angle sensor, is used to collect the crankshaft rotation angle and engine speed signal, and enter the electronic control unit (ECU) to determine the ignition time and fuel injection time.

• Camshaft Position Sensor (CPS) is also called Cylinder Identification Sensor (CIS). To distinguish it from CPS, it is generally expressed by CIS. The camshaft position sensor is to used to collect the position signal of the valve camshaft and input it to the ECU.

So, the ECU recognizes the compression top dead center of cylinder 1 to perform sequential fuel injection control, ignition timing control and knock control. In addition, the camshaft position signal is also used to identify the first ignition moment when the engine is started. Because the camshaft position sensor can identify which cylinder piston is about to reach the top dead center, it is called a cylinder position sensor.

(1) Photoelectric Crankshaft and Camshaft Position Sensors

1) Structural characteristics

The photoelectric crankshaft and camshaft position sensorsare mainly composed of a signal panel (that is, a signal rotor), a signal generator, a distributor, a sensor housing, and a harness plug.

The signal panel is the signal rotor of the sensor, which is pressed onto the sensor shaft, as shown in Figure 1. The inner and outer circles of transparent holes with evenly spaced arcs are made near the edge of the signal panel. The outer circle is made with 360 transparent holes (slits), and the interval arc is 1°. (The transparent hole and the blocking hole is 0.5° respectively) This is used to generate the crankshaft angle and speed signals.

The inner circle is made with 6 transparent holes used to generate the top dead center signal of each cylinder with an interval arc of 60°, one of which has a longer width, which is used to generate the top dead center signal of the cylinder 1°.

Figure 1. Working Principle of Photoelectric Position Sensors

The signal generator is fixed on the sensor housing, and it is composed of a Ne signal (speed and angle signal) generator, a G signal (top dead center signal) generator, and a signal processing circuit. The Ne signal and the G signal generator are composed of a LED and a phototransistor (or photodiode), and the two LEDs are directly opposite to the two phototransistors.

2) Working Principle

The working principle of the photoelectric sensor is shown in Figure 1. The signal panel is installed between the LED and the phototransistor (or photodiode).

When the transparent hole on the signal panel rotates between the LED and the phototransistor, the light from the LED will emit on the phototransistor. At this time, the phototransistor is turned on and its collector outputs a low level (0.1-0.3V).

When the blocking part on the signal panel rotates between the LED and the phototransistor, the light from the LED cannot emit on the phototransistor. At this time, the phototransistor is turned off and its collector outputs a high level (4.8-5.2V).

If the signal panel rotates continuously, the transparent hole and the blocking hole will alternately pass through the LED to transmit or block light, and the collector of the phototransistor will alternately output high and low levels. When the sensor shaft rotates with the crankshaft and the valve camshaft, the transparent holes and blocking parts on the signal panel will pass between the LED and the phototransistor, and the light from the LED will be alternately emitted to the phototransistor, a pulse signal corresponding to the position of the crankshaft and the camshaft is generated in the signal sensor.

Since the crankshaft rotates two turns, the sensor shaft drives the signal panel for one turn. Therefore, the G signal sensor will generate 6 pulse signals, and the Ne signal sensor will generate 360 pulse signals. Because the interval arc of the G signal transparent hole is 60°, each time the crankshaft rotates 120°, a pulse signal is generated, so the G signal is usually called 120°  signal. The 120° signal should be designed 70° before the top dead center. (BTDC70°), and the signal generated by the transparent hole with a longer width is corresponding to 70° before the top dead center of the engine cylinder 1°, so that the ECU can control the fuel injection advance angle and the ignition advance angle.

Because the interval arc of Ne transparent hole is 1°, (the transparent hole and the blocking hole is 0.5°respectively), so in each pulse period, the high and low levels account for 1° crankshaft rotation angle respectively, 360 signals indicate the crankshaft rotates 720°. Every 120° crankshaft rotates, G signal sensor produces a signal, and Ne signal sensor produces 60 signals.

(2) Magnetic Inductive Crankshaft and Camshaft Position Sensors

The working principle of the magnetic position sensor is shown in Figure 2. The line of magnetic force pass through:

the permanent magnet N-pole → the air gap between stators → rotor convex teeth → air gap between the rotor convex tooth and the stator magnetic head → magnetic head → concentrating flux plate → the permanent magnet N-pole

When the signal rotor rotates, the air gap in the magnetic circuit changes periodically, and the magnetic resistance of the magnetic circuit and the magnetic flux passing through the signal coil head change periodically. According to the principle of electromagnetic induction, the alternating electromotive force will be generated in the sensing coil.

Figure 2. Working Principle of the Magnetic Inductive Position Sensor

When the signal rotor rotates clockwise, the air gap between the rotor convex teeth and the magnetic head decreases, the magnetic resistance in the magnetic circuit decreases, the magnetic flux φ increases, the magnetic flux change rate increases (dφ/dt>0), and the induced electromotive force E Is positive (E>0), as shown by curve abc in Figure 3. When the convex teeth approach the edge of the magnetic head, the magnetic flux φ increases sharply, the magnetic flux change rate is the largest [dφ/dt=(dφ/dt)max], and the electromotive force E is the highest (E=Emax), as the point b in Figure 3. After the rotor rotates past point b, although the magnetic flux φ is still increasing, the change rate of magnetic flux decreases, so the induced electromotive force E decreases.

When the rotor rotates until the centerline of the convex teeth is aligned with the centerline of the magnetic head (see Figure 2-b), although the air gap between the convex teeth and the magnetic head is the smallest, the magnetic resistance of the magnetic circuit is the smallest, and the magnetic flux φ is the largest. However, since the magnetic flux is impossible to continue to increase, and the change rate of magnetic flux is zero, so the induced electromotive force E is zero, as the point c in Figure3.

Figure 3. Magnetic fluxφ Curve & the electromotive force E Curve

When the rotor continues to rotate clockwise and the convex teeth leave the magnetic head (see Figure 2-c), the air gap between the convex teeth and the magnetic head increases, the magnetic resistance of the magnetic circuit increases, and the magnetic flux φ decreases (dφ/dt< 0), so the induced electromotive force E is negative, as shown by curve cda in Figure 3. When the convex tooth turns to leave the edge of the magnetic head, the magnetic flux φ decreases sharply, the magnetic flux change rate reaches the negative maximum value [dφ/df=-(dφ/dt)max], and the induced electromotive force E also reaches the negative maximum value (E =-Emax), shown as point d on the curve in Figure 3.

It can be seen that each time the signal rotor rotates through a convex tooth, a periodic alternating electromotive force is generated in the sensing coil, which means the electromotive force will have a maximum value and a minimum value, and the sensing coil accordingly outputs an alternating voltage signal.

The outstanding advantage of magnetic position sensing is that no external power supply is required. The permanent magnet can convert mechanical energy into electrical energy, and its magnetic energy is not lost. When the engine speed changes, the speed of the rotor's convex teeth will change, and the change rate of the magnetic flux in the core will also change. The higher the speed, the greater the change rate of magnetic flux, and the higher the induced electromotive force in the sensing coil. When the rotation speed is different, the changes of magnetic flux and induced electromotive force are shown in Figure 3.

Since the air gap between the convex teeth of the rotor and the magnetic head directly affects the magnetic resistance of the magnetic circuit and the output voltage of the sensing coil, the air gap cannot be changed casually. If the air gap changes, it must be adjusted according to the regulations. The air gap is generally designed to be 0.2 - 0.4mm.

(3) Magnetic Inductive Crankshaft Position Sensor for Cars

1) Structural characteristics

The magnetic inductive crankshaft position sensor for cars is usually installed on the cylinder near the clutch of the crankcase, which is mainly composed of a signal generator and a signal rotor, as shown in Figure 4.

Figure 4. CPS Structure of Jetta Cars

The signal generator is fixed on the engine cylinder block with screws and is composed of a permanent magnet, a sensing coil, and a harness connector. The sensing coil is also called the signal coil, and the permanent magnet is equipped with a magnetic head directly opposite to the signal rotor mounted on the crankshaft. The magnetic head is connected with a magnetic yoke to form a magnetic circuit.

The signal rotor is of the toothed disk type, with 58 convex teeth, 57 small tooth gaps, and one large tooth gap evenly spaced on its circumference. The large tooth gap outputs a reference signal corresponding to a certain angle before the compression top dead center of engine cylinder 1 or cylinder 4. Therefore, the crank angle occupied by the convex teeth and tooth gaps on the circumference of the signal rotor is 360°.

2) Working Condition

When the position sensor rotates with the crankshaft, every time the signal rotor rotates through a convex tooth, a periodic alternating electromotive force is generated in the sensing coil, and the coil outputs a corresponding alternating voltage signal.

Because the signal rotor is provided with a large tooth gap generating a reference signal, when the large tooth gap passes through the magnetic head, the signal voltage takes a longer time, which means the output signal is a wide pulse signal corresponding to a certain angle before the top dead center of cylinder 1 or 4.

When the electronic control unit (ECU) receives the wide pulse signal, it can know that the top dead center of cylinder 1 or cylinder 4 is about to come. Whether cylinder 1 or cylinder 4 is coming is based on the signal input by the camshaft position sensor. Since there are 58 convex teeth on the signal rotor, every time the signal rotor makes a turn (the engine crankshaft makes one turn), the sensing coil will generate 58 alternating voltage signals and input them to the electronic control unit.

Whenever the signal rotor position sensor rotates one turn with the engine crankshaft, the sensing coil inputs 58 pulse signals to the electronic control unit (ECU). Therefore, every time the ECU receives 58 signals from the crank position sensor, it can know that the engine crankshaft has rotated once.

If the ECU receives 116000 signals within 1min, the ECU can calculate the crankshaft speed n as 2000 (n=116000/58=2000) r/rain. By analogy, the ECU can calculate the rotation speed of the engine crankshaft according to the number of signals received per minute.

The engine speed signal and load signal are the most important and basic control signals of the electronic control system. Based on these two signals, the ECU can calculate the three basic control parameters: basic injection advance angle, basic ignition advance angle, and ignition conduction angle.

(4) Hall-type Crankshaft and Camshaft Position Sensors

1) Structure and Working Principle

Hall-type crankshaft and camshaft position sensors and other forms of Hall-type sensors are all made based on the Hall effect, so they all belong to Hall effect position sensors.

Figure 5. Principle of Hall Effect

• Hall effect

The Hall effect was first discovered by Dr. E.H. Hall, a physicist at Johns Hopkins University in the United States, in 1879. He found that when a rectangular platinum conductor with current I was placed perpendicular to the magnetic lines in a magnetic field with an induction of B (see Figure 5), a voltage UH perpendicular to the direction of the magnetic field and current would be generated on the two lateral sides of the platinum conductor. When the magnetic field disappears, the voltage disappears immediately. This voltage is later called the Hall voltage, which is proportional to the current I and the magnetic induction B:

K_H一Hall Coefficient

d一Thickness of Platinum conductor

The element made with the Hall effect is called the Hall element, and the sensor made of Hall element is called the Hall sensor. The Hall effect can not only detect the voltage by switching on and off the magnetic field, but also detect the current flowing in the wire because the strength of the magnetic field around the wire is proportional to the current.

Since the 1980s, the number of Hall sensors used in automobiles has increased dramatically. This is mainly because of the two outstanding advantages of Hall sensors:

• the output voltage signal is similar to a square wave signal;

• the speed of the measured object is irrelevant to the rotation rate.

Different from magnetic induction sensors, Hall sensors usually require an external power supply.

2) Basic Structure of Hall Sensor

The Hall sensor is mainly composed of a trigger impeller, a Hall integrated circuit, a magnetic yoke, and a permanent magnet. The trigger impeller is installed on the rotor shaft, and the impeller has blades. In the Hall-type ignition system, the number of blades is equal to the number of engine cylinders. When the trigger impeller rotates with the rotor shaft, the blade rotates between the Hall IC and the permanent magnet. Hall IC is composed of the Hall element, amplifying circuit, voltage stabilizing circuit, temperature compensation circuit, signal conversion circuit, and output circuit.

3) Working Principle of Hall Sensor

When the sensor shaft rotates, the blades of the impeller passes through the air gap between the Hall IC and the permanent magnet. When the blade leaves the air gap, the magnetic flux of the permanent magnet passes through the Hall IC and the magnetic steel sheet to form a loop. At this time, the Hall element generates a voltage (UH = 1.9-2.0V), the transistor of the output stage of the Hall IC is turned on, and the signal voltage U0 output by the sensor is low. In the actual measurement, when the power supply voltage Ucc = 14.4V or 5V, the signal voltage U0 = 0.1-0.3 V).

When the blade enters the air gap, the magnetic field in the Hall IC is bypassed by the blade. Therefore, the Hall voltage UH is zero, the transistor of the IC output stage is cut off, and the signal voltage U0 output by the sensor is high. In the actual measurement, when the power supply voltage Ucc=14.4V, the signal voltage U0=9.8V; when the power supply voltage Ucc=5V, the signal voltage U0=4.8V.

4) Structure of Hall-type Camshaft Position Sensor

The Hall-type camshaft position sensor used in cars is installed at one end of the engine intake camshaft. The structure is shown in Figure 6. It is mainly composed of a Hall signal generator and a signal rotor. The signal rotor, also known as the trigger impeller, is installed on the intake camshaft with positioning bolts and a bezel.

Figure 6. Structure of Hall-type Camshaft Position Sensor

The septum of the signal rotor is also called a blade with a window on it. The signal corresponding to the window is a low-level signal, and the signal corresponding to the septum (blade) is a high-level signal.

The hall-type signal generator is mainly composed of a Hall integrated circuit, permanent magnet, and magnetic steel sheet. The Hall element is made of silicon semiconductor materials, and there is a gap of 0.2-0.4mm with the permanent magnet. When the signal rotor rotates with the intake camshaft, the septum and the window pass through the gap between the Hall IC and the permanent magnets.

The sensor connector socket has three lead terminals. Terminal 1 is the positive terminal of the sensor power supply and is connected to the ECU terminal 62. Terminal 2 is the sensor signal output terminal and is connected to the ECU terminal 76. Terminal 3 is the negative terminal of the sensor power supply connected to the ECU terminal 67.

5) Working Conditions

According to the working principle of the Hall sensor, when the septum (blade) enters the air gap, the Hall element does not generate a voltage, and the sensor outputs a high-level (5V) signal; when the septum leaves the air gap, the Hall element generates a voltage, and the sensor outputs a low-level signal (0.1V).

The relationship between the signal voltage output by the cam position sensor and the crankshaft position sensor is shown in Figure7. Every time the engine crankshaft makes two turns (720°), the Hall sensor signal rotor rotates one turn (360°), which generates a low-level signal and a high-level signal. The low-level signal corresponds to a certain angle before the compression top dead center of cylinder 1.

Figure 7. Relationship of Output Waveform of Camshaft Position Sensor and the Crankshaft Position Sensor

When the engine is working, the signal voltage generated by the magnetic induction crankshaft position sensor (CPS) and the Hall camshaft position sensor (CIS) is continuously inputted to the ECU. When the ECU simultaneously receives the low-level (15°) signal corresponding to the large tooth gap of the crankshaft position sensor and the low-level signal corresponding to the window of the camshaft position sensor, it can recognize that the piston of cylinder 1 is in the compression stroke, the piston of the cylinder 4 is in the exhaust stroke.

Besides, the ignition advance angle is controlled according to the output signal corresponding to the small tooth gap of the crankcase position sensor. After the ECU recognizes the position of the compression top dead center of cylinder 1, it can perform sequential fuel injection control and ignition timing control of each cylinder.

If the engine produces denotation, the ECU can also determine which cylinder has produced denotation based on the signal input from the denotation sensor, thereby reducing the ignition advance angle to eliminate denotation.

(4) Differential Hall Type Crankshaft Position Sensor

The differential Hall sensor is also called the dual Hall sensor, and its structure is similar to the magnetic inductive sensor, as shown in Figure 8-a. It consists of a signal rotor with convex teeth and a Hall signal generator. 

The working principle of the differential Hall sensor is the same as the ordinary Hall sensor. According to the working principle of the Hall sensor, when the missing teeth and convex teeth on the engine flywheel pass through the two probes of the differential Hall circuit, the air gap between the missing teeth or convex teeth and the Hall probe will change, and the magnetic flux will change accordingly.

An alternating voltage signal is generated in the Hall element, as shown in Figure 8-b. The output voltage is superimposed by two Hall signal voltages. Because the output signal is superimposed, the air gap between the convex teeth and the signal generator can be increased to 1± 0.5mm (common Hall sensor is only 0.2-0.4mm). So The signal rotor can be made into a toothed disc structure like the magnetic inductive sensor rotor, which is easy to install.

In automobiles, the convex tooth rotor is generally installed on the engine crankshaft or the engine flywheel.

Figure 8. Differential Hall Type Crankshaft Position Sensor

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