Operating mode of servo drive and test method of servo drive

The servo drive is a kind of controller used to control the servo motor. The servo drive functions like a frequency converter acting on a common AC motor and is part of the servo system. At present, the mainstream servo drivers adopt digital signal processor (DSP) as the control core, which can realize more complicated control algorithms, realize digitalization, networking and intelligence. The power device generally adopts the driving circuit designed with the intelligent power module (IPM) as the core. The IPM integrates the driving circuit internally, and has fault detection and protection circuits such as overvoltage, overcurrent, overheat and undervoltage, and adds soft in the main loop. Start the circuit to reduce the impact of the startup process on the drive.

The power driving unit first rectifies the input three-phase electric power or the commercial power through a three-phase full-bridge rectifying circuit to obtain a corresponding direct current power. After rectifying the three-phase electric or commercial power, the three-phase permanent-magnet synchronous AC servo motor is driven by the three-phase sinusoidal PWM voltage inverter. The entire process of the power drive unit can be simply the process of AC-DC-AC. The main topology circuit of the rectifier unit (AC-DC) is a three-phase full-bridge uncontrolled rectifier circuit.

Servo drives can generally be used in three control modes: position, speed and torque. They are mainly used in high-precision positioning systems and are currently the high-end of transmission technology. With the large-scale application of servo system, servo drive use, servo drive debugging and servo drive repair are all important technical issues of servo drives. More and more industrial control service providers have carried out in-depth technical research on servo drives.

Servo drives are an important part of modern motion control and are widely used in automation equipment such as industrial robots and CNC machining centers. Especially for servo drives for controlling AC permanent magnet synchronous motors, it has become a research hotspot at home and abroad. The current, servo, and position 3 closed-loop control algorithms based on vector control are commonly used in current AC servo drive design. The speed closed-loop design in this algorithm is reasonable or not, which plays a key role in the performance of the entire servo control system, especially the speed control performance.

In the servo drive speed closed loop, the real-time speed measurement accuracy of the motor rotor is essential to improve the dynamic and static characteristics of the speed loop. In order to balance the measurement accuracy with the system cost, an incremental photoelectric encoder is generally used as the speed measuring sensor, and the corresponding common speed measuring method is the M/T speed measuring method. Although the M/T velocity measurement method has certain measurement accuracy and wide measurement range, this method has its inherent defects, including:

(1) At least one complete code wheel pulse must be detected during the speed measurement period, limiting the minimum measurable speed;

(2) It is difficult to strictly maintain the synchronization of the two control system timer switches for speed measurement, and the speed measurement accuracy cannot be guaranteed in the measurement occasion where the speed changes greatly. Therefore, the conventional speed loop design using the speed measurement method is difficult to improve the servo drive speed following and control performance.

The servo drive can select operating modes: open loop mode, voltage mode, current mode (torque mode), IR compensation mode, Hall speed mode, encoder speed mode, tachometer mode, analog position loop mode (ANP mode). (The above modes are not all present in all models of the drive)

Open loop mode

The input command voltage controls the output load rate of the drive. This mode is used for brushless motor drives and has the same voltage mode as brushed motor drives.

Voltage mode

Enter the command voltage to control the output voltage of the driver. This mode is used for brushed motor drives and is the same as the open loop mode of brushless motor drives.

Current mode (torque mode)

The input command voltage controls the output current (torque) of the driver. The drive adjusts the load rate to maintain the command current value. This mode is generally included if the servo drive can operate at speed or position loop.

IR compensation mode

Enter a command to control the motor speed. The IR compensation mode can be used to control the speed of the motor without speed feedback. The driver adjusts the load rate to compensate for variations in the output current. When the command response is linear, the accuracy of this mode is inferior to the closed-loop speed mode in the case of torque disturbance.

Hall speed mode

Enter the command voltage to control the motor speed. This mode uses the frequency of the hall sensor on the motor to form a speed closed loop. Due to the low resolution of the hall sensor, this mode is generally not used for low speed motion applications.

Encoder speed mode

Enter the command voltage to control the motor speed. This mode uses the frequency of the encoder pulses on the servo motor to form a speed closed loop. Due to the high resolution of the encoder, this mode can be used for smooth motion control at various speeds.

Speedometer mode

Enter the command voltage to control the motor speed. This mode uses an analog tachometer on the motor to form a speed loop. Since the voltage of the DC tachometer is analog continuity, this mode is suitable for very high speed control. Of course, it is also susceptible to interference at low speeds.

Analog position loop mode (ANP mode)

Enter the command voltage to control the rotational position of the motor. This is actually a speed mode (such as an adjustable potentiometer, transformer, etc.) that provides a change in position feedback in an analog device. In this mode, the motor speed is proportional to the position error. And with a faster response and less steady state error.

Most of the domestic servo product technology research is still at the level of reliability, only reliable products can be recognized by the market, in order to truly bring value to its users. The lack of domestic servo reliability is reflected in the unstable power supply and insufficient device derating. These unreliable factors are mainly reflected in the reliability of electrical stress and thermal stress of key components, and secondly, the influence of electromagnetic disturbance on the performance of the circuit. A case study discusses power and device stress.

The most basic performance of the servo system is torque, speed, position accuracy and response speed. But when discussing servo performance, we must discuss it at the system level and include motor performance. In this paper, we discuss the test methods of torque response, speed response, positioning accuracy and repeat positioning accuracy.

1, auxiliary power supply short circuit protection test

The auxiliary power supply not only supplies power to the control chip, the driver chip, the interface circuit, and the fan, but also the servo driver supplies 24V power to the outside. Therefore, the short-circuit protection function of the switching power supply is particularly important. We take the minimum power supply voltage (DC200V), the normal power supply voltage (DC311V), and the highest power supply voltage (DC400V) to test the protection function of the auxiliary power supply.

During the test, the auxiliary power input is adjusted by adjusting the DC voltage regulator, and the bus power supply voltage is adjusted to DC200V, DC311V, DC400V respectively, and then the output is short-circuited accordingly. This article takes an actual product of 5V, 24V two-way output as an example. discuss. The test method is to short-circuit one of them and measure the other.

l 5V short circuit, measuring 24V output, as shown in Table 2-1:

l 24V short circuit, measuring 5V output, as shown in Table 2-2:

The test results show that when the 5V, 24V short circuit, the chip enters the snoring state, that is, meets the output short circuit protection test requirements.

2. Auxiliary power supply Topswitch voltage stress test

Topswitch device VDS voltage refers to the voltage between the drain and source of the internal IGBT of the integrated PWM controller. VDS is one of the main causes of damage. VDS directly affects the reliability and life of the servo driver. The test method is to regulate the voltage. The auxiliary power supply input voltage is adjusted to measure the VDS voltage. The higher the input voltage, the higher the VDS voltage, that is, the maximum VDS voltage at the busbar specification maximum (DC400V). If the maximum value is exceeded, it can be judged whether the Topswitch voltage stress is acceptable.

In another case, when the auxiliary power supply output is short-circuited, the VDS will be particularly high, and it is necessary to judge whether the Topsweitch voltage stress is qualified when the short circuit occurs.

l Test data when not short-circuited is shown in Table 2-3 below. The real shot waveform is shown in Figure 2-1:

When short-circuiting 5V, the VDS voltage is 650V less than 700 at the input of DC400V, which meets the specifications.

5V short circuit, VDS output waveform is shown in Figure 2-2:

3, auxiliary power start test

The auxiliary source startup time is very important for the reliability of the servo product, especially the impact of the power device and the power device driving power-on sequence. The power device must be powered on after the driver device is powered up. In order to ensure that the power device will not malfunction during power-on or power-off, and avoid serious short-circuit faults such as straight arm conduction.

In this example, when the input AC 220VAC, the test obtains a 5V output startup delay of 180ms, which is less than the IPM power-on startup time, which can ensure that the IPM driver chip works first, and the IPM internal IGBT works later to prevent power-on short-circuit and other faults. The delay waveform is shown in Figure 2-3:

l Sinusoidal signal is 50Hz input waveform

l Straight type signal is auxiliary source 5V output signal

4, auxiliary power supply ripple and noise test

(1) Output voltage test: Test the voltage values ​​of each channel under different busbars and full load conditions as shown in Table 2-4: (Unit: V)

Test result: qualified.

(2) Output voltage ripple test: The ripple is tested under the condition of different bus voltages, as shown in Table 2-5 (unit: mV)

Test result: qualified.

5, bus rectifier circuit test

(1) The rectification delay and the impact of the rectifier circuit on the grid are critical. The startup waveform of the rectifier circuit is shown in Figure 2-4. The startup delay time is 125ms, which satisfies the requirements.

(2) Figure 2-4 can reflect the charging time of the storage capacitor. From the perspective of safety, the discharge time is also critical. The discharge waveform of this design capacitor is shown in Figure 2-5. The discharge time of the capacitor is 7s, which satisfies the requirements.

6, IPM turn-off delay test

The turn-on and turn-off waveforms of the internal IGBT of IPM directly affect the reliability of IPM operation. If the turn-on and turn-off time is too long, there must be two situations. First, the short-circuit fault is caused by the vertical arm conduction of the upper and lower switch tubes, and the second is the IGBT. The turn-on and turn-off losses cause severe IPM heat generation. Long-term work will not only affect products other than servo drives, but also directly affect IPM life.

As shown in Figure 2-6, the above signal is the drive signal, and the IGBT turn-on signal delay is 500ns, which satisfies the requirements.

As shown in Figure 2-7, the above signal is the drive signal, and the IGBT turn-off signal is delayed by 500ns, which satisfies the requirements.

7, thermal stress test

As a product, users are most concerned about the reliability of the product. Reliability not only includes the electrical stress of each device, but also the thermal stress. It is especially important to study the temperature rise of each heating element.

Test Conditions:

l The entire servo drive is placed in an incubator environment.

l The ambient temperature is 22.5 °C.

l Test at full load full speed.

Temperature rise is the difference between the temperature of the measured component and the ambient temperature. The highest working ambient temperature defined by this product is 45 °C. This test is tested at an ambient temperature of 22.5 °C. It is known from the basic knowledge of heat that the temperature of the element at an ambient temperature of 45 ° C is 45 ° C plus the temperature rise at normal temperature. The test proves that the heating of the lower part of the whole machine in this design will not affect the heating of the upper half of the space device, and the heat transfer of the device in the switching power supply is not much different from the full load. The temperature and maximum temperature rise of each key component are shown in Table 2-6 below. The highest temperature rises to 26.8 degrees, which fully meets the design requirements. (in degrees Celsius)

1, torque response test

Test method: firmly fix the target motor and the motor shaft fixture (radially rotatable or fixed, similar to the indexing head commonly used in machine tools) on the test bench, and ensure that the motor shaft and the center of the fixture are concentric. Fix the motor shaft with the fixing device, as shown in Figure 3-1. The servo is enabled, the fixed device is rotated to maximize the U-phase current, and the U-phase current can reflect the magnitude of the torque. Under the condition of the torque command input of the step, the settling time of the U-phase current can reflect the torque response time.

Observation method: Observed with an oscilloscope, the observation time axis is set to 1ms, and the current rise time is the torque response time.

Experimental steps:

(1) The torque command is 30% rated analog torque, the fixed device is not fixed, the servo is ON, and the verification drive with the motor can run normally under the torque ring to ensure that the motor shaft rotates more than one turn.

(2) Servo OFF, the indexing head fixes the motor shaft, the current clamp is clamped on the U phase of the driver output, the U-phase current is observed by the oscilloscope, the servo is turned ON, the rotary fixture adjusts the motor shaft position, and the U-axis is displayed on the oscilloscope. When the phase current changes, when the U phase current is maximum, the rotary indexing head is stopped, the servo is turned OFF, and the fixing device is locked.

(3) The analog torque command is adjusted to 50% of the rated torque, the oscilloscope is set to the rising edge trigger, the servo ON is about 1 second, the servo is OFF, the oscilloscope captures the response current waveform and the torque waveform, the oscilloscope can not have the filter, save the experiment. Waveform and make a note. Repeat this experiment 5 times, and save the current response waveform under the same conditions 3 times.

(4) Adjust the analog torque command to 100% of rated torque and repeat step 3.

In this example, the torque loop response time is less than 4 milliseconds.

2, speed response test

Speed ​​bandwidth test method: Adjust the servo drive parameters to optimize the motor no-load response performance, limit the maximum speed to 3000RPM, and set the current to the rated motor current. The function signal generator sends a pulse signal whose frequency changes according to the sinusoidal law, and gradually increases the frequency of the sinusoidal change of the input signal. When the motor is blocked, the frequency of the sinusoidal change is defined as the servo driver speed response frequency, and the structure diagram of the speed bandwidth test platform is as follows. Figure 3-2 shows.

Acceleration performance test method: The test method using step response, in this example is to directly give a speed of 2500 rpm, observe the current waveform in the motor with an oscilloscope. As shown in Figure 3-3, the total acceleration to stabilization time in this example is less than 30ms.

3, positioning accuracy and repeat positioning accuracy test

The static error of the final positioning point and target value of the servo-driven AC permanent magnet synchronous servo motor is called positioning accuracy. Repeated positioning accuracy is the degree to which the motor continuously rotates at a certain angle under the same speed and acceleration and deceleration conditions.

Repeat positioning accuracy test method: The self-made pulse generator sends pulses to the servo driver at three different frequencies. The number of pulses is 30,000. The servo motor is controlled to rotate 10 turns, then reverse 10 turns, observe the error between the positioning position and the starting position, and the difference between each positioning position, and record three sets of data. Then control the servo motor to rotate 10 turns, then reverse 20 turns, and then record three sets of data.

Position deviation detection: As shown in Figure 3-4, the laser pointer is fixed on the motor shaft. When each operation stops, record the position of the test wall spot and record the error.

The test experiment is divided into the following two steps:

1) The transmission frequency of the test pulse is set to 500hz, and the generation period is 3s, that is, 1500 pulses are sent every 3s. At this time, the electronic gear ratio of the servo driver is 100/3; then the forward rotation is 10 revolutions, then the reverse rotation is 10 rotations. (After the electronic gear shifting, the motor needs one revolution per revolution, 10000 pulses, the motor will rotate half a turn every 3 seconds), the direct distance between the motor shaft and the wall surface is 3m, and the maximum deviation of the laser pen projected onto the wall surface is 2mm, after repeated tests, the results are consistent. The deviation angle can be calculated by using the trigonometric function relationship, and then 300 pulses corresponding to 360°, and the calculation result is that the positioning accuracy is less than 1 pulse. That is, the servo motor positioning accuracy is 1 pulse, which meets the design requirements.

2) The transmission frequency of the test pulse is set to 500hz, and the generation period is 3s, that is, 1500 pulses are sent every 3s. At this time, the electronic gear ratio of the servo driver is 100/3; then the forward rotation is 10 rotations, then the reverse rotation is 20 rotations. The direct distance between the motor shaft and the wall surface is 3m, and the maximum deviation of the laser pen projected onto the wall surface is also 2mm. After repeated tests, the results are consistent. The result of the calculation is that the positioning accuracy is less than 1 pulse. That is, the servo motor repeat positioning accuracy is 1 pulse, which meets the design requirements.

It is recalled that the domestic research on servo technology is very close to the foreign level, but these research results are mostly at the theoretical level and there is no productization. Due to the late start, the development of domestic servo drives has remained at the level of reliability and anti-interference considerations. The research on performance has gradually become the subject of domestic servo drive developers. With the development of electronic devices, the development of electronic processing technology, and the growth of domestic servo manufacturers, it is believed that servo drive products with higher reliability, better performance and more powerful functions will appear.