Introduction The division of labor in modern society is increasingly refined. Regardless of business or user, only focusing on the best job can maximize profits. Most of the users of stepper motors do not study the control and optimization of the motor, but apply it to a certain occasion. Based on this, foreign countries have begun to prevail in the integration of stepping systems, that is stepper motors, feedback devices, drive amplifiers, motion controllers combined into a whole. Its advantages are: small size, low failure rate, no need to match the motor and drive controller, easy to use, system design and maintenance convenience, greatly reducing product development time.
However, foreign integrated stepping systems are expensive, typically between $200 and $1,000/axis. In view of the actual situation in the country, Shanghai Youyibao Company has introduced integrated stepping system comprehensive technology from the United States to provide integrated stepper motor drive control systems for domestic customers at a price suitable for China. Its low-end products have a slightly lower price than similar domestic products. There is no similar high-end product in China, and its price is one-tenth that of foreign products. The key components are all purchased by U.S. partner companies in the United States.
For the current stepper motor user selection type integrated stepper motor micro drive controller some of the doubts are as follows.
The integrated stepper system consists of stepper motors, feedback systems (optional), drive amplifiers, motion controllers and other subsystems. If the user PC (PC, PLC, etc.) is compared to the company owner, the motion controller is the executive, the driver amplifier is the mechanic, and the stepper motor is the machine tool. The boss coordinates the cooperation among several senior managers through a certain communication method/protocol (telephone, telegraph, email, etc.). According to the boss’s intentions, the executives made detailed work plans to inform their technicians. The craftsman opened the machine to complete the production task according to the plan of the senior executive. At the same time, because the machine (stepping motor) is of good quality, it is generally not defective. However, in order to ensure quality, quality control personnel (ie, feedback systems) may be added to notify the senior management and the boss of the quality problems to determine the countermeasures. The boss will not open the machine or understand production, but with the help of executives and mechanics, the boss can still lead multiple executives to complete complex tasks. Executive + mechanic + machine (sometimes with quality inspection) is an integrated stepper motor system, while the user is the boss. The basic considerations for the selection of each subsystem are briefly described below.
Stepper motors Stepper motors can convert electrical pulse signals to angular or linear displacements. Within the rated power range, the speed of the motor depends only on the frequency and pulse number of the pulse signal, and is not affected by the load change, plus the small accumulated error of the stepper motor, making the stepping in the speed, position and other fields. Motor control becomes simpler. Stepper motors are divided into three types. Currently, hybrid stepper motors are widely used. When using a stepper motor, the following factory parameters must be noted (both provided with the motor):
1) Step Angle: The angle at which the motor rotates after receiving a stepping pulse. The actual step angle is related to the number of subdivisions of the drive. The accuracy of a typical stepper motor is 3-5% of the step angle and does not accumulate.
2) Number of phases: Number of coils inside the motor. The number of phases is different, and the step angle is different. If you use a subdivision driver, the 'phase number' is meaningless: you can change the step angle by changing the number of subdivisions.
3) Hold torque: Also called maximum static torque. Refers to the torque required by the external force to force the rotor to rotate when the speed is zero at rated current. The holding torque has nothing to do with the drive voltage, drive power, and so on. The torque of the stepper motor at low speed approaches the holding torque. Since the output torque and power of the stepper motor constantly change with increasing speed, the holding torque is one of the most important parameters for measuring the stepper motor. Although the holding torque is proportional to the number of electromagnetically excited ampoules, it is related to the air gap between the stators. However, it is not advisable to excessively reduce the air gap and increase the excitation amps to increase the static torque. This may cause heating and mechanical noise of the motor. The selection of holding torque is determined as follows:
The dynamic torque of a stepper motor is difficult to determine at once, and the static moment of the motor is often determined first. The choice of static torque is based on the load of the motor, and the load can be divided into two kinds of inertial load and friction load. A single inertial load and a single frictional load do not exist. The two types of loads are considered when stepping (abruptly) is started (usually by low speed). The inertial load is mainly considered during the acceleration (ramp) start, and only the friction load is taken into account during the constant speed operation. Under normal circumstances, the holding torque should be within 2-3 times of the friction load. Once the holding torque is selected, the seat and length of the motor can be determined.
4) Rated phase current: refers to the current per phase (per coil) when the motor implements various rated factory parameters. Experiments show that higher and lower than this current can cause certain indicators to exceed the standard while the other indicators are not up to standard.
Some other things worth noting are:
Vibration and noise at low speeds. Essentially, each pulse is an excitation to the motor system. When the excitation frequency is close to the motor's natural frequency, the rotor will have a large vibration. Therefore, vibration and noise are its inherent characteristics. The general speed is 1 rev/sec, and the maximum vibration is 2 rpm. Can be used to change the reduction ratio, subdivision drive and other methods to slow down.
When selecting the power supply, the voltage is usually selected according to the operating speed and response requirements of the motor. The higher voltage is selected when the rotation speed is higher or the response is faster. The current is generally determined based on the output phase current peak of the driver. The linear power supply takes 1.1 to 1.3 times the phase current, and the switching power supply takes 1.5 to 2 times the phase current.
The motor current is cut off when the machine is offline and the rotor is in a free state (offline state). If you want to turn the motor shaft manually when power is off, you can use it offline.
If the operating temperature is too high, demagnetization will result in a drop in torque and loss of synchronism. The general demagnetization point is greater than 130 degrees Celsius, so the motor surface is 80-90 degrees Celsius is normal.
The maximum pulse frequency that the motor can start normally without load is called the no-load start frequency. If it is higher than this value, it will not start normally. In the case of a load, the starting frequency should be lower.
The best advantage of the stepper motor is that the feedback system is different from all other motors. If the working process is known and the impeding torque is within the motor's capabilities, the feedback system is not required. The addition and signal acquisition processing of the feedback system will rapidly increase the cost and overlap with the specialty of the stepper motor, so unless it is necessary, generally do not feedback the system. In this case, some high-end drive controllers from Shanghai You Aibo also provide several feedback signal input ports, which can accept analog signals (with 12-bit A/D conversion) and digital inputs, and send them back to the corresponding communication protocol. user.
The driver amplifier driver amplifier includes a logic interpretation module and a current amplification module. The drive amplifier accepts input of step pulses, direction signals, etc., and outputs two (or several) sets of controlled currents to two (or several) windings of the motor.
The logic interpretation module makes the coil (winding) current switch in turn according to the step pulse, direction signal, and offline signal sent by the user's host computer, so that the motor rotor rotates in steps. There are only two types of motor operation, that is, full-step operation and half-step operation. The step angle is determined by the motor structure. When the basic step angle can not meet the requirements, subdivision drive is needed. That is, each time the input pulse is switched, the current is not turned on or off, but only a part of the current of the corresponding winding is changed, so that the rotor rotates only a part of the basic step angle. The subdivision function is generated by the driver by precisely controlling the phase current of the motor and has nothing to do with the motor. After the subdivision, the output current changes relatively smoothly, and the output torque is increased to some extent, the low frequency oscillation of the motor is weakened, and the step accuracy is improved. The specific degree of improvement depends on the degree of subdivision and the quality of the drive circuit. For motors of better quality, generally 16 to 32 subdivisions have no significant effect, but can slightly reduce vibration and noise.
The current amplification module amplifies the logic signal into operating current. More commonly used is an H-bridge circuit composed of field effect transistors. The FET can be an electronic switch, just like a light switch. The usual way to adjust the current using this on-off switch is chopping constant current. Constant current is constant current. The chopping wave is to cut the wave higher than needed. For example, add water to a leaking cup. If the rate of water addition is greater than the leak rate, stop and stop it to prevent overflow. When the FET is turned on, the current rises. When the current just exceeds the setting, the FET is turned off and the current is cut and dropped. When it falls below the setting, the FET turns on again and the current rises again. Repeatedly switch with the motor coil inductance, the current is maintained at the set value. It should be noted that some drive amplifiers generate heat (not temperature) and some are small. The main reason is that the internal resistance of most FETs is several hundred milliohms. Most of the Shanghai YOBO company's driver FET internal resistance is between 10-30 milliohms, so the heat is much smaller than the ordinary drive. The surface temperature of the driver is related to the amount of heat generated and the area of ​​heat dissipation. Because of the small size (matchbox size), Shanghai You Aibo Drives are small in heat (usually less than 1-2W) but still need to be installed in some high current (4-8A) working places. The flange plate is fixed to the motor or the cabinet to dissipate heat.
The motion controller motion controller includes a motion control module and a communication module. The motion controller is responsible for communicating with the user's host computer and controlling the drive amplifier to control the motor according to the requirements of the host computer.
In order for the user's upper computer to focus on high-level control, the motion controller must have sufficient intelligence. The function and principle of the motion control module involve more complicated control technology, which is simplified here. Here is the user-specific communication module. In most cases, the motion controller is responsible for implementing user instructions. The transfer of instructions involves software (protocols, instruction structures) and hardware. The hardware is similar to the phone/line in the previous example, PC/network cable, etc. The agreement (telephone or email) is set, and the hardware is basically set. The instruction structure is like the grammatical structure of speech, the structure of vernacular Chinese, the structure of classical Chinese, and so on, and the two sides understand it well.
At present, industrial common protocols include RS232, RS485, and CAN. RS232 is the simplest, but it has poor anti-interference and short transmission distance. RS485 and RS232 are almost simple, slightly stronger anti-interference, longer transmission distance. The CAN bus protocol is widely used in automobiles (engines, sensors, and electronic control systems, etc.), automated manufacturing, and transportation by its high speed (1 million bit rate), long distance (10,000 meters), and high anti-interference characteristics. Severe interference signals such as management are important for personal safety. The CAN bus requires only two wires to form the network. The network structure is similar to the festival lantern, and a total of two hundred power lights are hung from the beginning to the end of the two power cords. The CAN protocol guarantees that no node will crash in order to compete for the bus. However, the CAN protocol is relatively complicated and difficult to grasp.
Shanghai You Aibo's RS232 serial driver uses RS232 to communicate with the user's computer. It is worth noting that RS232 can reach very high speeds within a short distance. With suitable peripheral devices, the speed can reach 1M or more. The CAN bus series driver packages the CAN bus and protocol, and then uses an easy-to-use RS232 and user interface. Therefore, the user does not need to understand and deal with the complicated operation modes of the CAN bus, and can also fully enjoy the superior performance of the CAN bus such as high-speed, long-distance, high anti-interference, network functions, and simple connection. All instructions are passed through the CAN bus (in the work site, such as the plant, equipment), only in the import and export of the user's host computer CAN protocol and RS232 protocol bidirectional translation / conversion. Excluding the two power cords, there is no difference between the CAN series controllers and lanterns of Shanghai You Aibao: two signal lines can hold up to 100 drives.
In terms of instruction structure, the fool-type user interface makes it unnecessary for the user to understand the stepper motor, the driver, and the CAN bus. The instruction structure is simple, intuitive and highly fault-tolerant. For example, to execute speed = 1000 steps/sec, the following commands are all valid: "SPD=1000;", "SPD:1000;", "SPD 1000;", "SPD1000;" or even "SPD%$&*1000;" As soon as an erroneous instruction is entered, the driver returns an error message and the erroneous instruction is not executed.
In a nutshell, the characteristic of the integrated stepping system is that the boss (user machine) sends a sentence and the younger brother (all-in-one system) is all set and can report to the boss at any time.