Speed control and torque control are both achieved using analog signals. Position control is achieved by sending pulses. The specific control method used depends on the customer's requirements and the desired motion function.
If you don't have requirements for the speed or position of the motor , and only need to output a constant torque, then of course you should use torque mode.
If you have certain accuracy requirements for position and speed, but are not very concerned about real-time torque, torque mode is not very convenient, and speed or position mode is better.
If the host controller has good closed-loop control capabilities, speed control will be more effective.
If the requirements are not very high, or there are basically no real-time requirements, position control does not place high demands on the host controller.
In terms of servo drive response speed, torque mode has the least computational load and the fastest response to control signals; position mode has the greatest computational load and the slowest response to control signals.
When high dynamic performance is required during motion, real-time adjustments to the motor are necessary. If the controller's processing speed is slow (e.g., a PLC or a low-end motion controller), position control is used. If the controller's processing speed is fast, speed control can be used, moving the position loop from the driver to the controller, reducing the driver's workload and improving efficiency (as seen in most mid-to-high-end motion controllers). If a better host controller is available, torque control can also be used, moving the speed loop away from the driver as well. This is generally only possible with high-end dedicated controllers, and in this case, servo motors are completely unnecessary.
Generally speaking, when it comes to judging the quality of a driver control, each manufacturer claims theirs is the best. However, there's now a more intuitive way to compare them called response bandwidth. During torque or speed control, a square wave signal is sent to the motor via a pulse generator, causing it to continuously rotate forward and reverse, constantly increasing the frequency. The oscilloscope displays a frequency sweep signal. When the peak of the envelope reaches 70.7% of the maximum value, it indicates that synchronization has been lost. The frequency at this point reveals which product is superior. Typical current loops can achieve over 1000Hz, while speed loops can only reach tens of hertz.
To put it in a more professional way:
1. Torque Control: Torque control is achieved by setting the output torque of the motor shaft through external analog input or direct address assignment. For example, if 10V corresponds to 5Nm, then when the external analog input is set to 5V, the motor shaft output will be 2.5Nm. If the motor shaft load is below 2.5Nm, the motor rotates forward; if the external load is equal to 2.5Nm, the motor does not rotate; and if the load is greater than 2.5Nm, the motor rotates in reverse (typically occurring under gravity loads). The torque setting can be changed in real-time by altering the analog input, or it can be achieved by changing the corresponding address value via communication.
The main applications are in winding and unwinding devices where there are strict requirements on the stress on the material, such as wire winding devices or optical fiber drawing equipment. The torque setting must be changed at any time according to the change of the winding radius to ensure that the stress on the material does not change with the change of the winding radius.
2. Position Control: Position control typically determines the rotation speed by the frequency of externally input pulses and the rotation angle by the number of pulses. Some servo systems can also directly assign speed and displacement values via communication. Because position control allows for very strict control over both speed and position, it is generally used in positioning devices.
Application areas include CNC machine tools, printing machinery, etc.
3. Speed Mode: Rotation speed can be controlled via analog input or pulse frequency. With an external PID control system connected to a higher-level controller, speed mode can also be used for positioning, but the motor position signal or the position signal from the direct load must be fed back to the higher-level controller for calculation. Position mode also supports direct load external loop position signal detection. In this mode, the encoder at the motor shaft end only detects the motor speed, and the position signal is provided by the detection device at the direct final load end. This reduces errors in the intermediate transmission process and increases the overall positioning accuracy of the system.
4. Let's talk about the three-loop system. Servo systems generally use three-loop control, which refers to three closed-loop negative feedback PID control systems. The innermost PID loop is the current loop, which operates entirely within the servo driver. It uses Hall effect sensors to detect the output current of each phase of the motor and provides negative feedback to the current setting for PID adjustment, thereby achieving an output current as close as possible to the set current. The current loop controls the motor torque, so in torque mode, the driver's computation is minimized, resulting in the fastest dynamic response.
The second loop is the speed loop, which uses the signal from the motor encoder for negative feedback PID control. Its PID output is directly set by the current loop. Therefore, speed loop control includes both speed loop and current loop. In other words, the current loop must be used in any mode. The current loop is the foundation of control. While controlling speed and position, the system is also controlling the current (torque) to achieve corresponding control of speed and position.
The third loop is the position loop, which is the outermost loop. It can be built between the driver and the motor encoder, or between the external controller and the motor encoder or the final load, depending on the actual situation. Since the output of the position control loop is the setting of the speed loop, the system performs calculations for all three loops in position control mode. At this time, the system has the largest computational load and the slowest dynamic response speed.
What is the lifespan of a servo motor?
The rated speed is irrelevant. Servo motors have a concept of stall torque, which is actually measured at 1 rpm. As long as your motor is of good quality and the overload during use is not severe and is within the tolerance range, there should be basically no impact.
The lifespan of an electric motor mainly depends on its bearings, which are among the shortest-lived components. If you install it with high precision and good concentricity, you can use it with confidence. If it does burn out, it's the driver's fault for not having temperature protection.
The rotor of an AC servo motor is usually made into a squirrel-cage type. However, in order to make the servo motor have a wide speed range, linear mechanical characteristics, no "self-rotation" phenomenon and fast response performance, it should have the characteristics of high rotor resistance and low moment of inertia compared with ordinary motors.
Currently, there are two main types of rotor structures: one is a squirrel-cage rotor with high-resistivity conductor bars made of high-resistivity conductive material, which is made slender to reduce the rotor's moment of inertia; the other is a hollow cup-shaped rotor made of aluminum alloy with very thin walls, only 0.2-0.3 mm. To reduce the magnetic resistance of the magnetic circuit, a fixed inner stator is placed inside the hollow cup-shaped rotor. The hollow cup-shaped rotor has a very small moment of inertia, responds quickly, and runs smoothly, so it is widely used.
01
wiring
Power off the control card and connect the signal cables between the control card and the servo. The following cables are mandatory: the analog output cable of the control card, the enable signal cable, and the encoder signal cable output by the servo. After verifying that the wiring is correct, power on the motor and the control card (and PC). The motor should not move at this point and can be easily rotated with external force. If not, check the enable signal settings and wiring. Rotate the motor with external force to check if the control card can correctly detect changes in motor position; otherwise, check the encoder signal wiring and settings.
02
Try direction
For a closed-loop control system, if the direction of the feedback signal is incorrect, the consequences will be disastrous. Enable the servo via the control card. The servo should then rotate at a low speed; this is the so-called "zero drift." Most control cards have instructions or parameters to suppress zero drift. Use these instructions or parameters to see if the motor's speed and direction can be controlled. If not, check the analog wiring and control mode parameter settings. Confirm that a positive value results in the motor rotating forward and the encoder count increasing; a negative value results in the motor rotating backward and the encoder count decreasing. Do not use this method if the motor is under load and has limited travel. Do not apply excessive voltage during testing; below 1V is recommended. If the directions are inconsistent, modify the parameters on the control card or motor to make them consistent.
03
Suppress zero drift
In closed-loop control, zero drift can negatively impact control performance, and it's best to suppress it. Carefully adjust the zero drift suppression parameters on the control card or servo motor to bring the motor speed close to zero. Since zero drift itself has a degree of randomness, it's not necessary to require the motor speed to be absolutely zero.
Servo motors have the function of emitting pulses. So, for every angle a servo motor rotates, it will emit a corresponding number of pulses. This forms a response, or closed loop, with the pulses received by the servo motor. In this way, the system knows how many pulses were sent to the servo motor and how many pulses were received back. This allows for very precise control of the motor's rotation, thereby achieving precise positioning, which can reach 0.001mm.
DC servo motors are divided into brushed and brushless motors. Brushed motors are low in cost, simple in structure, have high starting torque, wide speed range, and are easy to control. However, they require maintenance, which is inconvenient (replacing carbon brushes), generates electromagnetic interference, and has environmental requirements. Therefore, they can be used in cost-sensitive general industrial and civilian applications.
AC servos are used in 17 fields.
AC servo drives borrow and apply frequency conversion technology. Based on the servo control of DC motors, they mimic the control method of DC motors through frequency conversion PWM. In other words, AC servo motors inevitably involve a frequency conversion stage. Similar to frequency converters, they first rectify the mains frequency AC power into DC power, and then use various gate-controllable transistors (IGBTs, IGCTs, etc.) to invert it into frequency-adjustable AC power through carrier frequency and PWM adjustment. The waveform resembles a pulsating sine and cosine wave.
Servo drives have developed variable frequency technology. The current loop, speed loop and position loop inside the drive (the variable frequency drive does not have this loop) have more precise control technology and algorithm calculations than general variable frequency drives. The main point is that it can perform precise position control.
The control part of modern AC servo systems uses digital signal processors (DSPs) as the control core. Its advantage is that it can implement relatively complex control algorithms to complete the closed-loop control of the servo system, including closed-loop control of torque, speed, and position.
Application areas of AC servos:
AC servo drives are suitable for applications requiring high precision in position, speed, and torque control. Examples include machine tools, printing equipment, packaging equipment, textile equipment, laser processing equipment, robots, electronics, pharmaceuticals, financial equipment, and automated production lines. Because servos are primarily used for positioning and speed control, they are also known as motion control systems.
1. Metallurgy and steel industry - continuous casting billet production line, copper rod upward continuous casting machine, inkjet marking equipment, cold continuous rolling mill, fixed length shearing, automatic feeding, converter tilting.
2. Electricity and cables—hydro turbine speed governors, wind turbine pitch control systems, wire drawing machines, twisting machines, high-speed braiding machines, winding machines, inkjet marking equipment, etc.
3. Petroleum and chemical industries—extrusion presses, film conveyor belts, large air compressors, oil pumps, etc.
4. Chemical fiber and textiles -- spinning machines, fine spinning machines, looms, carding machines, selvedge machines, etc.
5. Automotive manufacturing industry—engine parts production lines, engine assembly production lines, vehicle assembly lines, body welding lines, testing equipment, etc.
6. Machine tool manufacturing industry—lathes, gantry planers, milling machines, grinding machines, machining centers, gear making machines, etc.
7. Casting manufacturing industry—robotics, converter tilting, mold processing centers, etc.
8. Rubber and plastics manufacturing industry -- plastic calendering machines, plastic film bag sealing and cutting machines, injection molding machines, extruders, molding machines, coating and laminating machines, wire drawing machines, etc.
9. Electronics Manufacturing Industry – Printed Circuit Board (PCB) Equipment, Semiconductor Device Equipment (Lithography Machines, Wafer Processing Machines, etc.), Liquid Crystal Display (LCD) Equipment, Assembly and Surface Mount Technology (SMT) Equipment, Laser Equipment (Cutting Machines, Engraving Machines, etc.), General CNC Equipment, Robots, etc.
10. Paper industry – paper conveying equipment, specialty paper making machinery, etc.
11. Food manufacturing industry—raw material processing equipment, filling machinery, sealing machines, other food packaging and printing equipment, etc.
12. Pharmaceutical Industry – Raw material processing machinery, formulation machinery, processed medicinal materials machinery, printing and packaging machinery, etc.
13. Transportation—Subway platform screen doors, electric locomotives, ship navigation, etc.
14. Logistics, loading and unloading, handling—automated warehouses, pallet trucks, automated parking systems, conveyor belts, robots, lifting equipment, and handling equipment, etc.
15. Buildings – Elevators, conveyor belts, automatic revolving doors, automatic windows, etc.
16. Medical equipment—CT scanners, X-ray machines, MRI scanners, etc.
17. Testing Equipment—Automotive testing equipment, torque testing equipment, etc.
In short, mastering servo drive applications allows for the design of many high-end products, which is of great significance for career development. In the coming period, we will focus on learning servo drive applications.
What are the different control modes of a servo system?
Stepper motors, as an open-loop control system, are fundamentally linked to modern digital control technology. They are widely used in current domestic digital control systems. With the emergence of fully digital AC servo systems, AC servo motors are also increasingly being applied in digital control systems. To adapt to the development trend of digital control, most motion control systems use stepper motors or fully digital AC servo motors as actuators. Although they are similar in control methods (pulse trains and direction signals), they differ significantly in performance and application scenarios. A comparison of their performance is presented below.
1. Different control precision
Two-phase hybrid stepper motors typically have step angles of 1.8° or 0.9°, while five-phase hybrid stepper motors typically have step angles of 0.72° or 0.36°. Some high-performance stepper motors can have even smaller step angles after microstepping. For example, the two-phase hybrid stepper motors manufactured by SANYODENKI can have their step angles set to 1.8°, 0.9°, 0.72°, 0.36°, 0.18°, 0.09°, 0.072°, and 0.036° via DIP switches, making them compatible with both two-phase and five-phase hybrid stepper motors.
The control precision of an AC servo motor is ensured by a rotary encoder at the rear end of the motor shaft. Taking a Sanyo all-digital AC servo motor as an example, for a motor with a standard 2000-line encoder, due to the quadruple frequency technology used in the driver, its pulse equivalent is 360°/8000 = 0.045°. For a motor with a 17-bit encoder, the motor rotates once for every 131072 pulses received by the driver, meaning its pulse equivalent is 360°/131072 = 0.0027466°, which is 1/655 of the pulse equivalent of a stepper motor with a step angle of 1.8°.
2. Different low-frequency characteristics
Stepper motors are prone to low-frequency vibration at low speeds. The vibration frequency is related to the load and driver performance, and is generally considered to be half of the motor's no-load starting frequency. This low-frequency vibration, determined by the working principle of stepper motors, is very detrimental to the normal operation of the machine. When stepper motors operate at low speeds, damping techniques should generally be used to overcome low-frequency vibration, such as adding a damper to the motor or using microstepping technology in the driver.
AC servo motors operate very smoothly, without vibration even at low speeds. AC servo systems feature resonance suppression to compensate for insufficient mechanical rigidity, and internal frequency response testing (FFT) can detect mechanical resonance points, facilitating system adjustments.
3. Different torque-frequency characteristics
The output torque of a stepper motor decreases as the speed increases, and drops sharply at higher speeds. Therefore, its maximum operating speed is generally between 300 and 600 RPM. AC servo motors provide constant torque output, meaning they can output rated torque up to their rated speed (generally 2000 or 3000 RPM), and provide constant power output above the rated speed.
4. Different overload capacities
Stepper motors generally lack overload capacity. AC servo motors, on the other hand, have strong overload capacity. Taking the Sanyo AC servo system as an example, it has both speed and torque overload capabilities. Its maximum torque is two to three times its rated torque, which can be used to overcome the inertial torque of inertial loads at startup. Because stepper motors lack this overload capacity, a motor with a larger torque is often selected to overcome this inertial torque during selection. However, the machine does not require such a large torque during normal operation, resulting in wasted torque.
5. Different operating performance
Stepper motors are controlled in an open-loop manner. Excessive starting frequency or load can easily lead to missed steps or stalling. Excessive stopping speed can cause overshoot. Therefore, to ensure control accuracy, the acceleration and deceleration issues must be properly addressed. AC servo drive systems, on the other hand, use closed-loop control. The driver can directly sample the feedback signal from the motor encoder, internally forming position and speed loops. Generally, the missed steps or overshoot issues of stepper motors are not present, resulting in more reliable control performance.
6. Different speed response performance
Stepper motors require 200–400 milliseconds to accelerate from a standstill to their operating speed (typically several hundred revolutions per minute). AC servo systems offer better acceleration performance; for example, the Panasonic MSMA400W AC servo motor accelerates from a standstill to its rated speed of 3000 RPM in just a few milliseconds, making it suitable for control applications requiring rapid start and stop.
In summary, AC servo systems outperform stepper motors in many aspects. However, stepper motors are often used as actuators in less demanding applications. Therefore, the design of a control system must comprehensively consider factors such as control requirements and cost to select an appropriate control motor.
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