Automated precision manufacturing has driven the development and widespread use of many high-tech devices today. Stylish and sophisticated mobile phones rely on complex metal processing techniques and fine surface treatment capabilities to produce the chips and molds needed for manufacturing mechanical components. The production of tiny but powerful electronic devices in mobile phones depends on automated IC wafer processing and precision wire bonding equipment. Large equipment also requires high-precision and high-quality surface treatments. For example, modern jet engines rely on finely balanced and precisely matched turbine blades to achieve high fuel efficiency and quiet operation. Advanced electronic controls and complexly shaped precision engine components optimize the combustion process, improving the fuel efficiency of automobile engines.
A milling machine moves a high-speed rotating cutting tool along a predetermined path to cut solid metal blocks, producing metal parts. Precision machining is a multi-step process, starting with rough cutting followed by multiple fine cuts to achieve the desired finish. Multiple motors drive the feed spindle and multiple lead screws to position the tool head. The power and stiffness of the motor position and speed servo drives determine the maximum cutting rate that supports a specific level of surface finish precision. Therefore, high-performance motor drives can increase the cutting rate or reduce the number of cuts, directly impacting the efficiency of the milling process. Selecting the optimal motion scheme for each operation and minimizing tool change time also improves productivity and energy efficiency. Product quality depends on the accuracy of the lead screws and the position and speed control of the motor drive axes. The latest milling machines have five or more control axes, supporting the machining of complex shapes with a minimal number of workpiece setup operations. Specialized machining centers used in high-volume production lines even include more servo drives, supporting parallel operations of multiple metal processing steps and robotic-like functions, enabling full automation of the machining process. The challenge for machine designers is to synchronize the operation and motion schemes of multiple servo drive axes to maximize machine throughput while maintaining product quality.
Precision motion control
Figure 1 shows the various components controlling the automated machines used in modern factories. A central digital controller (CNC) or programmable logic controller (PLC) manages machine operation and generates motion trajectory planning for each servo motor axis in the machine. Each servo drive includes multiple control loops to manage the dynamic characteristics of the mechanical system, electromagnetic torque generation, and circuit dynamics. The performance of each control element is crucial to machine throughput efficiency and surface finish quality. Computer-aided manufacturing (CAM) tools generate the necessary machining operation combinations based on product drawings, material properties, machine and tooling capabilities. These plans are then executed by the automated machine to manufacture the product.
Figure 1. Automated machine control system
Complete machine control functionality comprises multiple cascaded control loops. Considering the transmission mechanism provided by the lead screw (used to convert rotation into linear motion), the CNC translates the machine space (x, y, and z) motion configuration into a (θ or ω) motion configuration for each motor axis. Each motion configuration is defined by a set of positions or velocities in time. Timing synchronization between axes is crucial because timing errors have the same impact on an axis as position and velocity errors.
The function of the servo drive speed loop is to calculate the motor torque command (T*) required to follow the target speed curve. The accuracy and surface quality of the finished product depend on the machine's ability to accurately guide the cutting tool along the target path. The challenge of machining operations lies in the discontinuous nature of metal cutting, as material is shed in fragments, resulting in rapid changes in the servo drive load. The speed loop must be able to maintain a constant speed during cutting operations unaffected by load variations and respond quickly to speed commands during tool changes. Control quality at low speeds is highly dependent on the resolution of the position feedback, necessitating a high-sampling-rate differentiator to generate a high-dynamic-range speed signal.
Precision encoders used in machine tool drives employ fast analog-to-digital converters to interpolate between encoder counts, providing higher resolution. For example, a 4096-line encoder with a simple digital interface can provide 14 bits/revolution position resolution, while using interpolation methods, its resolution can be extended to at least 22 bits/revolution. With the position resolution increased to 22 bits, a sampling rate of 4 kHz can be achieved at 4-bit speed resolution and 1 RPM, compared to only 1 kHz previously at 4-bit speed resolution and 60 RPM.
In permanent magnet AC servo motors, to generate torque efficiently and dynamically, the sinusoidal stator current must be aligned with the rotor magnet angular position, as shown in Figure 2. Current and field alignment control ensures that the motor torque meets the dynamic requirements of the speed loop. PWM and inverter feedback isolation modules are included in the circuit control function. The three-phase power inverter applies the required voltage to the motor windings to drive the target winding current. The current feedback function isolates the winding current measurement from the high-voltage inverter and provides a feedback signal to the field alignment module. The accuracy of the current feedback determines the quality of torque generation, as gain, offset, or nonlinear errors in the feedback will generate ripple torque, which in turn manifests as load interference to the speed controller. In some precision servo drives, an additional loop also compensates for the internal torque ripple of the servo motor caused by the interaction between the stator winding slots and the rotor magnets. All of these improve the low-speed performance of the motor, ultimately enhancing the precision and surface quality of the finished product.
Driver Architecture
As mentioned above, drive system performance is determined by multiple factors, such as control architecture, motor design, power circuitry, feedback sensors, and control processor. Faced with ever-increasing demands for drive performance, flexibility, and cost, as well as advancements in analog and digital electronic control components, control architectures are constantly evolving. Traditional servo control based on analog circuits has been replaced by digital control using embedded processors. Furthermore, CNC speed command signals, originally precision analog signals, are now data packets transmitted via real-time (RT) industrial networks. Therefore, in addition to control and power circuitry, modern servo drive systems also include communication interfaces.
A persistent circuit design challenge in drive systems is safely isolating high-voltage power circuitry from user-connected control and communication circuitry. A common architecture to mitigate inverter signal isolation difficulties is a direct ground connection between the power circuitry and the control processor, with an isolation barrier between the control processor and the communication interface. A more common architecture choice for servo drive applications is to place the safety barrier between the power stage and the control processor, while the control processor is directly connected to the communication interface. A less common architecture distributes the safety barriers among the power, control, and communication stages. This reduces the isolation standard requirements for each barrier and allows for a smaller overall system size.
Figure 3 illustrates an example of an isolated control architecture where the inverter gate drive, voltage feedback, and motor current feedback signals are isolated from the control processor but directly connected to the position feedback sensor, user interface, and communication interface. This architecture not only provides secure isolation for the control circuitry but also suppresses circuit noise generated by the high-voltage switching power supply inverter. The motor current feedback is generated by winding shunts and isolated Σ-Δ modulators, which provide gain matching, very low offset, and very high linearity. The complete current feedback signal path also includes the control processor, on which a programmable sinc3 filter also features output short-circuit detection. An analog signal isolator provides inverter bus voltage isolation; this signal is obtained from an embedded sampling ADC. The quadrature encoder peripheral (QEP) on the control processor supports a simple digital encoder interface, but higher-resolution encoders with interpolation circuitry typically use a high-speed serial interface to send position and speed information on demand.
Figure 2. Magnetic field alignment of a two-phase permanent magnet AC motor.
Figure 3. A dual-axis motor control system employing an isolated control architecture, using an ADSP-CM408 mixed-signal ASP and an AD7403 isolated modulator.
In the example above, the real-time (RT) Ethernet interface is provided by an FPGA circuit to flexibly support various industrial network protocols in the automation market. The FPGA manages real-time data packets from the network, while the control processor provides the bandwidth and memory to support protocol stack management. Many such protocols support synchronous real-time control with jitter requirements of less than 1μs, which places a very heavy processing burden on the communication interface. As mentioned earlier, this requirement for servo drive synchronization is as important as servo drive performance. In modern automated machining systems, both are indispensable for achieving high productivity and high-quality finished products. An emerging trend in automation systems is to use a single processor to control two or three servo motors and rely on a single real-time communication interface. High-speed dedicated signal processors (ASSPs) now support this trend, such as the ADPS-CM408, which includes a high-speed floating-point core and multiple sets of motor control and communication peripherals.
The diverse architectures exhibited in industrial motor drive applications highlight the fact that many significant design challenges for motor drive systems remain. With increasing bandwidth available for control processing and sensor feedback signals, the automation industry's demand for higher precision and dynamic response continues to grow. New materials, sensors, control and communication circuit architectures, and even more algorithms and software, are likely to continue to meet the automated manufacturing industry's demands for higher productivity and higher quality.
