Instantaneous mode switching can help smooth transitions between different servo motor control modes in complex motion applications.

Charan Bhamra
Mechatronics Engineer
EZ Motion

In closed-loop servo control systems, there are three essential control modes; torque, speed, and position. Each of these parameters can be individually controlled depending on the application. While a single control mode may be enough for some applications, other more advanced applications may require multiple control modes within the same motion operation.

In most traditional servo control systems, a given motion must be completed before switching between different control modes. Otherwise, the drive signal may experience a spike or significant drop in winding current that can adversely affect the performance of the motion profile. This creates complications for designs that require using multiple control modes without interruption.

Instantaneous mode switching
One solution to this challenge is servo motors and controllers with instantaneous mode switching. These motors and controllers provide instantaneous mode switching capability, enabling seamless switching between multiple control modes while the motor is in motion. Instantaneous mode switching can also be categorized as bumpless switching between servo control modes. While bumpless switching generally applies to mode switching between manual mode and automatic mode in proportional integral derivative (PID) control, it can be used similarly in servo motor control.

The feedback of all control signals is constantly monitored to develop the nested loop control structure of these types of servo motors and controllers. Typically, only one of the three control loops is connected to the output signal; however, unconnected control loops can also develop as if they are connected to the output signal. Therefore, when switching between control modes, the motor can smoothly transition between different control loops without any jumps in the output control signal.

Advantages of instantaneous mode switching
Instantaneous mode switching provides advantages that are highly relevant in various applications, particularly in fields that involve industrial automation, control systems, and manufacturing. Maintaining continuous and smooth operation is vital for product quality and consistency. These advantages help avoid disruptions that can affect overall motion-related process efficiency.

The benefits of instantaneous mode switching include:

• Allowing for seamless transitions between all control modes
• Preserving tuning and loop parameters during control mode switching
• Providing outstanding stability and performance during switching operation
• Offering additional flexibility and functionality to deploy advanced control algorithms for applications
• Useful in haptic force feedback control schemes

Definitions and comparison metrics
The current in motor windings is typically responsible for torque generation and rotation. In three-phase field-oriented control (FOC), the three-phase motor system is simplified and transformed into a two-axis direct-quadrature-zero (DQZ) transformation reference system. The minimized D-axis current (ID) is parallel to the rotor pole axis, and is not responsible for torque generation. The maximized Q-axis current (IQ) is perpendicular to the rotor pole axis, and is responsible for torque generation in the motor.

FOC with triple cascade loop control is the foundation for seamless mode switching in motor control. In a triple cascade loop configuration, each loop is responsible for regulating different aspects of the system such as current, speed, and position. The output of one loop is used as the setpoint for another loop, creating a cascading effect.

IQ feedback is a reliable indicator of the motor system’s performance and stability. Therefore, examining and comparing IQ feedback during mode switching can help evaluate the motor’s seamless mode switching performance.

With instantaneous mode switching, the motor starts in speed control mode with a target speed of 3,000 rpm and an acceleration of 10,000 rpm/sec. At 5 sec, the motor seamlessly switches to position control mode with the speed at 1,000 rpm. After a few seconds of operating in position mode, the motor transitions to torque control mode with the target torque set to 20% of the motor’s nominal torque. Finally, the motor switches back to speed control mode with a target speed of 3,000 rpm.

As Figure 2 shows, there are no bumps and spikes observed in the IQ signal during mode switching, and the respective control output signals are appropriately controlled to the reference signals after switching. Thanks to instantaneous mode switching, the motor is physically stable with no shaking or bouncing behaviors observed. In comparison, traditional motors that lack seamless mode switching capabilities shake or attempt to bounce off the ground during actual operation mode switching. IQ spiking may trigger a fault or protection, leading to an abrupt pause in motor operation.

Some application examples
A few examples illustrate how servo motors and controllers with instantaneous mode switching are used in applications requiring more than one control mode.

Cobot with feeding or insertion applications
Feeding and insertion applications require a cobot to accurately place parts and apply precise insertion force. In these types of applications, the servo can operate in position control mode to align the part, then switch to torque mode to apply the appropriate mechanical insertion force. Seamless mode switching allows the cobot to smoothly transition between position and torque mode while maintaining stability and performance. For example, consider a plastic clip insertion application, where the clip must be seated at a set location and depth before being pressed into the mating assembly with precise insertion force.

Automated centrifuge systems
Centrifuge systems use centrifugal force to separate liquid, gas, and solid substances based on the density. Automated centrifuge applications interact with pick-and-place machines, requiring precise speed control during the centrifuge process and accurate position control to stop. During the centrifuge separation process, the rotor holding the samples spins at a set speed for a given time, then seamlessly transitions to position mode. This stops the rotor at the target position in a single motion operation. The accurate stopping position allows the pick-and-place robot to load or transfer samples to their designated locations.

Spin rinse dryer (SRD) systems for wafers
Spin rinse dryers (SRDs) clean and dry semiconductor wafers in a series of cycles that includes rinsing, purging, and drying the wafers. A cassette holding a rack of wafers is loaded into the rotor inside the SRD chamber. This load-balanced rotor is responsible for the spinning motion in the wafer cleaning process. The rinse, purge, and dry cycles are driven in speed control mode for a set duration at given speeds. After all cycles are completed, the rotor holding the wafers requires position control mode to stop in an upright position without interruption. Instantaneous mode switching can enable seamless transition between speed and position control modes to achieve the SRD process in a single motion operation.

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