Feature: Processors and MCUs
Figure 1: Field-oriented control relies on current measurement, rotor position and PWM updates remaining synchronised throughout every control cycle
Partitioning the control loop in
humanoid robotics servo drives By Frank Malik, Chief Engineer, Toshiba Electronics Europe
I
n the past, the microcontroller (MCU) in a servo drive was primarily responsible for executing the motor control algorithm. Today, alongside the current control loop, it exchanges data with higher-level
machine controllers, monitors operating conditions, detects faults and executes the application soſt ware that coordinates the servo drive with the rest of the robot. Unlike these higher-level functions,
the motor control loop can’t wait until processor becomes available. It must acquire current and position measurements, calculate the next control outputs and update the inverter at defi ned intervals. Every control cycle has to complete before the next one begins, regardless of what else the processor is doing. Meeting these competing demands
has become an important aspect of servo drive design. Humanoid robots make these constraints particularly challenging because the requirements change from joint to joint. A wrist or a hand joint may require relatively little power and must fi t into an extremely restricted space, whilst an elbow or shoulder has to deliver considerably more torque and support a
28 October 2026
www.electronicsworld.co.uk
greater mechanical load. In each case, the electronics must combine precise control with compact dimensions and careful thermal management.
Precise timing for servo control Field-oriented control (FOC) is well established in robotic servo drives because it provides accurate control of torque and speed; see Figure 1. T ese characteristics are especially valuable for a robotic joint, where the motor may need to maintain holding torque and then respond immediately to a change in load. Smooth, low-speed operation and accurate positioning are equally important, whilst effi cient control reduces electrical losses and the heat that must be removed from a tightly-integrated joint. From the controller’s perspective,
however, the important point is not the algorithm itself but the timing it imposes. T e controller therefore works within
a fi xed timing window. Measurements acquired outside that window, or PWM values applied aſt er it has passed, no longer relate to the same motor state. Maintaining that timing relationship becomes increasingly demanding as other workloads compete for processor resources.
Partitioning the control loop Implementing the motor control loop entirely in soſt ware increases the number of time-critical operations executed by the processor. T ere is an alternative architecture that moves the vector control functions into dedicated hardware, allowing the processor to continue managing the functions that sit outside the control loop. T e Toshiba TXZ+ M4L family is a group
of compact, entry-class microcontrollers that use this alternative architecture. T e platform combines an Arm Cortex-M4 core with a motor control subsystem comprising Toshiba’s fourth-generation Vector Engine implementation, second-generation Advanced Vector Engine (A-VE2), A-PMD2 programmable motor driver, and A-ENC2 encoder interface. Here, current measurements and rotor
position information enter the motor control subsystem before the next PWM update is calculated. A-VE2 performs the vector control calculations. T e resulting PWM values are passed to the A-PMD2, which generates the inverter switching signals and synchronises current sampling. T e timing constraints become more demanding when a single shunt confi guration is used for current measurement. In this case, the Vector Engine,
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