Renesas and MinebeaMitsumi Design Stepping Motor Solution for Robots

December 12, 2019 by Gary Elinoff

The new high-torque resolver stepping motor and control circuity is aimed at robotic, automation, and medical applications.

Renesas Electronics and MinebeaMitsumi have teamed up to develop a resolver-based (angle sensor) stepping motor, a resolver-to-digital converter, and the driver circuitry necessary to animate the solution.


Stepping motor with a resolver, RDC, and MCU

Stepping motor with a resolver, RDC, and MCU. Image from MinebeaMitsumi and Renesas


Steppers have generally been controlled using open-loop control, a method vexed by excess current consumption and the concurrent generation of waste heat. It has previously been difficult to use resolver-based control with steppers because of the electromagnetic noise generated.

However, servo control employing resolvers has been used in the automotive and aviation fields, perhaps inspiring the solution offered by the two partners. Now, designers have more options than the larger motors otherwise necessary for the step-out margin. In addition, users can avoid the torque ripple that causes vibration and noise.  


Built for Robotic Applications

Renesas and MinebeaMitsumi claim that their joint efforts will be especially useful in small- to medium-sized robots. These automated robots, they posit, may save labor in industrial settings, facilitate automated driving, and provide better healthcare (especially in nursing-care) with robotic medical technology. 

The new motor with an angle sensor "resolver"—combined with motor control technology—is geared to meet these needs by offering precision control, low-speed rotation, and high torque. In addition, the two companies state that the new device is resistant to dust and heat and can be a good option for compact designs.


MinebeaMitsumi's Contribution: Resolver-based Stepping Motor

MinebeaMitsumi is an established supplier of stepping motors. The company asserts that because its new resolver-based (angle sensor) stepping motor, R17PMK440CNVA4438, experiences no losses due to the step-out margin, it can deliver two to three times as much torque when compared to existing products.

The new stepper features 200,000 P/R for high position precision. Additionally, the device includes servo control that responds to the load, allowing for precise current optimization.


Resolver-based stepping motor.

Resolver-based stepping motor. Image from MinebeaMitsumi


MinebeaMitsumi’s stepping motor is said to carry heavy loads without step-out. In the absence of the excess torque requirements (otherwise necessary to compensate for step-out), it is possible for designers to specify much smaller motors. This might be an advantage in robotics, where miniaturization and low-power consumption are key.


Maximization of motor torque possible with new system.

Maximization of motor torque is possible with the new system. Image from Renesas 


As illustrated above, servo control/available torque at all speeds is greater than what is attainable with open-loop control. In addition, the device allows rotation at higher speeds.


Renesas' Contribution

Renesas’ contribution is threefold. It has 1) released a resolver-to-digital converter (RDC) to mesh with the MinebeaMitsumi motor’s resolver, 2) a 32-bit RX MCU with software for control, and  3) an evaluation kit. In the image below, Renesas’ RDC and MCU interface with the MinebeaMitsumi motor’s resolver. 


Renesas’ RDC and MCU interface with the MinebeaMitsumi motor’s resolver.

Renesas’ RDC and MCU interface with the MinebeaMitsumi motor’s resolver. Image from MinebeaMitsumi and Renesas

Resolver-to-Digital Converter

The RAA306400XGFP is an RDC  that converts the resolver analog angle information to a more usable digital form. This device communicates using a 1MHz max SPI interface. It comes in a 7mm x 7mm x 0.5mm 48-pin plastic LQFP package. 


32-bit MCU

The RX24T designation represents a group of 32-bit MCUs. The units include Renesas’ 80 MHz RXv2 core. Members come with onboard memories of 512KB flash, 32KB SRAM, and 8KB data flash. These units come in LQFQ packages ranging in size from 10mm x 10mm to 14mm x 14mm.

The diagram below outlines the function of the Renesas’ MCU and the RDC ICs with the stepping motor with a resolver.


Renesas’ MCU and the RDC ICs

IC and software for motor control using resolver. Image from Renesas


Evaluation System and Development Support

Renesas’ Evaluation System for Stepping Motor with Resolver is currently under development.


Renesas Solution Starter Kit.

Renesas Evaluation System for Stepping Motor with Resolver. Image from Renesas


The kit includes: 

  • A board equipped with MinebeaMitsumi’s R17PMK440CNVA4438 stepping motor with a resolver sensor and Renesas’ RAA3064002GFP resolver-to-digital converter
  • A CPU card equipped with Renesas’ RX24T motor control microcontroller
  • A sample SW code
  • All necessary cabling


In addition, Renesas offers its support tool for motor control development, Renesas Motor Workbench 2.0, which is designed to aid in motor control debugging issues. 


24V Motor Control Evaluation System for RX23T
24V Motor Control Evaluation System for RX23T. Image from Renesas


When used in conjunction with the company’s 24V Motor Control Evaluation System for RX23T, it not only displays the waveform of internal microcomputer variables in real-time; it also extracts vital vector control parameters.



Have you worked with robotic technologies? What qualities do you look for in a component for such an application? Share your thoughts in the comments below.