What You Need to Know About Surgical Robot Motor

The accuracy and minimal invasiveness of today’s surgical equipment has truly revolutionized the modern operating room. Stepper, brush and brushless DC motors, gear motors, mechanical actuators and drives are all key components in this type of automated equipment. And DC motors are at the heart of the surgical robots.

When performing an operation with a robot, the surgeon sits at a console and looks at a 3D image of the operating area. They then use joysticks to move the robot’s arms, so that only the robot and surgical assistants stand over the patient. Precision is crucial in robotic surgery, and the 3D monitor means that the surgeon is able to see every detail and zoom in if required.

The slim, robotic arm offers more freedom of movement when cutting, repairing, or suturing compared to standard procedures. By converting the entries made at the console, the robot can make incisions in the range of a tenth of a millimeter, which can’t be done by hand. A tenth of a millimeter.

This is almost the size of a grain of sand. These smaller incisions mean less blood and smaller wounds, thus speeding up recovery times. And at the heart of surgical robots is a series of components that powers and interprets instructions from the surgeon to the output signals to the motors in the manipulators.

DC motors for surgical applications must also boast a full torque at low speeds. Since the manipulators need full range of motion to maximise their agility and operate effectively, it is possibly fundamental when it comes to precision mechanics. Compact motors make the robot more lightweight, which can in-turn makes it more agile and precise.

DC motors from ASSUN MOTORS are precision made with rare earth magnets and self-supporting rotors offering high copper fill for unrivaled performance. This makes them suitable for applications that require a compact drive solution with high torque, delivering up to 224 mNm without a gearhead in only 38mm diameter.

Because when it comes to the configuration of the motorized joints for surgical robot applications, the joint performance includes the analytics of driving motion, back drive motion, and holding position. Then there are common requirements for compact size, highly integrated electrical and mechanical systems, low weight, and tight thermal constraints. Where, the selection of control methods, feedback requirements, and mechanical attributes drive the motor selection.

In a highly controlled quality system, such as a precision machine, optimizing control of internal materials, design, or form factor gets you to the center of the requirements spectrum in performance through specification customisation.

Having complete control over the design, materials, and manufacturing process means every constant remains that way. Sourcing from a global manufacturer of precision micro motors ( such as ASSUN MOTOR) means you get to set controls in place to guarantee that materials and design will not change on the next delivery.

Here the most common motorized joint includes a brushless permanent magnet frameless torque motor kit, an absolute encoder kit, and a high ratio zero backlash gear.

Motion

A high precision absolute encoder is also required on the output of the gear to handle any lost motion and wind-up due to the low stiffness and lost motion in the gear systems.

Power

Low impedance motor characteristics and defines the input speed range. System voltage levels tend to be in the low voltage range < 50volts.

Sensors

Force and torque sensors may be necessary if the robot is used in a collaborative environment.

Peripherals

Field-oriented control sinusoidal servo drives with proper safety functionality and serial communication interfaces. Drives are getting smaller and located closer to the motor.

Motor Selection/ Integration

Torque Vs Speed

The only true indicator of a motor’s ability to output torque under thermal constraints. Km is a calculation = Kt/sqrt(R), where Kt is the torque constant NM/Amp, and R is the resistance in Ohms.

The most important attributes in a frameless motor kit are mechanical form factor, motor constant (Km), and torque vs. speed characteristics when operating within the power limits.

Dimensions

Diameter, Length and through hole size.

Smoothness of Operation

Cogging torque is not normally shown in a motor datasheet.

The absolute value and the frequency of this cogging torque is important to the dynamics. Less is always better in this case, zero is preferred.

Cogging torque is the torque needed to overcome the opposing torque created by the magnetic attractive force between magnets on the rotor and the iron teeth of the stator. There are multiple rotor positions within a revolution where the cogging torque is high.

Sinusoidal Torque versus Angle Curves and Phase Balance

Critical to smooth motion. These two items are not shown in the datasheets. It is only through experience and testing that a problem will be uncovered.

In all electromagnetic synchronous rotary devices, torque is a function of both currents applied to the winding and the position of the rotor w.r.t. the stator phase. Measurement of this typically yields a torque versus angle curve. It is the true representation of a motor’s ability to create torque at any specific angle.

It is also a map of how torque changes with angle and current, including the effects of phase balance, cogging torque, and saturation within the motor. As a motor is driven by a sinusoidal motor controller, it is applied to all three phases of the motor in a sinusoidal pattern with angle. While this method approaches an ideal system, i.e. sinusoidal current and sinusoidal torque versus angle, it can also be impacted from items like cogging torque or a mismatch between the control sine waves and the motor’s torque angle characteristics.

Electrical Time Constant

Surgical robots have very specific temperature requirements almost disconnected from how motors are typically rated.

Thermal Resistance

Servo system response and servo driver PWM frequency play a large role in system performance. A low inductance motor, (typically a result of smaller size, zero cogging, weight optimized , low voltage motor kits), requires higher driver PWM frequency to minimize current ripple. Current ripple can cause electrical noise as well as additional heating.

Sterilization

Today’s surgical robotics and robotically-assisted surgical devices often require Brushless Direct Current (BLDC) motors to meet demanding requirements. While motors and motion are core to all robotics, surgical robotics demand the capabilities of both traditional robotics and traditional surgical hand tools. In addition, devices used in surgery must be sterile. Often these devices must reliably and consistently function despite repeated steam sterilization during reprocessing, in addition to demanding field use.

Interesting Read

1. Brushless & DC Servo Motor Used in Biomedical Exoskeletons
2. Brushless DC Motors Used in Industrial Exoskeletons