Motion control for surgical and medical robots

In short:A surgical or medical robot is many small, quiet, backdrivable axes: arm joints, instrument drives behind a sterile barrier, and, in rehabilitation, wearable joints that must move with the patient. Choose the gear ratio for backdrivability as well as torque, decide early which parts must survive sterilisation, and size from the clinical motion, not the catalogue peak.

Typical requirements

Machinery we specify motion for

• Robotic-assisted surgical systems
• Robotic instrument and endoscope drives
• Surgeon consoles and haptic controllers
• Surgical microscopes
• Surgical tables
• Rehabilitation exoskeletons
• Powered prosthetics
• Rehabilitation and therapy devices

What surgical and medical robots demand from motion

This page goes deeper on robots than our medical and laboratory overview, which covers IEC 60601-1 component evidence, the basic frameless-motor and strain-wave joint, and lab automation. Here the focus is the rest of the robot, and the robots that work on or with the patient’s body.

A medical robot is rarely one big axis. It is many small ones, each with a different job:

• Arm joints position the instrument and must hold pose safely.
• Instrument drives move a wrist and jaws a few millimetres across, often through cables, behind a sterile barrier.
• Consoles and hand controllers read the surgeon’s hand and push back with haptic force.
• Wearable joints in exoskeletons and prosthetics move with a person, so they must be light, quiet and safe to push against.

maxon, whose motors are used in surgical systems, says several dozen motors are needed in one surgical robot to transmit the surgeon’s movements, and stresses zero cogging and haptic feedback. Harmonic Drive’s medical page lists surgical robots, surgical microscopes and exoskeletons as uses for zero-backlash, quiet gearing.

Where each component sits

Sterile barrier and sterilisation: decide first

The most important early decision is which parts must be sterilised. It changes the motor technology, the materials and the cost.

• Behind the sterile barrier. In many systems the arm is draped and the instrument attaches to its drive unit through a sterile adapter, so the motors stay outside the sterile field and need only to be cleanable. This is usually the cheaper route.
• In the sterile field. Handheld powered tools and some instruments carry their own motor. maxon publishes its sterilisable brushless motors for an autoclave cycle of 134 °C ±4 °C, 100% relative humidity, 2.3 bar and 18 minutes, with more than 1,000 cycles for drives with sensors and up to 2,000 without. Brushed motors are not autoclavable. maxon also states that ethylene oxide sterilisation is unrestricted for all its catalogue products.

Small motors for instruments run fast and drive through high-ratio gearheads. maxon’s ECX SPEED platform, for example, runs at up to 120,000 rpm at 4–22 mm diameter. Coreless and slotless designs have no cogging, which helps both haptics and slow motion.

Exoskeletons and prosthetics: torque, mass and backdrivability

Wearable robots turn the usual sizing upside down. The load is the person, the actuator rides on the person, and the joint must never fight them.

Published gait data gives the scale. In the hip exoskeleton study by Yu et al., the peak hip moment in walking is about 1.3 Nm/kg, or 97 Nm for a 75 kg adult at 1.6 m/s, with peak joint speed around 2.3 rad/s. Their exoskeleton targets partial assistance of 19.4 Nm, not the full moment, and its quasi-direct-drive actuator gives 17.5 Nm nominal and 42 Nm peak through an 8:1 gear at 0.77 kg. They measured backdrive torque of about 0.4 Nm and an actuator bandwidth of 62.4 Hz.

Worked example: strain-wave or low-ratio gear for a hip joint

Assume a 20 Nm continuous, 40 Nm peak assistance target at up to 3 rad/s (about 29 rpm), and a motor whose rotor inertia is 50 kg·mm². These are illustrative figures, not a design.

1. At 100:1 strain-wave. The motor needs about 0.2 Nm continuous and 0.4 Nm peak plus gear losses, at about 2,900 rpm. Reflected inertia at the joint is 50 × 100² = 500,000 kg·mm², or 0.5 kg·m². That is heavy for a leg to swing, and friction through a strain-wave gear adds to the backdrive torque.
2. At 8:1. The motor needs 2.5 Nm continuous and 5 Nm peak, at about 230 rpm, so it is larger and heavier. Reflected inertia is only 50 × 8² = 3,200 kg·mm², or 0.0032 kg·m², about 150 times less.
3. Decision. If the joint must be transparent to the user, as in assistive gait, the low-ratio route wins despite the bigger motor. If it must deliver high torque in a compact package and lead the patient’s movement, as in some rehabilitation devices, a strain-wave gear or a complete harmonic actuator is usually the better fit.
4. Check. Run both options through the real gait torque and speed cycle, and compare RMS motor heat, battery energy and mass at the joint.

Powered prosthetics face the same trade-off with tighter mass and battery limits. Size knee and ankle drives from published gait moments for the user’s mass and activity, and include the noise target, which users notice quickly.

Consoles, haptics and microscopes

Hand controllers read the surgeon’s hand and feed force back, so they need very low friction and cogging, and high-resolution encoders. Coreless motors, ironless direct drives such as the Akribis ACD and thin capacitive encoders suit these axes. Netzer lists tolerance to EMI, RFI and magnetic fields among its encoder features, which helps in a joint packed with motors and brakes.

Surgical microscopes move heavy optics that must not shake. Harmonic Drive cites backlash-free, smooth movement for optical stability in focusing and positioning. Use low-backlash gearing, brakes or balancing on the stand axes, and quiet micro motors or steppers for focus and zoom.

Surgical tables and patient positioning

Operating tables have their own particular standard, IEC 60601-2-46, which applies alongside IEC 80601-2-77 when a table forms part of a robotic system, according to the EVS listing of that standard. For motion, tables are slow, high-force, low-duty axes. Choose self-locking screw actuators such as the GT44, define what each axis does on power loss, and coordinate table moves with the robot so the patient and the instrument stay registered.

Typical pitfalls

1. Deciding sterilisation late. Moving a motor into the sterile field after the design freeze means a different motor.
2. Choosing ratio on torque alone. High ratios cost backdrivability and add reflected inertia, which matters for haptics and wearables.
3. Specifying encoder resolution, not accuracy. Tool-tip error comes from accuracy across every joint.
4. Forgetting the drive’s thermal path. A drive sealed in a joint may have no airflow at all.
5. Ignoring audible noise. PWM whine and gear noise are noticed in theatre and in daily prosthetic use.

What to send us

For each axis send the motion, range, torque or force, speed and duty, the envelope and mass budget, bus voltage, feedback interface and brake needs. Say whether the part sits in the sterile field and how it is sterilised, which safety standard the device follows, and the evidence your risk file needs. Our specialist engineering and supply partners will propose matched motor, gear, encoder and drive sets.

Standards to have in view

• IEC 80601-2-77 (robotically assisted surgical equipment, RASE and RASS)
• IEC 60601-1 (medical electrical equipment, basic safety and essential performance)
• IEC 60601-1-2 (EMC for medical electrical equipment)
• IEC 60601-2-46 (operating tables)
• ISO 14971 (medical device risk management)
• ISO 13485 (medical device quality management)
• ISO 13482 (personal care robots, for non-medical wearable robots)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.