Lightweight robot elbow and wrist joints

Robot arms, even medium size one with a 2m (6ft) reach endure tremendous torque at the joints.  The weights of the joints at the shoulder have little relevance as they are at the yaw axis.  But as the distance increases away from this axis, especially at full stretch, so does the torque due to the weight of the complete arm assembly from the shoulder , and is subtracted from the payload capacity at the manipulator at the ‘hand’ end.  Depending on the versatility, both elbow and wrist can employ two degrees of freedom.  The rotating axis for the elbow and wrist can be mounted at the shoulder elevation pivot and the elbow, after the elevation pivot respectively, reducing the torque somewhat.  The upper arm rotating motor/gearbox mounted near the yaw axis pivot has minimal torque loading on the shoulder but there is a serious trade off the further away the joints become from the shoulder.

There are two cost effective solutions at the elbow and wrist; these are smaller stepper/wormdrive combos and stepper/harmonic drive combos, although the wormdrive solution is about half the cost of the harmonic equivalent.  They are not created equal in two respects; firstly the wormdrive shaft is at right angles to the motor shaft, whereas the harmonic drive is concentric.  If the wormdrive has any combination of internal straight cut gears it will also have more backlash.  As in CNC machines backlash is a critical element in robotics; the further from the axis point the more backlash creates real world position errors.  One tenth of a degree error in the shoulder yaw equals about  1.5mm (60 thousandths of an inch) error at the ‘hand’ for an outstretched 2m (6ft) arm.  One degree backlash would probably render the arm useless.  Harmonic drives have almost negligible backlash due to their unique construction and are about half the weight of an equivalent stepper/wormdrive.

Harmonic drives combined with stepper motors can be a good choice for robot arm joints, particularly when you need precision (<100th of a degree), compactness, and high torque (tens of Nm / lb-ft in a hand size package). Harmonic drives, also known as strain-wave gears, provide advantages such as zero-backlash, high torque density, and compact design, making them suitable for robotic applications. Stepper motors, on the other hand, are known for their simplicity and cost-effectiveness.

When combining harmonic drives with stepper motors, it's crucial to ensure that the chosen stepper motor has sufficient torque to drive the harmonic drive, and the overall system meets the torque and precision requirements of your application.  A NEMA17 stepper and harmonic drive fit in the palm of a hand and cost just over US$100 with free shipping and up to 35N.m (25lb-ft) torque, but they will need a second bearing point.  As the size increases the cost of harmonic drive skyrockets.  The same torque is available with a worm drive with the same stepper at about US$50 but is about twice the weight and about 150mm (6 in) long.  The worm drive requires no extra bearing and can be dual right angle shafted, adding to arm stiffness.

Stiffness cannot be overlooked; money and effort spent on reducing backlash can be thrown away by an arm that is too flexible.  At 2m (6ft) resting your hand on the manipulator / hand or the arm at full reach is equivalent to a few Kg (some lbs) of torque at the shoulder and if it deflects the manipulator one cm (half inch) the arm is probably useless.

I talk a lot about torque; you hear a lot about it with trucks – hp and torque.  HP is the total power but torque is what gets you up a hill without gearing down.  Power is proportional to rotational speed x torque.  Torque is the twisting strength of a shaft.  It's important to note that while harmonic drives and stepper motors are suitable for certain robotic joints, the choice depends on specific application requirements.  Factors such as payload capacity, speed, precision, and available space should be considered when selecting the appropriate joint type for a robotic system.  The higher the gear ratio in the harmonic or worm drive the slower the fastest rotation will be but with greater torque.  The cost and weight will hardly change if at all.

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