Low cost, high torque drives for medium size robot arms

Small robots and collaborative robots (cobots) often require precise and powerful joint movements to perform various tasks efficiently.   The torque is the rotational force at a pivot point.  The torque due to load of an extended arm is due to gravity.  Torque is the product of multiplying the load (weight) by the distance (length).  One popular and effective solution for achieving high torque in such applications is the use of stepper motors paired with worm drives. This combination offers several advantages that make it an optimal choice for small robotic systems.  The stepper provides a simple and accurate source of movement, and the worm drive a cost effective and robust torque multiplier, albeit at the cost of speed.  But with the angles of robot arm movement mostly much less than one revolution this merely provides more resolution (accuracy of position).

When paired with worm drives, which are known for their high gear ratios, the torque output from a stepper motor is greatly amplified easily up to 10-30 lbft 13-40 Nm), so at 6ft (2m) reach it could control a load of 5lb (2.5 Kg) depending on the stepper motor torque. Stepper motors have a habit of losing all torque if they lose sync, but not having enough torque to move or sustain a load.  In elevation (vertical movement) if they lose sync it is lost period, until motion is again started from standstill.  In simple terms the arm collapses under the influence of gravity.  The high torque gained from the worm drive enables the robot arm to handle heavy payloads and all but eliminate loss of sync and subsequent joint collapse under gravity as is possible with direct drive using steppers.  Their design usually also eliminates joint collapse under gravity when power is removed, especially with higher gear ratios due to the fact of being hard or impossible to back drive a worm gear.

Nema23 Stepper motors and single or dual shaft right angle worm drives about 4” (100mm) sq are well-suited for small but powerful roboti arms. Paired they cost under US$100 including shipping from AliExpress.com.  The integration of these components allows for a space-efficient design with incredible torque and strength.  Having cast alloy housings and wide spaced mounting points , they are well suited for the azimuth (rotation in horizontal plane).  The dual shaft type are equally suited to shoulder and elbow elevation (rotation in vertical plane).  The strong integral bearings ensure stability, low flex and ruggedness of the arm under load and extension.

Stepper motors operate in discrete steps and can be operated without feedback as required by DC servos.  This precision is vital for applications such as pick-and-place tasks, assembly operations, and any scenario where exact positioning is critical to the robot's functionality.  Feedback is an essential element of servo systems.  RC type servos, including most of the very high torque ones up to 300Kg.cm use a potentiometer to derive a feedback voltage controlled by servo angle.  The tracks of these potentiometers wear unevenly over time and the positional accuracy will drift.  There are ones based on hall-effect devices that are much more stable but their overall linearity is questionable.  I don’t of any that use a digital encoder of any type like the larger and more expensive DC/AC servos use.  A geared DC motor with a pulsed encoder, like some linear actuators use is better suited to robotics.

Backlash (slop due to the tiny space between gear teeth) is always a concern with geared systems, it worsens the more gears are in the gear train.  Worm drives only have two gears with one space between teeth.  In some worm drives even this can be minimized.  In a robot type arm, the worst case scenario is probably in the azimuth (horizontal rotation).  This can be minimized with a spring type mechanism , which exerts a rotation force in one direction always keeping the gears meshed on one face.  Alternatively a simple DC motor can exert a rotational force by belt of gears to do the same.  The vertically inclined axes have gravity to do this, although the wrist axes can get complicated.

Stepper motors are generally more cost-effective than alternative motor types, such as servo motors. The simplicity of the stepper motor design and its ease of control contribute to a lower overall cost. Worm drives, too, are known for their cost-effectiveness compared to other gearing mechanisms. The combination of these components offers an economical solution without compromising performance, making it an attractive choice for small robot arms in various industries.

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