FIRGELLI APPLICATION CALCULATOR
Robot Hand Actuator Calculator
Size five forearm-mounted actuators for a tendon-driven robot hand: one drive for the thumb and one for each finger. Set each digit's open and closed angles, moving mass and fingertip load, then compare the required tendon travel and actuator force. Feedback Micro Pen actuators are the starting selection.
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Loading the five-finger tendon calculator…
How the five-actuator robot hand works
Each actuator moves a small guided crosshead connected to two tendons. One tendon curls its finger; the opposing tendon opens it. The opening tendon passes around a fixed pulley near the base of the forearm, so the two tendon paths respond in opposite directions to the same crosshead movement. Actuator retraction curls the digit, and extension opens it. Both tendons pull; neither pushes the finger.
The five digits can move independently, but the joints within each digit follow a fixed curl pattern. The three joints of each finger rotate in the ratio 1:1.15:0.75. The thumb uses two joints in the ratio 1:0.9. The hand needs mechanical coupling that enforces these ratios; a loose tendon alone does not do this. A single actuator does not independently position every joint or make the finger conform automatically to an object.
Enter the movement and load for each finger
- Set the open and closed base-knuckle angles. The other joints follow that digit's curl ratio.
- Enter the moving finger mass, excluding the fixed palm and forearm.
- Enter the load applied at that fingertip as a mass in grams (or ounces with imperial units). Each digit is calculated separately.
- Compare the required travel and force with the selected actuator. Check the actual actuator length, tendon routing and mounting space before building the forearm.
The animation shows the selected curl path. Real tendons need suitable guides, tension adjustment and secure terminations. Slack, stretch, pulley friction and joint play can change the relationship between actuator position and finger position.
What the force and stroke results mean
Tendon travel is the effective tendon radius multiplied by the sum of the joint angle changes, with angles measured in radians. The two opposing routes use equal constant joint radii, giving a one-to-one relationship between crosshead travel and tendon take-up. Check that the physical guides maintain this geometry throughout the curl.
Actuator force is estimated from the fingertip load and the moving finger's weight throughout its travel. The calculation uses each load point's movement per degree of curl to form a conservative, worst-direction force envelope. The displayed demand includes a fixed 1.5× design allowance and 90% mechanical efficiency. It is the net force required at the actuator, not the absolute tension in a pre-tensioned tendon. It is a slow-motion estimate, not a prediction of impact force or high-speed performance.
A fingertip load is not an object-gripping rating. Contact location, surface friction, object shape, thumb opposition and control behavior determine whether a grasp holds. Do not add the five fingertip entries together and treat the total as the hand's lifting capacity.
Using feedback Micro Pen actuators
The feedback Micro Pen actuator provides Hall signals for tracking motor movement. Those signals are not fingertip force measurements and do not directly detect object contact. Each of the five drives needs its own motor-control and feedback channel, with a position reference and appropriate travel limits.
Check the exact voltage, force, stroke and retracted length for every selected variant. Allow unused stroke only with suitable control limits. The Micro Pen's 10% duty-cycle rating also matters: continuous animated movement is not permission to run the physical actuators continuously.
Keep fingers clear while setting travel limits and testing the hand. Use suitable current limiting, mechanical stops and an accessible means of stopping motion. Tendon tension and actuator position feedback alone do not establish a safe contact force.