Parameter |
Value |
Unit |
Max. force possible when the hand is open |
10 |
N |
Max. force possible when the hand is closed |
17 |
N |
Max. distance between the two bars (hand open to closed) |
5 |
cm |
Spring constant |
131,5 |
N/m |
Gear ratio of worm drive |
20 |
|
Diameter of the pulley were the cord is wind up |
2 |
cm |
Max. needed couple of the motor |
0,085 |
kg.cm |
The manufacturer of the motor we selected is Futaba.
The manufacturer number is S3003. This servo can draw a relatively large current from its power supply.
We have to make sure the power supply is capable of delivering sufficient amps. Relevant specifications are summarized in the table below.
The servo can only turn over an angle of 180 degrees.
A mechanical stop prevents the servo from rotating any further. This is insufficient for our application thus the mechanical stop needs to be removed.
The position of the servo is controlled by a rotational potentiometer, which can also only measure 180 degrees.
We thus need to replace this potentiometer as well. The servo is used to set the displacement of a spring, which is a linear movement.
This can easily be measured by a linear potentiometer, which we connect to the servo instead of the rotational one.
The potentiometer will now rotate until the lever is in the correct position, allowing for an easy control of the spring displacement ergo spring tension. |
Specification |
Value |
Unit |
Torque |
3,2 |
kg.cm |
Speed |
0,23 |
sec/60° |
Voltage |
4,8 |
V |
Dimensions |
40 x 20 x 36 |
mm |
Weight |
37 |
g |
The Arduino board is powered by a simple adapter (AC/DC converter) at 12V DC.
The servo can operate on a voltage of 5V, meaning that the 5V output of the Arduino can be used to power the servo.
However, a servo can draw considerable amounts of current. It is therefore wiser to connect it to an external power supply.
We already have the 12V coming from the adapter, which can easily be subtracted from the Arduino board by soldering two wires to the pins shown in the figure on the right.
Unfortunately, 12V is too much for the servo. It is therefore converted to a 5V DC voltage by a DC/DC converter on the PCB. |
The pinion and worm gear are from the manufacturer RS-components.
Specifications are given in the table below.
Unlike with ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible.
This can be an advantage when it is desired to eliminate any possibility of the output driving the input.
We use this in our advantage because in this way the motor does not have to deliver a torque during the squeezes, but only when the force level is changed.
This decreases energy consumption. |
Specification |
Value |
Unit |
Material pinion gear |
Bronze |
|
Material worm gear |
Steel, Bronze |
|
Module |
0,8 |
|
Number of teeth pinion gear |
20 |
|
Overall diameter pinion gear |
17,6 |
mm |
Overall diameter worm gear |
15,6 |
mm |
Pitch diameter pinion gear |
16,03 |
mm |
Pitch diameter worm gear |
14 |
mm |