Motor Unit

In a motor hand, every degree of freedom is driven from the array of twenty motors mounted on the forearm frame. Each motor drives two tendons to give a pull/pull control.

Force sensing is integrated into the tendons at the motors, and is used to provide compliant movements. Each pair of tendons couples a motor to a joint.

Each motor is managed by the Hand PC and completely controlled by the on-board electronics.

Small vs Large

Two types of motors are used in this hand design.

Small Motors: Sixteen small motors are used for all finger joints, and most thumb joints.

Part	Value	Units
Motor	Maxon 118608
Gear	Maxon 352367
Motor power	3	W
Gear ratio	131:1
Max continuous safe tendon load	65	N
Tendon Load at full power	110	N
Max continuous current	217	mA

Large Motors: Four large motors are used for the two wrist joints, and thumb joints 4 and 5.

Part	Value	Units
Motor	Maxon 110151
Gear	Maxon 143988
Motor power	6	W
Gear ratio	128:1
Max continuous safe tendon load	190	N
Tendon Load at full power	190	N
Max continuous current	350	mA

Tensioner

Each motor unit (except for the wrist) includes a tensioner unit. These exist purely to maintain a little tension on each tendon at all times, so that the tendons wind tidily onto the motor spool. The tension applied by the tensioner is very slight, and does nothing to compensate for backlash.

Torque measurement

The tendons exit the motor spool, and take a 90º turn around a metal bar. This bar is held at each end by a load cell, which is set up to measure the vertical component of any load exerted on the bar. The torque is calculated as the difference between the two readings.

If the tendons are not correctly adjusted, the tensioners may not be able to maintain tension on the spool. In this case, the tendons can become tangled around the spool.

Slack adjustment and tensioning.

Refer to the section: Maintaining the hand.

The spool is split into two halves which can rotate relative to each other, but are normally held by a bolt fixedly with respect to each other. To take up any tendon slack, simply loosen the bolt, rotate the top half of the spool, and re-tighten the bolt.

Only one tendon is tight at any one time, while the other tendon is fairly loose. However, the loose tendon is kept under very slight tension by the tensioner to help it wind around the spool tidily. The wrist motors do not have tensioners.