Design+Changes

toc =Design Changes=


 * Usage of a force sensor at the bottom of the arm to detect the additional weight of the object and produce a force in the opposite direction of movement of the user-controlled arm.
 * Usage of an additional motor (dc motor) at the elbow joint to produce the Haptic force for weight detection.
 * Usage of an additional servo at the wrist joint for vertical movement of the wrist.
 * Usage of ONLY potentiometers at the controlling arm for position feedback instead of the servos for the controlling arm. This reduces the complexity of the programming and the cost by a lot.

=Additional Electronics=

This force sensor works similar to the Parallax FlexiForce sensor proposed for Gripper Haptics, but bigger in size. The properties of a force sensor and how it works can be found here. The brushed DC motor generates torque directly from the DC power supplied to the motor by using internal commutation, stationary permanent magnets, and rotating electrical magnets. It works on the principle of Lorentz force, which states that any current carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force.
 * Force sensor - Interlink Electronics 1.5" Square Force Sensor - $9.79 each**
 * DC motor - Banebots FF-050SK-1490 6V 15025 RPM 0.93 oz-in Brushed DC Motor - $3.73 each**

The specifications of this DC motor are as follows: Operating Voltage - 4.5V - 8V Nominal Voltage - 6V No Load RPM - 15025 No Load Current - 0.2A Stall Torque - 0.93 oz-in / 6.6 mN-m Stall Current - 1.9A Kt - 0.49 oz – in / 3.5 mN-m/A Kv - 2504 rpm/V Efficiency - 52% RPM - Peak Efficiency - 11730 Current - Peak Efficiency - 0.5A Weight - 0.6 oz (17g) Length of the motor - 1.06 in (26.9mm) Diameter - 0.61 in (15.5mm) Width across flat - 0.47 in (12mm)

This servo is the similar one that is going to be used for gripper rotation. The properties of a servo and how it works can be found here. = = = = =Gripper Haptics=
 * Servo - Hitec HS-422 Servo Motor** **- $14.12**

This gripper consists of the modified ‘Lynxmotion Little Grip Kit’ with 1 Hitec HS-422HD standard servo and a 10K potentiometer. The servo is used for the haptic feedback received from the output arm (basically used for gripping). This servo will be modified to give the position feedback to the output arm. Ideally, the driving gear would have to be removed from the servo in order to achieve free rotation of the shaft without any signal, as the gripper is controlled manually. But removing the driving gear will not produce the haptic feedback from the output arm. As a result, a very low torque servo has been chosen that can be moved ‘almost freely’ at no signal with the driving gear intact. The initial design idea for the rotational movements of the gripper was to use a modified servo with the driving gears removed for position feedback. This would be completely unnecessary as the job can be done perfectly using just a potentiometer instead of the servo. The potentiometer will be supplied a 5V voltage from the power supply and the control pin would be connected to the analog port of the Arduino Mega. As the pot is directly connected to the gear, the rotation would cause a change in voltage, which would be sent directly to the Arduino to control the output arm gripper. This change in design would reduce the cost and complexity by a lot. The software would be controlled in such a way that a null signal would be sent to the servo while the output arm is not holding any object. This would let us have free motion of the gripper when controlled manually and a blocking motion when a rigid object is held.
 * Input Arm:**

This gripper consists of the original ‘Lynxmotion Little Grip Kit’ with 2 Hitec HS-422HD standard servos and a 0.2” Circular Force Sensor. The servos are used for gripping and rotational movements of the gripper. The force sensor will be providing the force feedback to the input arm to produce the haptic effect. This detects the amount of force applied on the gripper, which is indirectly the change in resistance again. This produces a change in voltage as the force is exerted on the gripper. This voltage will then be sent to the input arm to act accordingly.
 * Output Arm:**

=Weight Haptics=

The elbow joint consists of a Brushed DC motor and a 10K potentiometer. This potentiometer is similarly connected to the elbow joint gears. So, any movement in the elbow joint would cause a change in voltage, which is sent to the Arduino to control the output arm. The brushed dc motor that is connected to the elbow joint using a low profile axis will be used to produce the haptic effect. According to the corresponding force recorded at the output arm, a certain voltage will be applied to the dc motor. A small amount of force at the output arm would apply a small voltage on the dc motor rotating it a low speed. Since the dc motor has a very low holding torque, we can manually move it in the opposite direction while it is rotating at some particular speed. So the higher the speed is, the larger the opposing force it can create on the input arm.
 * Input Arm:**

The elbow joint consists of a Hitec HS-755HB giant scale servo for the arm movements according to the position received from the input arm. A 1.5” force sensor would be placed below the arm in an appropriate location, where the whole weight of the arm is exerted. Depending on the structure of the arm base, more than one force sensor could be used to get an overall average force the arm is exerting downwards. This overall force is then calculated with no load and will act as the threshold voltage. So any external force acting on the arm (when a certain object is lifted) would cause an additional force on the force sensor, which is then measured and sent to the Arduino. This measured force is transferred to the dc motor at the elbow joint of the input arm to exert a force in the opposite direction.
 * Output Arm:**

For example, these are following changes that could occur:
 * Note:** Depending on the accuracy of both the principles, the complexity of the hardware/software could be increased while testing.

//Hardware:// Since the accuracy level for the analog servos are not very impressive, some modifications would probably be necessary. The servos on the output arm would be modified to record the position feedback. The position of the servos will then be smoothened accordingly. //Software:// The force measured on the 1.5” force resistor placed at the bottom of the arm might be varying a lot at no load when arm is at different locations. In this case, the axis of the arm would have to divided into various axes in order to produce an accurate haptic feedback.

=Building Materials=


 * Lynxmotion Low Profile Axis LPA-01 – This part could be used to attach the dc motor at the elbow joint for the Haptic feedback (weight) - $22.76 each


 * Lynxmotion Aluminum Long “C” large Servo Bracket Two Pack (Brushed) ASB-203B – This part can be used for elbow bracket. They are long enough to carry both the Servo and the DC motor – $20.49 for a pair
 * Lynxmotion Large Aluminum Multi-Purpose Servo Bracket Two Pack ASB-201 (Blk) – These brackets can be used as an additional casing to support the elbow servo - $17.34 for a pair
 * Lynxmotion Aluminum “L” Connector Bracket Two Pack ASB-06 – These connectors can be used to connect the elbow bracket to the servo bracket - $6.47 for a pair
 * Lynxmotion Aluminum Multi-Purpose Servo Bracket Two Pack ASB-04 – These brackets can be used to support the wrist servo - $12.99
 * Lynxmotion Aluminum "C" Servo Bracket Two Pack ASB-03 – These brackets can be used to carry the wrist servo - $8.64
 * Lynxmotion SEA-03 Servo Extender Cable – 24”