Daryl

= = =1. Force Sensors= toc = = There are many types of sensors available and each of them have their own functionality. We are to find a sensor that is to be placed on the gripper of our robotic arm. The sensor shall detect if the robotic gripper has gripped an object. The following are the requirements that need to be considered:
 * Requirement for Selection**

1. Measures load and force with reasonable range (in lbs) 2. Flexible material for easier attachment to the gripper 3. Rigid material that can sense various shaped objects 4. Reasonable price

Proximity sensors cannot be used since they detect objects at a distance. Slip sensors also have a different functionality as it detects shear force. Furthermore, force sensors measure force or load. There are two general classifications of force sensors: hard and thin film sensors. The former (e.g. Phidgets Force Sensors) are not suitable for robotic grippers for they are not flexible and they are bigger in size. Thin film sensors on the other hand are lighter, smaller, and flexible, although some are still only optimized for human touch (e.g. Interlink Force Sensors). Taking into account all the requirements for selection, the Parallax Flexiforce Sensor, which is a thin film force sensor, is best suited for our design project
 * Choosing the Right One**


 * FlexiForce A201 Force Sensor**



• A piezoresisitve force sensor that is flexible in terms of shape and size • Measures force or load • Ultra thin material • Can be trimmed into smaller length
 * Features**

Thickness: 0.008" Length: 8" Width: 0.55" Sensing Area: 0.375" diameter Connector: 3-pin male square pin Response Time: <5 microseconds Force Range: 0-25 lb
 * Specifications**

When the force sensor has no load, its resistance is very high (>5MΩ). When a force or load is applied, the resistance decreases. This resistance is measured by connecting an ohmmeter to the two outer pins. This device is designed such that force is inversely proportional to the resistance. Conversely, force is directly proportional to conductance (1/R).
 * How It Works**

A table must be completed to test the precision of the A201 Force Sensor. Resistances are to be measured with respect to various sample forces. Conductance is then calculated from these resistance values (G=1/R). If the device is working correctly, the plot of conductance vs force should be linear and should show a straight line with a positive slope as seen below.
 * Testing the Sensor**



To process the input signal (force) from the A201 force sensor, it has to be connected to a force-to-voltage circuit. This drive circuit is used to calibrate and convert the force signal into a proper engineering unit (voltage). Also, it is used to adjust sensitivity of the sensor. The drive circuit is shown below.
 * Integration**

• Supply Voltages should be constant • Resistance RF is 1kΩ to 100kΩ • Max recommended current is 2.5mA • Aim is to get a Vout of 0V when no force is applied and 4.2V when max force is applied.

1. If the footprint of the applied load is larger than the sensing area, it may be necessary to use a "puck." A puck is a piece of rigid material (smaller than the sensing area) that is placed on the sensing area to ensure that the entire load path goes through this area. The puck must not touch any of the edges of the sensing area, or these edges may support some of the load and give an erroneous reading.
 * Maintenance and Operation**

2. Avoid shear forces on the sensor (since sensor reads forces that are perpendicular to the sensor plane)

3. Use tapes or adhesives that will not degrade the substrate (polyester) material of the sensor.

4. Saturation force - when maximum force of sensor is achieved and no change in output occurs. This can be altered by changing the sensitivity (reference resistance). A lower RF will make the system less sensitive, and increase its active force range. It is essential that the sensor(s) do not become saturated during testing. If the sensor saturates at a lower load than desired, adjust the "Sensitivity."

5. Conditioning the sensors - As with most measurement devices, it is customary to exercise, or "condition" a sensor before calibrating it or using it for measurement. This is done to reduce the amount of change in the sensor response due to repeated loading and unloading. A sensor is conditioned by loading it to 110% of the test weight four or five times.

6. Hysteresis is the difference in the sensor output response during loading and unloading, at the same force. For static forces, and applications in which force is only increased, and not decreased, the effects of hysteresis are minimal. If an application includes load decreases, as well as increases, there may be error introduced by hysteresis that is not accounted for by calibration.

7. Drift is the change in sensor output when a constant force is applied over a period of time. If the sensor is kept under a constant load, the resistance of the sensor will continually decrease, and the output will gradually increase. It is important to take drift into account when calibrating the sensor, so that its effects can be minimized. The simplest way to accomplish this is to perform the sensor calibration in a time frame similar to that which will be used in the application.

8. Linearity error - FlexiForce standard sensors are linear within +/- 3%.

=2. Robotic Gripper with Servo= 1. One SG-01 Gripper 2. Two HS-422 Servos
 * Lynxmotion Little Grip Kit (With Servos)**
 * Kit Includes

SG-01 Gripper Features** • Supports a maximum of two mechanical movements all at the same time (Gripping and Wrist Rotation) • Can be easily connected to two separate servos (one for each mechanical movement) • Compatible with several HiTec servo motors like the HS-422, HS-322, HS-325, etc.



Max accommodated object thickness: 0.9” (23 mm)
 * Gripper Specifications**

The Servo Gripper SG-01 can be directly connected to a maximum of two servo motors (HS-422 shown below). One motor is used to open and close the gripper and the other to rotate the wrist. The figure below is the HS-422 servo motor and its dimensions.
 * Servo Motor for the Gripper**



Control System: +Pulse Width Control 1500usec Neutral Required Pulse: 3 - 5 Volt Peak to Peak Square Wave Size: 41 x 20 x 37 mm Torque: 3.3 kg-cm (45.82 oz-in) to 4.1 kg-cm (57 oz-in) Speed: 0.21 sec/60 degrees to 0.16 sec/60 degrees at no load Weight: 45.5 g (1.6 oz) Angle Rotation: 360 degrees Operating Voltage Range: 4.8V to 6V Connections: Black wire - ground, Red wire - power, Yellow wire - signal
 * Servo Motor HS-422 Specifications**

**Integration** The two figures below show how the SG-01 connects to two HS-422 servo motors. The left figure shows how to connect one servo motor to control the grip (open and close) and the right figure shows how another servo is connected to control the wrist movements.



Combining the above configurations will yield a gripper that can not only grip objects but can also rotate for enhanced flexibility (see figure below).