Lesson 2

The reading module

Read the text: Technical description

Defining parameters

Numbers of axes – two axes are required to reach any point in a plane; three axes are required to reach any point in space. To fully control the orientation of the end of the arm (i.e. the wrist) three more axes (roll, pitch and yaw) are required. Some designs (e.g. the SCARA robot) trade limitations in motion possibilities for cost, speed and accuracy.

Degrees of freedom which is usually the same as the number of axes.

Working envelope – the region of space a robot can reach.

Kinematics – the actual arrangement of rigid members and joints in the robot, which determines the robot's possible motions. Classes of robot kinematics include articulated, cartesian, parallel and SCARA.

Carrying capacity or payload – how much weight a robot can lift.

Speed – how fast the robot can position the end of its arm. This may be defined in terms of the angular or linear speed of each axis or as a compound speed i.e. the speed of the end of the arm when all axes are moving.

Acceleration - how quickly an axis can accelerate. Since this is a limiting factor a robot may not be able to reach its specified maximum speed for movements over a short distance or a complex path requiring frequent changes of direction.

Accuracy – how closely a robot can reach a commanded position. Accuracy can vary with speed and position within the working envelope and with payload (see compliance). It can be improved by Robot calibration.

Repeatability - how well the robot will return to a programmed position. This is not the same as accuracy. It may be that when told to go to a certain X-Y-Z position that it gets only to within 1mm of that position. This would be its accuracy which may be improved by calibration. But if that position is taught into controller memory and each time it is sent there it returns to within 0.1mm of the taught position then the repeatability will be within 0.1mm.

Motion control – for some applications, such as simple pick-and-place assembly, the robot need merely return repeatably to a limited number of pre-taught positions. For more sophisticated applications, such as arc welding, motion must be continuously controlled to follow a path in space, with controlled orientation and velocity.

Power source – some robots use electric motors, others use hydraulic actuators. The former are faster, the latter are stronger and advantageous in applications such as spray painting, where a spark could set off an explosion.

Drive – some robots connect electric motors to the joints via gears; others connect the motor to the joint directly (direct drive). Using gears results in measurable 'backlash' which is free movement in an axis. In smaller robot arms with DC electric motors, because DC motors are high speed low torque motors they frequently require high ratios so that backlash is a problem. In such cases the harmonic drive is often used.

Compliance - this is a measure of the amount in angle or distance that a robot axis will move when a force is applied to it. Because of compliance when a robot goes to a position carrying it's maximum payload it will be at a position slightly lower than when it is carrying no payload. Compliance can also be responsible for overshoot when carrying high payloads in which case acceleration would need to be reduced.

After text activity

I. Reading Exercises:

Exercise 1.Read and memorize using a dictionary:

Exercise 2. Answer the questions:

1) Why do some designs trade limitations in motion?

2) How must motion be controlled for arc welding?

3) How do robots connect electric motors to the joints?

Exercise 3.Match the left part with the right:

Exercise 4. Open brackets choosing the right words:

Accuracy – how closely a robot can (reach/get) a commanded position. Accuracy can (differ/ vary) with speed and position (within/in) the working envelope and with payload (see compliance). It (can/must) be improved by Robot calibration.

The speaking module

II. Speaking Exercises:

Exercise 1. Describe acceleration, repeatability, compliance, accuracy, drive using the suggested words and expressionsas in example:

Exercise 2. Ask questions to the given answers:

1) Question: ___________________________________?

Answer: Classes of robot kinematics include articulated, cartesian, parallel and SCARA.

2) Question: _____________________________?

Answer: Using gears results in measurable “backlash” which is free movement in an axis.

3) Question: _____________________________?

Answer: Working envelope-the region of space a robot can reach.

The writing module

III. Writing exercises:

Exercise 1. Complete the sentences with the suggested words: to, as, within, by, if, into.

Repeatability - how well the robot will return ___ a programmed position. This is not the same____ accuracy. It may be that when told to go ___ a certain X-Y-Z position that it gets only to ________ 1mm of that position. This would be its accuracy which may be improved___calibration. But ____that position is taught _____ controller memory and each time it is sent there it returns to ______ 0.1mm of the taught position then the repeatability will be _____0.1mm

Exercise 2. Fill in the table with words and expressions from the text

:

Exercise 3. Compose a story on one of the topics (up to 100 words):

“Accuracy and repeatability”

“Motion control and power source.”

“Drive and compliance.”