Sensor Sample Questions

1. Consider the following reasons for using resolvers as the position sensors for permanent magnet brushless motors:

1. They are physically robust sensors.
2. They are the less expensive sensing systems available.
3. Absolute sensors are required to commutate these motors.
4. Their high resolution permits smooth commutation algorithms.

Which of these reasons is valid?

1. 1 and 4
2. 1 and 3
3. 1, 3 and 4
4. 3 and 4
5. 1, 2, 3 and 4

2. The following are possible means by which to protect the integrity of an incremental encoder signal.

1. differential line drivers for the signals
2. twisted-pair wiring
3. shielding the encoder cable, grounding at the controller end only
4. routing the encoder cable away from the motor cable
 

Which are valid?

1. 1, 2, 3 and 4
2. 1 and 3
3. 1, 2 and 4
4. 2 and 4
5. 1 and 2

3. The primary purpose of the index pulse in a rotary encoder is to provide the controlling software with

1. a known encoder rotational reference
2. a place to zero the system counters
3. a single mechanical reference position
4. an indication of operation sequence
5. a direction indicator

4. A disk with radius 2.5 meters is rotated slowly using a motor and gear system. It is necessary to know the angular position of the disk within 2 cm. What must the resolution of an optical encoder attached directly to the shaft (without gearing) be?

1. 393 pulses per revolution
2. 786 pulses per revolution
3. 197 pulses per revolution
4. 590 pulses per revolution
5. 295 pulses per revolution

5. A DC motor that drives a conveyor belt has a maximum speed of 1720 rpm. It is necessary to monitor the speed with a resolution of 1 revolution per minute and provide an update every 100 ms. How many pulses per revolution are required?

1. 600
2. 300
3. 450
4. 750
5. 900

6. An absolute encoder is to be used for measurements that require a resolution of at least one minute of arc. The minimum number of bits required is

1. 15 bits
2. 12 bits
3. 18 bits
4. 21 bits
5. 24 bits

7. Brush type tachometers produce a voltage that is

1. directly proportional to the angular velocity of the armature
2. inversely proportional to the angular velocity of the armature
3. directly proportional to the back emf.
4. inversely proportional to the back emf.
5. directly proportional to the position of the armature

8. The major problem with an incremental encoder is that it

1. is not able to determine the absolute position
2. is not able to determine the direction of motion
3. rarely provides sufficient resolution
4. requires a grey code to implement
5. is not usable for determining velocity

9. Following is a list of potential pitfalls in eliminating the tachometer from a  positioning axis and relying on only digital incremental encoder feedback:

1. loss of long-term accuracy due to loss of velocity sensor
2. impossibility of achieving stability due to loss of damping
3. loss of stiffness due to sample rate limitations
4. roughness at low speeds due to quantization errors

Which are valid concerns?

1. 3 and 4
2. 1 and 2
3. 1 and 4
4. 1 and 3
5. 2 and 3

10. Which of the following is a valid criticism on the use of a Hall-effect position sensors as the commutation sensor for a permanent magnet brushless motor?

1. motion is subject to high torque ripple due to low commutation resolution
2. a phasing-search move is required at power-on
3. the commutation algorithm will not recover from a noise glitch on the sensor line
4. low frequency inaccuracies
5. high frequency inaccuracies

11. A servo positioning system has a motor with a 1000 line encoder attached to a 5 pitch ballscrew. Using one of the many commercially available servo motion controllers, what is the system resolution?

1. 0.0002 inches/step
2. 0.0001 inches/step
3. 0.00125 inches/step
4. 0.0005 inches/step
5. 0.0008 inches/step

12. Long cable runs may be a problem for resolvers due to the

1. cable capacitance per foot
2. cable resistance per foot
3. cable inductance per foot
4. termination methods
5. signal attenuation

Page Revised: July 14, 2003