Lecture 8

Question 1

What is a servo?

Answer 1

It is an automati device that uses error-sensing negative feedback to correct the action of a mechanism

It usually incldes a built-in encoder or other position feedback mechanism to enure the output is achieving the desired effect

Question 2

What are the colours of ther servo leads?

Answer 2

The +ve signal is always in the middle and always red

The 0V signal is either black or brown

Question 3

What do servos require?

Answer 3

They require a PWM signal that has a frequeny of 50Hz

Question 4

What does the show duty cycle?

Answer 4

The width of the pulse that determines the direction of rotatiin, the position of the arm mechanism or, in the cause of continuous servos, the speed of the continuous rotation of the arm mechanism

Question 5

What is duty cycle?

Answer 5

It is the percentage that the signal is high relative to the period of the waveform

Question 6

How does a servo use the PWM signal to define position?

Answer 6

Inside the IC:
○ Takes the PWM signal in and turns it into an analogue value
○ Compares analogue value with position feedback from the potentiometer
○ Then control a secondary PWM controller that drives the H-bridge
○ H-bridge controls the motor

If there is an error signal a PWM signal is created and motor is moved

Question 7

What is the dead band?

Answer 7

Due to the nature of the conversion from the primary PWM input through the secondary motor drive PWM output, there is a quantisation within the conversion and this conversion is known as the dead band

It is where a change can occur in the PWM input but no change will be applied to the PWM motor drive output i.e. there is a quantised transfer characteristic

Question 8

How do continuous servos work?

Answer 8

They will rotate continuously at a speed and direction that is proportional to the change in the duty cycle relative to the centre duty cycle of 7.5%

Question 9

How do continuous servos adjust the centre duty cycle?

Answer 9

There is no position feedback in these servos, but there is a potentiometer that is used to adjust the centre duty cycle at which the servo stops moving, this is adjustable by the user

Question 10

What are digital servo signals?

Answer 10

The incoming PWM is converted into frequency PWM signal with a much high duty cycle

This new signal has a higher resolution but it also drives the motor with a PWM signal that is based around a higher frequency and significantly raises the torque and response time of the servo

Question 11

What is PPM?

Answer 11

Pulse Position Modulation (Multiplexed analogue protocol using PWM)

Question 12

What is PCM?

Answer 12

Pusle Coded Modulation (Multiplexed digital protocol)

Question 13

What is SBUS?

Answer 13

Serial Bus (Serial communications digital protocol)

Question 14

What is servo torque?

Answer 14

Dependent on the voltage applied to the servo

Measued in oz.in or Kg.cm

Question 15

What is servo speed?

Answer 15

Depends on the voltage applied to the servo

Measured in sec/60 degrees or rpm

Question 16

What happens to the torque when you move further away fro the centre of rotation?

Answer 16

Torque decreases

Question 17

What happens to linear displacement when you move away from the centre rotation point?

Answer 17

Speed of linear displacement increases

Question 18

What happends to the activation distance when you move away from the centre rotation point?

Answer 18

Activation distance increases

Question 19

What are speed and torque dependent on?

Answer 19

Voltage

Question 20

Are all servos controlled by PWM?

Answer 20

No

Question 21

Compare digital servos to analogue servos

Answer 21

Digital servos have a better performance than analogue servos, but are significantly more expensive

Question 22

What is a solenoid?

Answer 22

A cylindrical coil of wire acting as a magnet when carrying an electrical current?

Question 23

How do solenoids work?

Answer 23

When a electrical current is applied to the coil, a magnetic field is created which will attract an armature/plunger towards the centre

Question 24

Why are springs used with solenoids?

Answer 24

Often used to get the required return action when no voltage is applied to the coil

Question 25

When is a plunger solenoid used?

Answer 25

Often used in valve control for both fluids and gases, i.e. hydraulic or pneumatic control systems

Question 26

Why are diodes used in DC non-latching solenoids?

Answer 26

Dc solenoids have huge magnetic field storage and thereforse, when they are switched off the back EMF can be significant and can easily damage DC drive circuits and generate electrical noise. Power diodes are used to consume the energy released during field collapse

Question 27

What are latching solenoids?

Answer 27

They use a permanent magnet to hold the armature in one position state whilst using a spring to hold the armature in the other position state By applying a bi-directional pulse signal to the energising coils it is possivle to move the armature from one state position to the other i.e. they consume very little power whilst holding either position state

Question 28

Where are solenoids used?

Answer 28

Pinball machines Gas and fluid valves Pinch valves Hydraulic control valves Pneumatic control valves

Question 29

Which way round must the coils be connected in the relay?

Answer 29

The diode polarises the voltage connection and therefore the cathode of the diode must be connected to the positive end of the supply

Question 30

What are the relay parameters?

Answer 30

``` Contact configuration (SPST etc) Nominal coil voltage (DC or AC) Coil operation voltage Coil release voltage Operate/release time (ms) Bounce time (ms) Contact current Contact voltage Frequency of operation (loaded and unloaded) Diode protected ```

Question 31

Why do we use relays?

Answer 31

Circuit isolation Power switching High power system signal detection Medium voltage logic control systems - a means by which to interconnect relays that perform functionality