Servos and Solenoids

Question 1

What is a solenoid?

Answer 1

Uses DC current to produce linear movement

Question 2

How do solenoids work?

Answer 2

> When an electric current is applied to the coil, a magnetic field is created which will attract a armature towards the centre (the armature is made from ferrous metal).
Since the ferrous metal has no defined magnetic polarity it does not matter which direction the current flows, it will always attract the armature.
They operate on DC.
Springs are used to obtain the required return action.

Question 3

What is the circuit diagram for a solenoid?

Answer 3

[Picture24]

Question 4

What is a relay?

Answer 4

This is a component that allows a small voltage to control a large voltage using physical connections

Question 5

What are the different types of relay contact configurations?

Answer 5

SPST
SPDT
DPST
DPDT

Question 6

What are some relay parameters?

Answer 6

> Contact configuration
> Nominal coil voltage
> Coil operating voltage
> Coil release voltage
> Operate/release time
> Bounce time
> Contact current
> Contact voltage

Question 7

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Answer 7

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Question 8

Define ‘servos’

Answer 8

A motor that uses error sensing negative feedback to correct the action of a mechanism to ensure that the output achieves the desired effect. Often has an inbuilt encoder.

Question 9

How are servos controlled?

Answer 9

> Using a 50Hz PWM signal (2ms period)
A 1.5ms pulse (0.75 duty cycle) = 0°
Changing the duty cycle changes the direction of the servo in either direction

Question 10

How does a servo work?

Answer 10

> A DC motor is connected to a gearbox that controls the output
A potentiometer is connected to the output and provides feedback
An internal PWM signal is controlled by this feedback
The internal PWM signal is compared with the input PWM signal and if there is no error then there is no PWM signal to the motor

Question 11

What is the problem with servos?

Answer 11

There is a large ‘dead band’ which is when there is a difference in the time period but the servo does not respond to the difference. It can be as high as ±2% duty cycle

Question 12

Define ‘Continuous servos’

Answer 12

They will rotate at a continuous speed and direction. Both these are proportional to the change in the duty cycle relative to the centre duty cycle of 75%

Question 13

What is the feedback for continuous servos?

Answer 13

There is no position feedback in these servos. The potentiometer is used to adjust the centre duty cycle at which the servo stops moving.

Question 14

What are digital servos?

Answer 14

> Similar to normal servos but they combat the dead-band problem
The incoming PWM signal is converted to a 300Hz PWM signal with a much higher duty cycle.
The new cycle has a much higher resolution.
The output signal to the DC motor, has a much higher torque and response time because the PWM to the motor has a 19kHz frequency.

Question 15

What are 3 servo protocols?

Answer 15

> PPM – Pulse Position Modulation (Multiplexed analogue protocol using PWM)
PCM – Pulse Code Modulation (Multiplexed digital protocol)
SBUS – Serial Bus (Serial communications digital protocol)

Question 16

How is torque calculated and what is its relationship to distance with an equal force?

Answer 16

Torque = Force × Distance

Torque decreases the further you move away from the centre of rotation

Question 17

How is servo speed calculated?

Answer 17

Speed = Sec / 60°