Programming A Driverless Car Chapter 11: Robotics actuators, Sensors, and Batteries
Sub System in robot control has:
Fig- Subsystem in Robot Control
In this robot control, there is an output encoder which processes the actual output and gives the feedback of the actual output to the controller. Controller then analyzes the desired output and the actual output and therefore controls the input to make the actual output equal to desired output. The motor that is generating some output is called actuator and the part sensing the data is called sensors.
Actuator is something that converts energy into motion. That converts an electrical energy, hydraulic or pneumatic energy into motion. This is actually the part that makes the motion. The type of output can be different like linear, oscillatory or rotary motion.
Types of Actuators:
- Electrical: Motors that convert energy into motion like DC motor, Stepper motor, DC servo.
- Pneumatic: Air pressure
- Hydraulic: Fluid pressure
- Advanced Actuators: Ultrasonic motors, artificial muscles, molecular motors.
Electrical Actuator- Rotational or Linear Actuator-
DC GEARLESS MOTOR: It converts electrical energy to rotational motion
DC GEARED MOTOR: It uses geared assembly over gearless motor
BRUSHLESS MOTOR: UAV< quadcopters
STEPPER MOTOR: Fixed angular rotation ex. 3D printers
SERVO MOTOR: To provide exact torque
DC LINEAR ACTUATOR: to provide linear moment
SOLENOID: Open and close the valve to stop the process.
DC GEARLESS MOTOR:
- Moderately high speed (rpm)
- Less Torque
- Can be used for low power application
- Usually used as propeller in small boats, in beam bots and other solar bots.
- Good torque
- Relatively less speed due to the presence of geared box
- Used when the torque is the main criteria
- Low speed application
- Usually used to drive a robot and for robotic arm
- Very high speed
- Low torque
- Used as propellers in UAV, aerial robots and as air propeller. RC controlled boats
- Requires a good power source( like LiPo batteries)
- Good torque
- Speed is variable
- Used when precise rotation is required
- Requires special circuit to work
- Good Torque
- Rotates maximum of 180 degree
- Rotates to a particular position depending on the duty cycle of pwm
DC LINEAR ACTUATOR:
- Provides linear movement
- Made up of DC motor
- Similar to DC motor and hence speed can be controlled using pwm
- Electromechanical, hydraulic, pneumatic
- Stroke is usually very small but very fast
- Can be made to work using MOSFET or transistor
Choosing An Actuator:
It depends on certain questions:
- Is the actuator being used to move a wheeled robot?
- Is the motor being used to lift or turn a heavy weight?
- Is the range of motion limited to 180 degrees and need good torque?
- Does the angle need to be very precise?
- Is the motion needs to be a straight line?
We can use H-Bridge circuit:
Fig: H-Bridge Circuit
L293D is a dual H bridge motor driver integrated circuit. Motor drivers act as current simplifiers since they take a low-current control signal and provide a higher 0 current signal. This higher current signal is used to drive the motors.
Using motors you can make the robots move forward and back:
Fig- Motors to make movements in robots
Sensors in Robotics
Sensors are transducers which convert one form of energy into another form of energy. It can sense the temperature of the environment or the light around and convert it into electrical energy.
Sensors measure a physical quantity and convert it into a useable signal for your robot.
When designing your robot it is important to choose the correct sensors to enable it to be aware of its environment and perform tasks required.
It is important for the robots to understand the environment and act rationally. There are two types of sensors:
- Internal Sensors: Required for basic working of the system ex. Position, velocity, acceleration.
- External Sensors: Interaction with the environment ex. Vision, force, proximity, pressure, touch.
Various types of sensors are: Infrared LEDs, Photodiodes, Photo Interpreters, Photoresistors, IR-Receiver modules, Light Sensors, Digital Hall sensors, Thermistors, Vibration sensors, Ultrasonic sensors etc.
LIDAR (Light detection and ranging):
LIDAR enables a self driving car (or any robot) to observe the world with a few special powers:
-Continuous 360 degrees of visibility – Imagine if human eyes allowed you to see in all the directions all the time.
-Insanely accurate depth information- Imagine if, instead of guessing, you could always know the precise distance of objects in relation to you.
Ex. used by UBER.
Fig: Representation of cameras on Self Driving Cars
LIDAR helps in real time mapping of objects in the street/road and mapping the distance between the object and car.
For robots, we use Ultrasonic sensors.
It is a 4-pin sensor. The 2 pins are VCC and Ground. The other two pins in the left most are Trigger and Echo. Trigger is the output of sensor where we generate microsecond digital pulse of type 40 kilohertz and trigger it and wait on the echo pin for the pulse to comeback. We then calculate the time of coming back of the pulse and the recipient and based on that we calculate distance between the sensor and object.
Types of batteries used are:
How do we select a battery?
-Current rating (in terms of mAh)
-Load that it takes
-Weight of the battery
Alkaline batteries are the most common, easiest to get and cheapest to use.
Low power capacities
Short battery life
Ni-Mh & Ni-Cd
These batteries are good for small to medium size range robots.
They have memory effect problem
To prevent memory effect, whenever you wish to recharge your NiCad, you must first fully discharge it.
NiMh batteries last many more cycles than typical batteries.
- Very small in size and weight compared to Ni-Cd, Ni-Mh and Lead Acid batteries.
- Fully charge in 180 minutes with special charger
- Long life with full capacity for up to 1000 charge cycles
- Low maintenance
- For ex.: 3*Li-Po 4.2V 2200mAh cells
Discharge Current:20*2200 mAh=44 Amp
Recommended Reading: Programming A Driverless Car Chapter X: Robotic System Architecture and Robotic Controls
Next: Programming A Driverless Car Chapter 12: Programmable Systems and Microcontrollers
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