Basic Mechanics of Robots

 

The meaning of this word deals with the motion of the ROBOT. In basic mechanics we will try to understand motors, gear mechanism and physical structure of a robot. Depending upon area of uses ROBOT size may differ from each other. It should have the proper equipment to complete or perform a task. Like we have hands, fingers and arm to hold a thing similarly our ROBOTs require.

Our hand, fingers and arm move at a certain angle and in a certain direction which is known as degree of freedom. For movement, our body contains several joints likewise we provide joints in robots too. At joints we fix different types of motor for movement. These motors can be servo, DC geared and stepper.

Motors

So first we will try to understand about motors, their functionality and their different uses. The motor uses electrical energy and convert it into mechanical movement with the help of inbuilt gears.

DC Motors

As the name indicates, these motors work with DC only. DC motor can be brush or brushless. DC motor continues rotation till the supply is working. As DC motors are non-polarized, we can reverse its direction. DC motors work with voltage range of 6v to 12v. It draws current according to the load applied. If you are lifting something heavy, you will need more energy as compared with light weight things; similarly this case works with every motor. DC motor consumes ~150 MA without load and with the load it can consume current up to several Amps.

For mounting fans, propeller and wheels these are widely used.

Fig. 1: Typical Image of DC Motor

Fig. 2: Typical Image of DC Motor

Servo Motors

These are DC motors with gears and control loop circuitry. No additional motor drivers are required for Servo motors. Servos are extremely popular with robot, RC plane, and RC boat builders. Most servo motors can rotate about 90 to 180 degrees. Some rotate through a full 360 degrees or more. However, servos are unable to continually rotate. Yes! It’s correct, you can’t use servo motors in wheels, fans and places where continues rotations are required. Then what is the use?? The answer is here; you can use servo in arms, legs and in the fingers of Robot where limited movement is required.

Fig. 3: Typical Image of Servo Motor

Fig. 4: Typical Image of Servo Motor

Stepper Motors

Stepper Motors are DC motor with more than one coil. At the time of programming, we have to give commands. These commands select one of coil in several and decide motions of the motor. For example, a 10 degree step angle stepper motor would require 36 commands to rotate 360 degrees. Some notable points are there to remember when you try to do science with stepper motor.

Notes on Stepper Motors

• Stepper motors can be easily found in any 3.5″ disk drive
• Require special stepper motor controllers
• Have a set resolution, higher resolutions mean higher accuracy, but lower holding torque
• If torque applied to stepper is greater than holding torque, the stepper will lose accurate position measurements
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• Fig. 5: 

  
  Typical Image of Stepper Motor

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• Fig. 6: 

  
  Typical Image of Stepper Motor

• Gears

Motors are not enough for ROBOTs. Sometimes we have to make shaft structure and additional mechanical arrangements. Generally, we move those structures with the help of motors only. But we provide interfacing between parts and motor shaft through Gear Assembly.

Gears works on the principle of mechanical advantage. This means that by using different gear diameters, you can exchange between rotation (or translation) velocity and torque. As with all motor, by looking at the motor datasheet you can determine the output velocity and torque of your motor. But unfortunately for robots, motors commercially available do not normally have a desirable speed to torque ratio. That is the reason why we use gear assembly.

Gearing Ratios

If we want to change our velocity and torque, gearing ratio can be helpful. Just multiple your velocity and torque by gearing ratio. Suppose the gearing ratio is 3/1. This would mean we would multiple our torque by 3 and our velocity by the inverse, or 1/3.

Example; TorqueOld = 10 kg.cm, VelocityOld = 100rpm

Gearing ratio = 2/3
Torque X 2/3 = 6.7 kg.cm
Velocity X 3/2 = 150rpm

Fig. 7: Representational Image of Gears

Achieving a Particular Gearing Ratio

If we wanted a simple gearing ratio of say 2 to 1, we would use two gears, one being twice as big as the other. It isn’t really the size as much as the diameter ratio of the two gears. If the diameter of one gear is 3 times bigger than the other gear, we would get a 3/1 (or 1/3) gearing ratio. We can easily figure out the ratio by hand measuring the diameter of the gears we are using.

For a much more accurate way to calculate the gearing ratio, calculate the ratio of teeth on the gears. If one gear has 28 teeth and the other has 13, you would have a (28/13=2.15 or 13/28=.46) 2.15 or  .46 gearing ratio.

Gear Efficiency

By using gears, we lower our input to output power efficiency. This is due to obvious things such as friction, misalignment of pressure angles, lubrication, gear backlash (spacing between meshed gear teeth between two gears) and angular momentum, etc. Different gear setups, different types of gears, different gear materials, and wear and tear on the gear, will all have different efficiencies.

Direction of Gear Rotation

When designing our gear setup we should understand how gearing changes the rotation direction of our output. Two gears touching will always be counter rotated; meaning if one rotates clockwise, the other will always rotate counterclockwise. The rule is, an odd number of gears always rotate in the same direction, and even numbers of gears are counter-rotational.

Fig. 8: Representational Image of Gear Directions

Types of Gears

All gears, no matter the type, work on the same principles above. However the different types let us accomplish different things. Some types of gears have high efficiencies, or high gearing ratios, or work at different angles, for example. Below are the main common types. This is not a complete list. It is also possible to have a combination of types.

Spur Gears (~90% efficiency)

Fig. 9: Typical Image of Spur Gear

Due to their simplicity and the fact that they have the highest possible efficiency of all gear types, Spur gears are the most commonly used gears. For very high loads as gear teeth can break more easily, hence it is not recommended.

Helical Gears (~80% efficiency)

Fig. 10: Typical Image of Helical Gear

Helical gears operate just like spur gears, but offer smoother operation. You can also optionally operate them at an angle, too. As we can see,due to the complex shape, they are generally more expensive.

Bevel Gears (~70% efficiency)

Fig. 11: Typical Image of Bevel Gear

For changing the rotation angle, bevel gears are good. They suffer low efficiencies, so we avoid use if possible.

Rack and Pinion (~90% efficiency)

Fig. 12: Typical Image of Rack and Pinion

Rack and Pinion is the type of gearing found in steering systems. This gearing is great if you want to convert rotational motion into translational. Mathematically, use radius = 1 for the straight ‘gear’.

Worm Gears (~70% efficiency)

Fig. 13: Typical Image of Worm Gears

Worm gears have a very high gearing ratio. To calculate mathematically, consider the worm gear as a single tooth. Another advantage of the worm gear is that it is not back-drivable. What this means is only your motor can rotate the main gear, so things like gravity or counter forces will not cause any rotation. This is good say if we have a robot arm holding something heavy, and we don’t want to waste power on holding torque. The efficiency is low, but lubrication really helps.

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Filed Under: Electronic Projects
Tagged With: Bevel Gears, DC Motors, Helical Gears, Rack and Pinion, robot, Servo Motors, Spur Gears, Stepper Motors, Worm Gears
 

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