# Sizing DC Motor for Large Robot (350 lb)

• Northbysouth
In summary, the individual is trying to size a motor for a large robot, and is having some confusion with the specs. They have found a motor that they think will work, but would appreciate input from others. The motor is for a smooth indoor flooring and has a power output of 200 watts.
Northbysouth
I'm working on a senior design project. Currently I'm having some issues with sizing a motor for a large robot; everywhere I look I get different advice. I have a motor that I think will work but would appreciate some input.

The specs are for our robot are as follows:

mass = 350 lb = 158.8 kg
Max velocity = 4 mph = 1.788 m/s
angle = 4.8 degrees = 0.0837 radians
acceleration = (2*v_max)/t + v_i = 1.788 m/s^2
wheel radius = 10 inches = 0.254 m
time = 5 seconds

Surface = smooth indoor flooring

where v_max is the maximum velocity and v_i is the initial velocity (0 m/s), t is the time it takes for the robot to accelerate from 0 to v_max

I did a free body analysis (FBD) of the robot on an inclined plane (0.0837 radians) and got the following equation for the torque:

Torque = m*r*(a+g*sin(theta))

where m is the mass, a is the acceleration, g is gravity (9.81 m/s^2) and theta is the angle of the inclined plane.

This gives me a torque of 31.7 N-m. I think that this motor:

http://www.superdroidrobots.com/shop/item.aspx/dg-158-24vdc-135-rpm-wheel-chair-motor-pair/1531/

may work (the specs given are for the motors together). But the specs are confusing me a little. I don't understand where the 'Power Output (W) =200' spec is coming from or what the 'Gear Rated Motor Torque = 16.95 N-m)' means. As I understand it:

Power = torque*angular velocity

So, for the specs given for the motor:

w_motor = 135 RPM *2*pi/60 = 14.13 rad/s

Power_motor = 16.95 N-m*14.13 rad/s = 239.5 W

The issues is that for us to move the robot we need the following power:

w_robot= v_max/r = (1.788m/s)/(0.13 m) = 13.75 rad/s

where we rounded the radius of the wheel from 0.254 m to 0.13m.

Power_robot = 31.7 N-m*13.75 rad/s = 435.875 W

Additionally, in the images given for this motor there is a speed (RPM) vs torque graph which plots efficiency, current and speed (RPM). Based on the speed curve I re-plotted the points in excel and used a curve of best fit to find that the stall torque was approximately 60 N-m. So, we'd be operating these motors at 50% of their stall torque, but most sources I've read indicate that you should operate dc motors between 10 and 33% of their stall torque.

Any input about whether I'm looking in the right direction would be appreciated. Thanks

I know nothing of math or even actual engineering. When faced with a high-torque application, I always fall back on the good old automobile starter motor.
Also, you could look into the motors used on my power-chair (Jazzy Jet 3). It's like a combination of easy chair and motorized wheelchair. 24VDC gearhead motors. You can get them through eBay for a couple of hundred bucks, but they cost over \$1,500 new.

## 1. How do you determine the appropriate size of a DC motor for a 350 lb robot?

The size of a DC motor for a large robot is determined by several factors, including the weight of the robot, the desired speed and torque, and the type of terrain the robot will be operating on. A general rule of thumb is to choose a motor that is at least 10-20% larger than the estimated weight of the robot.

## 2. What is the recommended power rating for a DC motor for a 350 lb robot?

The recommended power rating for a DC motor for a 350 lb robot is typically between 1-2 horsepower. However, this can vary depending on the specific requirements of the robot and its intended use.

## 3. How do I calculate the torque required for a DC motor for a 350 lb robot?

The torque required for a DC motor can be calculated by multiplying the weight of the robot by the desired acceleration and the radius of the wheels. It is also important to consider any external forces that may affect the robot's movement, such as inclines or obstacles.

## 4. What type of DC motor is best suited for a 350 lb robot?

There are several types of DC motors that can be suitable for a 350 lb robot, including brushed and brushless motors. The type of motor to use will depend on the specific requirements of the robot and its intended use. Brushless motors are generally more efficient and durable, while brushed motors are typically more affordable.

## 5. How do I ensure the DC motor is compatible with the robot's power source?

It is important to ensure that the DC motor is compatible with the power source of the robot. This includes checking the voltage and current requirements of the motor and matching them with the power supply. It is also important to consider the battery life and capacity, as well as any additional components that may be needed to regulate the power supply and protect the motor.

• Mechanical Engineering
Replies
1
Views
733
• Mechanical Engineering
Replies
23
Views
3K
• Mechanical Engineering
Replies
8
Views
2K
• Mechanical Engineering
Replies
1
Views
1K
• DIY Projects
Replies
3
Views
2K
• Mechanics
Replies
7
Views
1K
• Mechanical Engineering
Replies
5
Views
3K
• Mechanical Engineering
Replies
42
Views
12K
• Introductory Physics Homework Help
Replies
3
Views
2K
• Mechanical Engineering
Replies
4
Views
9K