Motor Trade Off Study
By Shaun Pasoz – Electronics & Control Engineer
Table of Contents
Based on the level 1 requirement that all power shall be supplied via the 3DoT board, the motor choices have certain limitations including: must draw 450mA or less, and have an operating voltage of 5V or less.
Following the Preliminary Design Review the customer requests that SpiderBot’s overall size needs to be reduced. Our initial build had a mass of 858 grams. The SpiderBot team has decided to reduce the overall mass to 500 grams or less. Since our mass has changed, so has the motor torque requirement to make SpiderBot walk. In a future post we will present a systematic process used to determine the torque requirement.
Due to the delicate nature of servos, SpiderBot’s walking mechanism will be driven by two DC motors. DC motors are basic electromechanical devices. They consist of two wires, one for V+ and one for V–. The speed, or power level, of the motor is controlled via pulse width modulation (PWM). PWM controls the motor by allowing, for example, full voltage for 50% of a duty cycle to create 50% of the rated RPM. Since the DC motor only has two wires, it provides no feedback. When compared to a servo motor, the servo has: a DC motor, a position sensing unit, and feedback control. However, servos are prone to gear degradation under heavy loads, and size is proportional to cost. This post attempts to deliberate the factors in choosing the correct dc motor for SpiderBot.
Types of DC Motors:
- Stepper Motor: A type of DC motors that moves in a predefined step size. They are much more precise, and allow for more control than say a brushless motor. This will not be used in the robot as they are much too bulky for our desired size and weight.
- Brushless Motor: A motor that utilizes electronic communication to control the current flow through the motor. This will not be used in our robot as it is more complicated to control.
- Brushed Motor: These are the most common motors and are useful for many applications. They can be used in geared motors which will provide more torque. The increased torque and price are what makes the brushed motor ideal for our robot.
The three main factors in choosing the correct motor comes down to: cost, size, and operating parameters. Out of the three types of DC motors, brushed motors became the immediate choice for the reasons listed under types of DC motors. After the PDR review, it became evident that we needed to shrink the robot and cut as much weight as possible, therefore we will be trying to choose motors that weigh less than 20g. However, smaller motors tend to have less power. Our operating parameters require that the motor operate at 6V or less and have an operating current of under 450mA as this is what the 3DoT can power. After conducting research this is the list of motors that most closely suit what we are looking for:
|Motor:||Rated Voltage||Speed @Rated Voltage||Free-run Current @Rated Voltage||Stall Current @Rated Voltage||Stall Torque @Rated Voltage||Weight:||Size (mm)|
|Pololu 298:1 Micro Metal Gearmotor MP||6V||75 RPM||40 mA||700 mA||125 oz-in||10.5 g||10x12x26|
|SparkFun Micro Gearmotor||6V||45 RPM||30 mA||360 mA||40 oz-in||17 g||26x12x10|
|Solarbotics GM3 224:1 Gear Motor 90 deg. Output||3V-6V||46 RPM||50 mA||733 mA||57 oz-in||31 g||69.4×22.2×18.6|
|Hobby Motor||3V||6600 RPM||110 mA||800 mA||Not Specified||26 g||27x27x33|
Table 1: Motor Trade Off Study Data
In conclusion, the most suitable type of DC motor for our robot is the brushed DC motors. From there the guiding factors for deciding which motors to look at were decided from the limitations of the 3DoT board. For the time being, the motors that are going to be the most viable are the SparkFun Micro Gearmotor, or the Pololu 298:1 Micro Metal Gearmotor. To conduct our initial testing of the miniaturized SpiderBot we will be using the SparkFun Micro Gearmotors as it meets the correct parameters even with a load large enough to stall. Some of the testing will include average current draw under load, and max power test.
Motor 1: https://www.pololu.com/product/2371
Motor 2: https://www.sparkfun.com/products/12285
Motor 3: https://solarbotics.com/product/gm3/
Motor 4: https://www.sparkfun.com/products/11696