GARP Article Map<\/summary>\n\n- Motivations and Design of a UGV for Robotics Research<\/a><\/li>\n\n\n\n
- GARP Power Subsystem\n
\n- Design<\/a><\/li>\n\n\n\n
- Hardware<\/a><\/li>\n\n\n\n
- Software<\/a><\/li>\n<\/ul>\n<\/li>\n\n\n\n
- GARP Mobility Subsystem\n
\n- Motor Selection<\/a><\/li>\n\n\n\n
- Subsystem Design<\/a><\/li>\n\n\n\n
- Motor Controller\n
\n- Design<\/a><\/li>\n\n\n\n
- Hardware Abstraction Layer (HAL)<\/a><\/li>\n\n\n\n
- Motor Controller Interfaces<\/a><\/li>\n\n\n\n
- Alpha Implementation<\/a><\/li>\n<\/ul>\n<\/li>\n\n\n\n
- C&DH\n
\n- Design<\/li>\n\n\n\n
- Alpha Implementation<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n\n\n\n
- GARP E-Stop\n
\n- Design<\/li>\n\n\n\n
- Alpha Implementation<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/details>\n\n\n\n
Requirements and Constraints<\/h2>\n\n\n\n
I don’t have any hard-and-fast performance requirements for the GARP, so I’ll take an iterative motor selection strategy for GARP. I’ll aim for an initial motor selection with emphasis on ease, integrate it, and then use the resultant performance of the integrated platform to help refine requirements and select a more capable motor. That said, I’ll want to select a cheaper motor initially as (1) I’m buying four, and (2) I expect to trade them out for another model in the future.<\/p>\n\n\n\n
To make the initial motor selection, I’ll make a few basic assumptions about the design of GARP and draw up some rough estimates of the motor specifications needed. This will help seed the search (and will likely provide a comical review once the GARP is integrated and rolling.) The starting list of assumptions are:<\/p>\n\n\n\n
\n- GARP will use a wheel with diameter of 6″ (152.4 mm)<\/li>\n\n\n\n
- GARP will use one motor per wheel (i.e. no differentials, etc.)<\/li>\n\n\n\n
- GARP will weigh 50 lbs (23 kg)<\/li>\n\n\n\n
- GARP will have a ground speed threshold of 1 m\/s (2.2 MPH) and an objective speed of 2 m\/s (4.5 MPH); This puts GARP between a Clearpath Jackal and Husky<\/li>\n\n\n\n
- GARP will have a target acceleration of 1 m\/s2<\/sup> (i.e. will reach maximum speed in 1-2 seconds)<\/li>\n<\/ul>\n\n\n\n
Furthermore, I’ll neglect loss of traction, and assume no losses in the drive train.<\/p>\n\n\n\n
Torque<\/h3>\n\n\n\n
To arrive at a torque target, I’ll need to make some further assumptions about the use cases of the GARP. To make things easy at this initial motor selection, as a threshold use case, I’ll assume GARP is operating in grass with a rolling friction factor of 0.15, and on level terrain. For an objective use case, I’ll have GARP push a reel mower (because adding rapidly spinning blades to the front of a robot really ups the stakes.) Beginning with the threshold case, this means that the body-centered force required to move the GARP is 33.4 N:<\/p>\n\n\n
![\"F_L^{(T)} = f_{grass} F_W = 0.15 \cdot 23 \text{ kg} \cdot 9.81 \text{ m}/\text{s}^2 = 33.4 N\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-429d9f55879eae12708b7127ac410a13_l3.png\" "\\"Rendered")
<\/p>\n\n\n\n
This is 8.3 N per wheel and with 6″ wheels, is 0.64 Nm of torque per wheel:<\/p>\n\n\n
![\"T_{wheel}^{(T)} = \frac{1}{4} F_L^{(T)} \cdot \frac{0.1524 \text{ m}}{2} = 0.64 \text{ Nm}\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-c571fc5075da37462826c958b5d1fd11_l3.png\" "\\"Rendered")
<\/p>\n\n\n\n
For the accelerating case, the force required to accelerate the GARP at 1 m\/s2<\/sup> is 23 N, or 5.8 N per wheel, and an additional 0.43 Nm of torque per wheel (assuming the rolling friction is independent of velocity and acceleration):<\/p>\n\n\n![\"T_{wheel}^{Accel, (T)} = \frac{1}{4} \cdot 23 \text{ kg} \cdot 1 \text{ m}/\text{s}^2 \cdot \frac{0.1524 \text{ m}}{2} = 0.432 \text{ Nm}\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-6518cfe8267a4d722afcc63f8e4b8091_l3.png\" "\\"Rendered")
<\/p>\n\n\n\n
Summing these values places the estimated required torque per wheel at 1.1 Nm.<\/p>\n\n\n\n
For the objective case of GARP pushing a reel mower, assume the coefficient of rolling friction for the reel mower is twice that of the GARP (i.e. 0.3), and that the weight of the reel mower is 52 lbs. This weight is from the Fiskars Staysharp 18″ Reel Mower<\/a> (but the doubled rolling friction is entirely arbitrary). In this case the static torque required per wheel is 1.96 Nm (an additional 1.3 Nm), the dynamic torque is 0.88 Nm (+0.45 Nm), and the total torque required per wheel is 2.8 Nm (+1.8 Nm).<\/p>\n\n\n\nIn summary, the torque required per wheel for the constant velocity and accelerating cases is:<\/p>\n\n\n\n<\/td> Constant Velocity<\/strong><\/td>Accelerating<\/strong><\/td>Total<\/strong><\/td><\/tr>Threshold (GARP only)<\/strong><\/td>0.64 Nm<\/td> 0.43 Nm<\/td> 1.1 Nm<\/td><\/tr> Objective (GARP and Reel Mower)<\/strong><\/td>2.0 Nm<\/td> 0.88 Nm<\/td> 2.8 Nm<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nSpeed<\/h3>\n\n\n\n
Assuming the GARP has a wheel diameter of 6″, the circumference becomes 0.48 meters or inversely 2.1 revolutions per meter, and the threshold and objective wheel rotational speeds (1 and 2 m\/s, respectively) in revolutions per second become 2.1 and 4.2 RPS:<\/p>\n\n\n
![\"\omega^{(T)} = v^{(T)} \cdot \frac{1}{\pi D} = 1 \frac{\text{m}}{\text{s}} \cdot 2.1 \frac{\text{rev}}{\text{m}} = 2.1 \frac{\text{rev}}{\text{s}}\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-a3fa5e38be2a3c100761233b6ed5623d_l3.png\" "\\"Rendered")
<\/p>\n\n\n
![\"\omega^{(O)} = 2 \frac{\text{m}}{\text{s}} \cdot 2.1 \frac{\text{rev}}{\text{m}} = 4.2 \frac{\text{rev}}{\text{s}}\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-af108043970907bd792bd1fabe1372fb_l3.png\" "\\"Rendered")
<\/p>\n\n\n\n
In summary:<\/p>\n\n\n\n<\/td> Threshold (1 m\/s)<\/strong><\/td>Objective (2 m\/s)<\/strong><\/td><\/tr>Rotational Velocity<\/strong><\/td>2.1 RPS<\/td> 4.2 RPS<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nMounting<\/h3>\n\n\n\n
While not a performance requirement, mounting of the selected motor will need to be considered. A matched mounting option is desirable, but is secondary to other considerations, and the introduction of pillow blocks is expected.<\/p>\n\n\n\n
Electrical Characteristics<\/h3>\n\n\n\n
When selecting a motor, several electrical characteristics will need to be considered. The motor assembly will need to include an encoder and a driver or electronic speed control (ESC) so when selecting a motor, a motor driver will need to be identified as well. I expect to make the motor controller as a learning experience, so a matched\/OEM motor controller isn’t necessary.<\/p>\n\n\n\n
The motors are expected to dominate the GARP’s power budget (with the possible exception of the Perception Subsystem that will likely include a GPU); To support the power budget calculations and Power Subsystem design, we can calculate the power and currents consumed by the motors at various voltage levels. A rough estimate of the power for the motor can be estimated as:<\/p>\n\n\n
![\"P = 2 \pi \cdot T [\text{Nm}] \cdot \omega [\text{RPS}]\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-d774385f3062f5777f9b6a36df2e5b2b_l3.png\" "\\"Rendered")
<\/p>\n\n\n\n
which for the threshold and objective cases become:<\/p>\n\n\n
![\"P^{(T}} = 2\pi \cdot 1.1 \text{ Nm} \cdot 2.1 \text{ RPS} = 14 \text{ W}\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-95334ef0d2c6bf821e0f8ba076ad3357_l3.png\" "\\"Rendered")
<\/p>\n\n\n
![\"P^{(O)} = 2\pi \cdot 2.8 \text{ Nm} \cdot 4.2 \text{ RPS} = 74 \text{ W}\"](\"https:\/\/michael-stinger.com\/wp-content\/ql-cache\/quicklatex.com-aa0f44132642243a73a218dd2780d424_l3.png\" "\\"Rendered")
<\/p>\n\n\n\n
The size (and cost) of the power converters used to produce the GARP’s motor voltage rail is expected to scale with the current drawn from the converters. To understand the magnitudes of current draw (neglecting conversion efficiencies) the threshold and objective powers become:<\/p>\n\n\n\n
- \n
- Design<\/a><\/li>\n\n\n\n
- Hardware<\/a><\/li>\n\n\n\n
- Software<\/a><\/li>\n<\/ul>\n<\/li>\n\n\n\n
- GARP Mobility Subsystem\n
- \n
- Motor Selection<\/a><\/li>\n\n\n\n
- Subsystem Design<\/a><\/li>\n\n\n\n
- Motor Controller\n
- \n
- Design<\/a><\/li>\n\n\n\n
- Hardware Abstraction Layer (HAL)<\/a><\/li>\n\n\n\n
- Motor Controller Interfaces<\/a><\/li>\n\n\n\n
- Alpha Implementation<\/a><\/li>\n<\/ul>\n<\/li>\n\n\n\n
- C&DH\n
- \n
- Design<\/li>\n\n\n\n
- Alpha Implementation<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n\n\n\n
- GARP E-Stop\n
- \n
- Design<\/li>\n\n\n\n
- Alpha Implementation<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/details>\n\n\n\n
Requirements and Constraints<\/h2>\n\n\n\n
I don’t have any hard-and-fast performance requirements for the GARP, so I’ll take an iterative motor selection strategy for GARP. I’ll aim for an initial motor selection with emphasis on ease, integrate it, and then use the resultant performance of the integrated platform to help refine requirements and select a more capable motor. That said, I’ll want to select a cheaper motor initially as (1) I’m buying four, and (2) I expect to trade them out for another model in the future.<\/p>\n\n\n\n
To make the initial motor selection, I’ll make a few basic assumptions about the design of GARP and draw up some rough estimates of the motor specifications needed. This will help seed the search (and will likely provide a comical review once the GARP is integrated and rolling.) The starting list of assumptions are:<\/p>\n\n\n\n
- \n
- GARP will use a wheel with diameter of 6″ (152.4 mm)<\/li>\n\n\n\n
- GARP will use one motor per wheel (i.e. no differentials, etc.)<\/li>\n\n\n\n
- GARP will weigh 50 lbs (23 kg)<\/li>\n\n\n\n
- GARP will have a ground speed threshold of 1 m\/s (2.2 MPH) and an objective speed of 2 m\/s (4.5 MPH); This puts GARP between a Clearpath Jackal and Husky<\/li>\n\n\n\n
- GARP will have a target acceleration of 1 m\/s2<\/sup> (i.e. will reach maximum speed in 1-2 seconds)<\/li>\n<\/ul>\n\n\n\n
Furthermore, I’ll neglect loss of traction, and assume no losses in the drive train.<\/p>\n\n\n\n
Torque<\/h3>\n\n\n\n
To arrive at a torque target, I’ll need to make some further assumptions about the use cases of the GARP. To make things easy at this initial motor selection, as a threshold use case, I’ll assume GARP is operating in grass with a rolling friction factor of 0.15, and on level terrain. For an objective use case, I’ll have GARP push a reel mower (because adding rapidly spinning blades to the front of a robot really ups the stakes.) Beginning with the threshold case, this means that the body-centered force required to move the GARP is 33.4 N:<\/p>\n\n\n
<\/p>\n\n\n\nThis is 8.3 N per wheel and with 6″ wheels, is 0.64 Nm of torque per wheel:<\/p>\n\n\n
<\/p>\n\n\n\nFor the accelerating case, the force required to accelerate the GARP at 1 m\/s2<\/sup> is 23 N, or 5.8 N per wheel, and an additional 0.43 Nm of torque per wheel (assuming the rolling friction is independent of velocity and acceleration):<\/p>\n\n\n
<\/p>\n\n\n\nSumming these values places the estimated required torque per wheel at 1.1 Nm.<\/p>\n\n\n\n
For the objective case of GARP pushing a reel mower, assume the coefficient of rolling friction for the reel mower is twice that of the GARP (i.e. 0.3), and that the weight of the reel mower is 52 lbs. This weight is from the Fiskars Staysharp 18″ Reel Mower<\/a> (but the doubled rolling friction is entirely arbitrary). In this case the static torque required per wheel is 1.96 Nm (an additional 1.3 Nm), the dynamic torque is 0.88 Nm (+0.45 Nm), and the total torque required per wheel is 2.8 Nm (+1.8 Nm).<\/p>\n\n\n\n
In summary, the torque required per wheel for the constant velocity and accelerating cases is:<\/p>\n\n\n\n
<\/td> Constant Velocity<\/strong><\/td> Accelerating<\/strong><\/td> Total<\/strong><\/td><\/tr> Threshold (GARP only)<\/strong><\/td> 0.64 Nm<\/td> 0.43 Nm<\/td> 1.1 Nm<\/td><\/tr> Objective (GARP and Reel Mower)<\/strong><\/td> 2.0 Nm<\/td> 0.88 Nm<\/td> 2.8 Nm<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n Speed<\/h3>\n\n\n\n
Assuming the GARP has a wheel diameter of 6″, the circumference becomes 0.48 meters or inversely 2.1 revolutions per meter, and the threshold and objective wheel rotational speeds (1 and 2 m\/s, respectively) in revolutions per second become 2.1 and 4.2 RPS:<\/p>\n\n\n
<\/p>\n\n\n<\/p>\n\n\n\nIn summary:<\/p>\n\n\n\n
<\/td> Threshold (1 m\/s)<\/strong><\/td> Objective (2 m\/s)<\/strong><\/td><\/tr> Rotational Velocity<\/strong><\/td> 2.1 RPS<\/td> 4.2 RPS<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n Mounting<\/h3>\n\n\n\n
While not a performance requirement, mounting of the selected motor will need to be considered. A matched mounting option is desirable, but is secondary to other considerations, and the introduction of pillow blocks is expected.<\/p>\n\n\n\n
Electrical Characteristics<\/h3>\n\n\n\n
When selecting a motor, several electrical characteristics will need to be considered. The motor assembly will need to include an encoder and a driver or electronic speed control (ESC) so when selecting a motor, a motor driver will need to be identified as well. I expect to make the motor controller as a learning experience, so a matched\/OEM motor controller isn’t necessary.<\/p>\n\n\n\n
The motors are expected to dominate the GARP’s power budget (with the possible exception of the Perception Subsystem that will likely include a GPU); To support the power budget calculations and Power Subsystem design, we can calculate the power and currents consumed by the motors at various voltage levels. A rough estimate of the power for the motor can be estimated as:<\/p>\n\n\n
<\/p>\n\n\n\nwhich for the threshold and objective cases become:<\/p>\n\n\n
<\/p>\n\n\n<\/p>\n\n\n\nThe size (and cost) of the power converters used to produce the GARP’s motor voltage rail is expected to scale with the current drawn from the converters. To understand the magnitudes of current draw (neglecting conversion efficiencies) the threshold and objective powers become:<\/p>\n\n\n\n
- Hardware Abstraction Layer (HAL)<\/a><\/li>\n\n\n\n
- Subsystem Design<\/a><\/li>\n\n\n\n
- Hardware<\/a><\/li>\n\n\n\n
![\"\"](\"https:\/\/michael-stinger.com\/wp-content\/uploads\/2025\/05\/image.png\")
<\/figure>\n\n\n\n![\"\"](\"https:\/\/michael-stinger.com\/wp-content\/uploads\/2025\/05\/sample-motors-rpm-vs-torque.png\")
<\/figure>\n\n\n\n