From b9fc5a9677e0f3e805ef13713dae36b408bb78a3 Mon Sep 17 00:00:00 2001 From: Danny Diaz Date: Mon, 24 Jun 2024 00:43:53 -0400 Subject: [PATCH] Techtips (#275) * Added 6/24/2024 Tech Tip Content --- docs/source/tech_tips/tech-tips.rst | 452 +++++++++++++++++++++++++++- 1 file changed, 451 insertions(+), 1 deletion(-) diff --git a/docs/source/tech_tips/tech-tips.rst b/docs/source/tech_tips/tech-tips.rst index 7a61a212..01bc8ce6 100644 --- a/docs/source/tech_tips/tech-tips.rst +++ b/docs/source/tech_tips/tech-tips.rst @@ -11,9 +11,453 @@ on this page chronologically, with the newest content at the top of the page. Just click to expand the Tech Tip you'd like to read. -.. dropdown:: Week of 06/10/2024 "Updating the SDK Manifest" +.. dropdown:: Week of 06/24/2024 "Calculating Motor and Servo Power" :open: + .. _calculatepower: + + In this Tech Tip of the Week we’ll be exploring mechanical and electrical + power, why some types of power are calculated differently for motors versus + servos, and how to use this calculated power to compare servos. This Tech + Tip was written and fact-checked with the help of Google Gemini 1.5 Flash + using Google AI Studio. + + The fundamental concept we need to understand is power. We are generally + concerned with two similar but different kinds of power, so let’s look at + the two most common types. In a motor, **electrical power** is the energy + supplied by the electrical current flowing through the motor's windings. + This electrical energy is transformed into **mechanical power**, which is + the rate at which the motor performs work by rotating a shaft. Both kinds of + power are measuring different aspects of the motor; electrical power deals + with the movement of electrical charges, and mechanical power deals with the + movement of objects due to forces. Both of these measurements are expressed + in the same unit, Watts (W), because power, in general, is defined as the + rate of energy transfer or work done. No matter the form of energy + (electrical, mechanical, thermal, etc.) the fundamental concept of power + remains the same. Even though these two power measurements carry the same + unit, they are calculated differently and **cannot be used interchangeably + (or together!)**. + + Motors and servos are constructed similarly - both are electromechanical + devices that convert electrical energy into mechanical energy - but there + are big differences in how they’re used. Motors are often used in + applications requiring continuous power, such as pumps, fans, and conveyor + systems. Motors are typically rated for **continuous power output**, meaning + they can sustain that power level indefinitely without overheating. Servos + are commonly used in robotics and precision positioning systems, where + controlled movement and precise positioning are essential. Servos are + designed for intermittent operation - typically cycling through on/off + periods to control movement - and are often rated for their **stall torque** and + **no-load speed** reflecting their ability to hold a position against a force + and how fast they move when unloaded. While electrical power is calculated + generally the same for both types of devices, these design and use + differences have a big impact on how mechanical power is calculated. + + Both motors and servos calculate **electrical power** the same, using the + standard electrical power formula: + + - *Electrical Power(W) = volts(V) x amps(A)* + + For example, a typical REV Smart Servo is supplied with 6V when used with a + REV Servo Power Module (SPM) or 5V when used with a Control or Expansion + Hub. Per the servo’s specs, at 6V the servo will pull at most 2A at stall + (when the servo cannot physically move to the position it’s being commanded + to). This means the maximum electrical power the servo will consume is + 12Watts of power when plugged into the REV SPM and being commanded to a + position it cannot reach. The REV SPM supplies 90W of maximum electrical + power, so the maximum number of fully-stalled REV Smart Servos the SPM can + supply full power to is 7 (90W divided by 12W, ignoring the remainder). + + Motors and servos calculate mechanical power differently. Because motors are + rated for continuous power output, and thus generally convert electrical + energy into pure mechanical power, motor mechanical power and electrical + power are calculated the same. + + - *Motor Mechanical Power(W) = volts(V) x amps(A)* + + Servo mechanical power is calculated a bit differently due to the + fact that servos convert electrical energy into mechanical motion, not pure + mechanical power. Because of this, the torque, speed, load, efficiency, and + duty cycle of the servo has to be accounted for, making it very complicated + to calculate perfectly. Instead, a reasonable approximation is: + + - *Servo Mechanical Power(W) = 0.25 x stall torque(N-m) x no-load speed(rad/s)* + + It’s important to understand that this formula is often written in an + equivalent form representing the product of half the maximum stall torque + (when the servo is unable to move) and half the no-load angular speed (when + the servo is not pushing against any force other than its own internal + friction). Using this approximation the REV Smart Servo, when being + provided 6V, produces a maximum Stall Torque of 13.5kg-cm (1.33N-m) and a + time of 0.14s per 60 degrees of travel (7.48rad/s) yielding an approximate + servo mechanical power of 2.48W. + + One of the most difficult parts of calculating Servo Mechanical Power is + working with unit conversions, especially since servo manufacturers use lots + of different units. In order to calculate servo mechanical power correctly + the speed unit MUST be converted to radians-per-second and the max stall + torque unit MUST be converted to Newton-meters. Below is a handy calculator + that you can use to automatically perform the necessary conversions and + calculate Servo Mechanical Power (*Thank you to Orion DeYoe for providing + this tool*). + + .. raw:: html + + + + + + + + + + +
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+ + + .. tip:: + + - For Speed, use the radio button to choose the unit type that the + manufacturer has provided - for most servos this will be listed in a + period of time per 60 degrees (such as with the REV Smart Servo + example) or perhaps the manufacturer may provide an angular velocity, + such as rotations-per-minute (RPM). Enter the no-load speed value and + unit as the manufacturer has provided. + + - For stall torque, provide the value and select the unit as specified by + the manufacturer. If the manufacturer merely provides kg, assume kg*cm. + + The calculator automatically recalculates on any + changes, there is no button to press in order to trigger a calculation. + + Here is a handy table of some common servo mechanical power values: + + .. list-table:: Common Servo Mechanical Power Values ( @6V ) + :widths: 50 20 20 20 20 20 + :header-rows: 1 + :align: center + + * - Description + - Speed + - Torque + - Stall Current + - Power + - Cost ($USD) + * - `Tetrix MAX Standard (HiTec HS-485HB) `__ + - 0.18 s/60° + - 6 kg-cm + - 1.2 A + - 0.86 W + - $29.50 + * - `REV Smart Servo `__ + - 0.14 s/60° + - 13.5 kg-cm + - 2.0 A + - 2.48 W + - $30.00 + * - `goBILDA 2000 Series Speed Servo `__ + - 0.09 s/60° + - 9.3 kg-cm + - 2.5 A + - 2.65 W + - $33.99 + * - `Axon Robotics Micro+ `__ + - 0.075 s/60° + - 7.8 kg-cm + - 2.2 A + - 2.67 W + - $63.79 + * - `goBILDA 2000 Series Torque Servo `__ + - 0.20 s/60° + - 300 oz-in + - 2.5 A + - 2.77 W + - $33.99 + * - `Studica Multi-Mode Smart Servo 200 `__ + - 0.046 s/60° + - 5 kg-cm + - 2.7 A + - 2.79 W + - $24.99 + * - `goBILDA 2000 Series Super Speed Servo `__ + - 0.043 s/60° + - 4.7 kg-cm + - 2.5 A + - 2.81 W + - $33.99 + * - `AndyMark am-4954 High Torque Servos `__ + - 0.20 s/60° + - 22 kg-cm + - 1.7 A + - 2.82 W + - $34.00 + * - `Studica Multi-Mode Smart Servo `__ + - 62 RPM + - 20 kg-cm + - ?? + - 3.18 W + - $23.99 + * - `AndyMark am-4955 High Speed Servos `__ + - 0.05 s/60° + - 7 kg-cm + - 2.7 A + - 3.59 W + - $30.00 + * - `FeeTech FT5335M-FB `__ + - 0.20 s/60° + - 35 kg-cm + - 4.0 A + - 4.49 W + - $52.95 + * - `HiTec HS-805BB `__ + - 0.14 s/60° + - 24.7 kg-cm + - 6.0 A + - 4.53 W + - $49.99 + * - `HiTec HSR-M9382TH `__ + - 0.17 s/60° + - 34 kg-cm + - 2.7 A + - 5.13 W + - $199.99 + * - `Power HD GTS3 `__ + - 0.083 s/60° + - 20 kg-cm + - 4.0 A + - 6.19 W + - $120.00 + * - `Axon Robotics MINI+ `__ + - 0.09 s/60° + - 25 kg-cm + - 3.8 A + - 7.13 W + - $79.99 + * - `Axon Robotics MAX+ `__ + - 0.115 s/60° + - 34 kg-cm + - 4.0 A + - 7.59 W + - $79.99 + +.. dropdown:: Week of 06/10/2024 "Updating the SDK Manifest" + .. _updatingthesdkmanifest: This week’s Tech Tip of the Week comes to us from an amalgamation of emailed @@ -1304,6 +1748,8 @@ Just click to expand the Tech Tip you'd like to read. .. dropdown:: Week of 09/18/2023 "Technical Update video by AJ Foster" + .. _techvideo9182023: + This week’s Tech Tip of the Week is a Video Tech Tip of the Week from AJ Foster, *FIRST* Tech Challenge World Championship FTA and Orlando Robotics League All-Star Volunteer. AJ gives a great synopsis on many of the key @@ -1313,6 +1759,8 @@ Just click to expand the Tech Tip you'd like to read. .. dropdown:: Week of 09/11/2023 "Updating the Robot Controller App" + .. _updateRobotControllerApp: + This week’s Tech Tip of the week is all about updating software on your Control Hub. If you use :ref:`Android Studio `, @@ -1326,6 +1774,8 @@ Just click to expand the Tech Tip you'd like to read. .. dropdown:: Week of 09/04/2023 "Battery Charging" + .. _batteryCharging: + The *FIRST* Tech Challenge Tech Tip of the week this week is all about Battery Charging. There are :ref:`three robot main batteries `