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 + +
+ + + + + + + + +