diff --git a/docs/source/tech_tips/images/dc-motor-curve.jpg b/docs/source/tech_tips/images/dc-motor-curve.jpg new file mode 100644 index 00000000..e2262b42 Binary files /dev/null and b/docs/source/tech_tips/images/dc-motor-curve.jpg differ diff --git a/docs/source/tech_tips/images/dc-motor-curve.png b/docs/source/tech_tips/images/dc-motor-curve.png new file mode 100644 index 00000000..c29c7306 Binary files /dev/null and b/docs/source/tech_tips/images/dc-motor-curve.png differ diff --git a/docs/source/tech_tips/tech-tips.rst b/docs/source/tech_tips/tech-tips.rst index 01bc8ce6..0a23e4ce 100644 --- a/docs/source/tech_tips/tech-tips.rst +++ b/docs/source/tech_tips/tech-tips.rst @@ -17,10 +17,10 @@ Just click to expand the Tech Tip you'd like to read. .. _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. + power, why some types of power are calculated differently, 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 @@ -51,7 +51,7 @@ Just click to expand the Tech Tip you'd like to read. **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. + differences have an impact on how mechanical power is determined. Both motors and servos calculate **electrical power** the same, using the standard electrical power formula: @@ -68,29 +68,41 @@ Just click to expand the Tech Tip you'd like to read. 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)* + Motors and servos also generally calculate mechanical power similarly. + + - *Mechanical Power(W) = torque (N-m) x angular speed (rad/s)* + + Mechanical Power for a DC motor generally follows a very specific curve, + based on its efficiency, stall current, stall torque, speed, and a bunch of + other factors. The general performance curve of a DC motor can be seen in + Figure 1. + + .. figure:: images/dc-motor-curve.* + :width: 75% + :align: center + :alt: DC Motor Performance Curves - 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: + Figure 1: General DC Motor Performance Curve - - *Servo Mechanical Power(W) = 0.25 x stall torque(N-m) x no-load speed(rad/s)* + From this we can see that the Peak Power is found at the intersection of 1/2 + Stall Torque and 1/2 Speed. Even though a servo is used different than a + generic motor, this approximation is still good for calculating the maximum + mechanical power of a servo. Simplified, we can use this formula: + + - *Servo Max Mechanical Power(W) = 0.25 x stall torque(N-m) x no-load speed(rad/s)* + + 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 max servo + mechanical power of 2.48W. + + .. tip:: - 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. + It's important to point out that a high speed motor or servo that is + loaded past its maximum power point will actually do worse than a + slower motor or servo with the same load. It's all about getting the + maximum mechanical power by operating the motor at the max power + point. One of the most difficult parts of calculating Servo Mechanical Power is working with unit conversions, especially since servo manufacturers use lots @@ -357,7 +369,7 @@ Just click to expand the Tech Tip you'd like to read. - Speed - Torque - Stall Current - - Power + - Max Power - Cost ($USD) * - `Tetrix MAX Standard (HiTec HS-485HB) `__ - 0.18 s/60°