Changes between Version 2 and Version 3 of Playbook/Engineering/MotorSelectionPhysics

Timestamp:

Feb 10, 2018, 3:25:29 PM (7 years ago)

Author:

Timothy Lin

Comment:

--

Playbook/Engineering/MotorSelectionPhysics

@@ -1 @@
-= Motor Selection =
+= A General Method for Choosing Motors and Gearings
+
+{{{
+#!div style="border:3pt dotted; color:red; text-align:center; font-size:150%"
+Work in Progress
+
+
+}}}
+
+The example/case study below (in the green boxes) are probably more intuitive and easier to follow than the theory presented first in each section, but the theory discusses the why and how.
+
 == 1. Define the problem. ==
 Typically, you have the following overarching constraints and requirements in FRC:
@@ -10 +20 @@
  * mass budget
  * space
+ * specific allowable motors
 
 '''Requirements'''
@@ -16 +27 @@
    * distance - what is the range of the mechanism?
 
-On this page, we'll ignore mass and space constraints.
-
+On this page, we'll ignore mass and space constraints for the most part.
+
+{{{
+#!div style="color:red; text-align:left;"
+First, let's consider power and efficiency.
+}}}
 ----
+
 From physics, power is equivalently defined by the following: \\
 {{{
-#!div style="border:1pt dotted; color:blue; text-align:center; font-size:120%;"
+#!div style="border:1pt dashed; color:blue; text-align:center; font-size:120%;"
 P = W/t = Fd/t = Fv
 }}}
@@ -148 +164 @@
 
 
-== 2. Determine torque and actual speed (using motor curves).
-
-
-== 3. Find a gearing: gearbox, etc.
+== 2. Understand the requirements of the system.
+In section (1), we considered power and efficiency as the basis for understanding the problem. Next, we consider mechanical systems that can accomplish our needs.
+
+Since we don't exactly know the efficiency of the final motor+gearing combination, we still need to consider our {{{effective power}}} = {{{required power}}} * {{{efficiency}}}.
+
+We also need to consider
+* torque requirements
+* speed requirements
+* current limitations
+for the
+* motor
+* gearing, etc.
+
+{{{
+#!div style="border:1pt solid; color:green; text-align:left; font-size:100%;"
+Back to lifting the 600 N robot.
+
+Let's assume we're going to go with a 775pro. Useful information about the problem and the motor from the above box:
+
+{{{#!th align=center
+'''Max Power of the 775pro'''
+}}}
+{{{#!td align=center
+347 W
+}}}
+|----------------
+{{{#!th align=center
+'''Desired Linear Velocity'''
+}}}
+{{{#!td align=center
+1 ft/s = 0.305 m/s
+}}}
+|----------------
+{{{#!th align=center
+'''Required Power to Lift'''
+}}}
+{{{#!td align=center
+~ 180 W
+}}}
+|----------------
+
+''Rationale for Desired Linear Speed:'' Climbing in about 1 s is decently fast, and even if the efficiency is about 50%, the motor+gearing will be able to accomplish that.
+
+This next part will require some trial and error, trying different combinations of gearing and output radius:
+
+* Suppose our design involves a winch with a 4 in. (.1 m) diameter drum --> radius of drum is 2 in. (.05 m).
+    The force available to lift the 600 N robot at the output will be divided by the radius (read: smaller radii enable more force with less deflection).
+* The following gear ratios are available in stock (either in the shop or from a manufacturer) in the form of a "standard" gearbox:
+  * 1:16
+  * 1:10
+  * 1:8
+  * 1:4
+
+Let's take a loop at the motor curve for the 775pro:
+
+[[Image(http://content.vexrobotics.com/motors/217-4347-775pro/775pro-motor-curve-20151208.PNG)]]
+
+'''Stall Torque Analysis'''\\
+From the design of a 2 in. (.05 m) radius drum at the output, we need an output torque of at least 30 Nm
+
+'''Peak Power Analysis'''\\
+
+'''Peak Efficiency Analysis'''\\
+
+}}}
 
 
 == Other Considerations
 * mass
-* 
+* lead times