Changes between Version 3 and Version 4 of Playbook/Engineering/MotorSelectionPhysics

Timestamp:

Feb 11, 2018, 12:56:09 AM (7 years ago)

Author:

Timothy Lin

Comment:

torque analysis case study

Playbook/Engineering/MotorSelectionPhysics

@@ -7 +7 @@
 
 }}}
+
+With the plethora of requirements, motors, gearboxes, pulley systems, etc., how do you choose which motor and gearbox(es) to use? How do you know they will
 
 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.
@@ -177 +179 @@
 * gearing, etc.
 
+----
+
+We'll need to analyze:
+1. required torque and available torque
+    --> torque determines motor speed, which determines your actual output/linear speeds
+2. current draw
+based on a choice of
+* motor
+* gear ratio
+
+'''Torque Analysis'''\\
+The torque/force induced by your load is the driving force for the kinematics of the mechanism. From the system, you can figure out the particular force 
+
 {{{
 #!div style="border:1pt solid; color:green; text-align:left; font-size:100%;"
@@ -212 +227 @@
     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
+  * 1:100
+  * 1:80
+  * 1:60
+  * 1:20
 
 Let's take a loop at the motor curve for the 775pro:
@@ -221 +236 @@
 [[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'''\\
+'''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'''. Note that {{{output torque}}} is related to {{{motor torque}}}:
+    {{{output_torque}}} = {{{motor torque}}} / {{{gear ratio}}}
+where in this case gear ratio is expressed as 1:n (sometimes it is represented as the opposite; sometimes it ''is'' the opposite)
+
+@ Stall Torque of 0.71 Nm: A 1:60 gearbox would get us there.
+    {{{motor torque}}} = 30 Nm * (1/60) = 0.50 Nm needed
+
+But stall torque generally serves only as an upper limit (if stall torque + gear ratio isn't enough, it can't be done; but if it is enough, you still need to crunch numbers a bit).
+
+For the hypothetical gear ratios available for this motor:
+
+{{{#!th align=center
+'''Torque at Output'''
+}}}
+{{{#!th align=center
+'''Gear Ratio'''
+}}}
+{{{#!th align=center
+'''Torque at Motor'''
+}}}
+{{{#!th align=center
+'''Speed at Motor'''\\
+(from Motor Curve)
+}}}
+{{{#!th align=center
+'''Speed at Output'''
+}}}
+{{{#!th align=center
+'''Linear Speed'''
+}}}
+|----------------
+{{{#!td align=center
+30 Nm
+}}}
+{{{#!td align=center
+1:100
+}}}
+{{{#!td align=center
+0.3 Nm
+}}}
+{{{#!td align=center
+10,800 RPM
+}}}
+{{{#!td align=center
+108 RPM
+}}}
+{{{#!td align=center
+0.57 m/s = 1.87 ft/s
+}}}
+|----------------
+{{{#!td align=center
+30 Nm
+}}}
+{{{#!td align=center
+1:80
+}}}
+{{{#!td align=center
+0.375 Nm
+}}}
+{{{#!td align=center
+8500 RPM
+}}}
+{{{#!td align=center
+106.25 RPM
+}}}
+{{{#!td align=center
+0.57 m/s = 1.87 ft/s
+}}}
+|----------------
+{{{#!td align=center
+30 Nm
+}}}
+{{{#!td align=center
+1:60
+}}}
+{{{#!td align=center
+0.5 Nm
+}}}
+{{{#!td align=center
+5500 RPM
+}}}
+{{{#!td align=center
+91.67 RPM
+}}}
+{{{#!td align=center
+0.48 m/s = 1.57 ft/s
+}}}
+|----------------
+{{{#!td align=center
+30 Nm
+}}}
+{{{#!td align=center
+1:20
+}}}
+{{{#!td align=center
+1.5 Nm
+}}}
+{{{#!td align=center
+0 RPM
+}}}
+{{{#!td align=center
+0 RPM
+}}}
+{{{#!td align=center
+0 m/s = 0 ft/s
+}}}
+|----------------
+
+From this table, we see that we can't use the 1:20 gearbox, as it doesn't provided enough output torque given our load and design. But we ''can'' use any other others: 1:100, 1:80, and 1:60. These give us 2 distinct speeds in this to choose from.
+
+'''Current Analysis'''\\
+The other factor we need to take into account is the current draw of the motor. Circuit breakers will trip if both
+1. a current threshold is reached, and
+2. current is above for a certain amount of time.
+
+That is, breakers won't trip for short peaks.
+
+For each of the motor speeds above, there is an associated current
 
 }}}