Difference between revisions of "DEWBOT XIV Drive Train"

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[[image:CVT_swerve_2018.jpg|300px|right]]One drawback of swerve drive vis-à-vis tank drive is the relative penalty paid for incorporating gear shiftingTank drive, with two independent powerplants, requires two gear shifting mechanisms; Swerve, with four independent powerplants, requires four.  This is a serious design hurdle for a drive train which is already a little ''avoirdupois''.
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[[image:CVT_swerve_2018.jpg|300px|right]]Sab-BOT-age's swerve focus for 2018 was to enable straight and accurate driving, in both autonomous and teleoperated modesAccurate driving and navigation in autonomous was quickly identified as a critical performance requirement for FIRST Power-Up.  This performance requirement is based on our need to:
 +
# Score in either side of the scale during autonomous; and
 +
# Score two or more cubes in the switch or scale during autonomous
  
==CVT Reduction==
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CVT swerve, first employed in [[DEWBOT XIII Drive Train | DEWBOT XIII]], was again used for the 2018 robot. Fast is good.
In past swerve systems, reduction had been managed in two stages: a 3.11:1 1<sup>st</sup> stage reduction via HTD5 pulleys (18T & 56T) from CIM to coaxial drive shaft; and a 2.67:1 2<sup>nd</sup> stage reduction from the miter gear to wheel via sprockets and chain.
 
  
[[image:1640_Swerve_2017_-_CVT.png|300px|left]]The current system replaces the HTD5 pulley reduction with a spring-loaded constant velocity pulley (Torque Transmission VPS-10 and a V-belt pulley (Torque Transmission PO-3.5-3/8) connected with a V-belt (Gates 2L 180).  The constant velocity pulley had a pitch diameter which varies between 1.375 and 2.75 inches; the other pulley has a fixed 3.4 inch pitch diameter.  These provide a 1<sup>st</sup> stage reduction which can be varied between 2.47:1 and 1.24:1.  In combination with the old 2<sup>nd</sup> stage reduction, the overall reduction can be varied over the range 6.59:1 to 3.30:1.
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==Backlash Reduction==
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[[image:DB14_swerve_gearbox-1.jpg|200px|left|thumb|Steering gearbox]]Backlash in 1640's swerve were traced back to:
  
The constant velocity pulley comprises two v-pulley flanges forced together by spring loading.  Under low belt tension, the pulley flanges remain together and the v-belt rides in the high-diameter position.  As belt tension is increased, the pulley flanges are pushed apart, bringing the belt into a lower-diameter path.  Since the drive pulley is constant velocity, increasing the belt tension increases the reduction ratio.
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1) the reduction gearbox;
  
The V-belt is tensioned variably using a servo which controls the positions of a pair of tensioning pulleys.  The servo and tensioning pulleys are mounted on an upward extension of the coaxial drive shaft.
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2) undersized keys on pulleys.
 +
 
 +
We have used 3-stage 132:1 BaneBot P60 gearboxes for steering since 2012 ([[DEWBOT VIII]]).  These gearboxes are not re-used year-to-year, but are purchased new for each generation of swerve drives.  The key cause of backlash in the P60 gearboxes is the double-D connection between the final planet carrier and the output shaft.  This backlash tends to increase over time.
 +
 
 +
The team identified a 100:1 planetary gearbox (17HS19-1684S-PG100) from Steppermotorsonline.com with integral output shaft/planet carrier.  This gearbox has extremely low backlash and testing in 2017 indicated that they were robust enough for FRC service.  These gearboxes are purchased with stepper motors attached; these stepper motors are removed and replaced with AndyMark 9015 motors (mated via printed mounts). In spite of having to purchase and discard the stepper motors, this gearbox is a cost savings vis-à-vis the P60 it replaces.
 +
 
 +
The weak point of the new gearbox is its unfortunate D output shaft.  A great deal of development went into making the connection to this D-shaft reliable and durable.  This effort is likely to be ongoing.
 +
 
 +
An alternative gearbox for consideration is VexPro's Versaplanetary gearbox.  These also have low backlash (with spline connections between output shaft and planet carriers and between stages).  The form factor of the Versaplanetary offers challenges and it would add weight and cost to 1640's swerve modules.
 +
 
 +
To reduce backlash at the pulley/shaft connections:
 +
# The drive pulley is now printed with a 1/2" Hex hub.  A 1/2" Hex shaft mates precisely with this pulley.
 +
# On the driven side, a 1/4" key fits precisely in a keyway in the 1" pivot shaft.  A printed pulley with a 1/4" broached keyway fits snugly over this.
 +
 
 +
We also aggressively stopped a bad practice of cutting keyways too small, thereby requiring keys to be sanded to fit.  This poor practice led to systematically undersized keys.  In addition to introducing backlash, these undersized keys had a tendency to fall out.
 +
 
 +
==Quadrature Encoder==
 +
[[image:DB14_swerve_gearbox-7.jpg|300px|right|thumb|Encoder mounted on swerve module]]After years of employing makeshift encoder devices (none of which were satisfactory) on our swerve modules, we finally took the action of adding a real quadrature encoder.  This was driven by the need to navigate and maneuver accurately during autonomous (also to make teleoperated drive easier).
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 +
The encoder is on its own axle, driven by round belt by a 3-D printed pulley piggy-backed under main driven pulley.  A second printed pulley is mounted on the encoder shaft and driven by the round belt.  The printed encoder mount houses the bearing races holding the encoder axle.
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 +
==Proliferation of Printed Parts==
 +
[[image:DB14_180211-17.jpg|200px|left|thumb|2018 swerve module on calibration stand.  piggy-back encoder pulley (yellow) and round belt (orange) under the driven pulley is visible.]]Our 2017 swerve modules were the first to employ a significant number of 3-D printed parts.  The use of printed parts expanded significantly on the 2018 modules, to include:
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 +
25mm Bearing Mount (since 2017)
 +
 
 +
(2) HTD5 32t pulleys with flanges (new)
 +
 
 +
(2) Encoder round pulleys (new)
 +
 
 +
(2) CVT tensioning pulleys (since 2017)
 +
 
 +
CVT tensioning arm (since 2017)
 +
 
 +
Steering motor lead protecting ring (since 2017)
 +
 
 +
Steering motor mount to gearbox (new)
 +
 
 +
Steering motor/gearbox mount to swerve module (new)
 +
 
 +
[[image:Value_Engineering_2018.jpg|440px|right]]Angle encoder mount (since 2016)
  
 
==Motors==
 
==Motors==
 
A CIM motor provides the drive power for the wheels.
 
A CIM motor provides the drive power for the wheels.
  
Our old standard steering motor, the BaneBots RS540, is no longer on the FRC approved list.  It was replaced by the AndyMark 9015 motor (am-0912) at a weight penalty of 0.16 lb<sub>m</sub>.  This motor is mounted on a BaneBots P60S-555-5 132:1 reduction gearbox (same gearbox used since 2013).
+
An AndyMark 9015 motor (am-0912) served as steering motor.  This motor is mounted on a 17HS19-1684S-PG100 100:1 reduction gearbox from Steppermotorsonline.com.  Mounts for the steering motor are 3-D printed.
  
 
The belt tensioning servo is a Rev Robotics Smart Robot Servo (REV 41-1097).  Its metal gears provide good durability.
 
The belt tensioning servo is a Rev Robotics Smart Robot Servo (REV 41-1097).  Its metal gears provide good durability.
  
==Other Developments==
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==Software==
* In 2013, the very heavy 1" top flanged bearing (McMaster 6384K373) [[image:DB13_170204-34.jpg|right|thumb|300px|25mm 6805-2Z bearing and printed holder]]was replaced with an Igus polymer bushing and a bronze sleeve.  This arrangement occasionally failed either in having the Igus bushing become unseated from the module top plate or through increasing rotational frictionIn 2017, this was replaced once more with a bearing, but a much lighter, 25mm bearing (6805-2Z) with a printed bearing retainer. The top of the steering tube is turned down accordingly and a boring head was used to assure the bearing's proper fit in the modules' top platesSo far, this has been a positive change with no issues observed.  The original 1" top flanged bearing (McMaster 6384K373) remains in service in the bottom position.
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[[image:DB14_180128-16.jpg|200px|left|thumb|Swerve assembly line]]The quadrature encoder enabled a change in CVT Shifting logic from motor current to velocity.  This provides more accurate and consistent shifting.
* There's a new encoder scheme: a Hall-Effect sensor employed as a tooth sensor under the 3.5" pulleyThis is not satisfactory.
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 +
DEWBOT's lift raises the robot's center-of-mass and therefore can create problems of instabilityTo help compensate for this, the robot's maximum velocity is reduced as the lift raises.
 +
 
 +
==Quality Control==
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The team established a defined manufacturing process for swerve modules in 2018 to assure quality and consistency in the final product.  Inspections were introduced at specific points in the process.  Team Co-Captain Laura developed this process and oversaw its execution.
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 +
==Value Engineering==
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[[image:DB14_180211-23.jpg|200px|left|thumb|2018 swerve modules showing finished wiring harnesses]]{{#ev:youtube|K5AVrWWAhuk|300|right|Synchronised autonomous at Westtown}}As in 2017 (with the introduction of CVT technology), 2018 value engineering focused on performance improvements relating to allowing straight driving with swerve.  We were able to realize this benefit through cross-field scale scoring at Westtown.  These changes address and correct a key swerve weakness.   
 +
 
 +
While not a purposeful objective, there has been a significant growth of printed parts on the swerve module, facilitating manufacture and reducing costs.
 +
 
 +
We took a serious effort in 2018 to sensibly organize the swerve wiring harnessWhile this had always been a consideration, the 2018 swerve wiring harness can be considered to be designed.
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<br><br><br><br><br>
 
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[[Category:Robot]][[Category:DEWBOT XIV]][[Category:Drive-train]][[Category:Swerve Drive]][[Category:Photo Galleries]][[Category:Engineering]]
 
[[Category:Robot]][[Category:DEWBOT XIV]][[Category:Drive-train]][[Category:Swerve Drive]][[Category:Photo Galleries]][[Category:Engineering]]

Latest revision as of 23:43, 28 March 2018

CVT swerve 2018.jpg
Sab-BOT-age's swerve focus for 2018 was to enable straight and accurate driving, in both autonomous and teleoperated modes. Accurate driving and navigation in autonomous was quickly identified as a critical performance requirement for FIRST Power-Up. This performance requirement is based on our need to:
  1. Score in either side of the scale during autonomous; and
  2. Score two or more cubes in the switch or scale during autonomous

CVT swerve, first employed in DEWBOT XIII, was again used for the 2018 robot. Fast is good.

Backlash Reduction

Steering gearbox
Backlash in 1640's swerve were traced back to:

1) the reduction gearbox;

2) undersized keys on pulleys.

We have used 3-stage 132:1 BaneBot P60 gearboxes for steering since 2012 (DEWBOT VIII). These gearboxes are not re-used year-to-year, but are purchased new for each generation of swerve drives. The key cause of backlash in the P60 gearboxes is the double-D connection between the final planet carrier and the output shaft. This backlash tends to increase over time.

The team identified a 100:1 planetary gearbox (17HS19-1684S-PG100) from Steppermotorsonline.com with integral output shaft/planet carrier. This gearbox has extremely low backlash and testing in 2017 indicated that they were robust enough for FRC service. These gearboxes are purchased with stepper motors attached; these stepper motors are removed and replaced with AndyMark 9015 motors (mated via printed mounts). In spite of having to purchase and discard the stepper motors, this gearbox is a cost savings vis-à-vis the P60 it replaces.

The weak point of the new gearbox is its unfortunate D output shaft. A great deal of development went into making the connection to this D-shaft reliable and durable. This effort is likely to be ongoing.

An alternative gearbox for consideration is VexPro's Versaplanetary gearbox. These also have low backlash (with spline connections between output shaft and planet carriers and between stages). The form factor of the Versaplanetary offers challenges and it would add weight and cost to 1640's swerve modules.

To reduce backlash at the pulley/shaft connections:

  1. The drive pulley is now printed with a 1/2" Hex hub. A 1/2" Hex shaft mates precisely with this pulley.
  2. On the driven side, a 1/4" key fits precisely in a keyway in the 1" pivot shaft. A printed pulley with a 1/4" broached keyway fits snugly over this.

We also aggressively stopped a bad practice of cutting keyways too small, thereby requiring keys to be sanded to fit. This poor practice led to systematically undersized keys. In addition to introducing backlash, these undersized keys had a tendency to fall out.

Quadrature Encoder

Encoder mounted on swerve module
After years of employing makeshift encoder devices (none of which were satisfactory) on our swerve modules, we finally took the action of adding a real quadrature encoder. This was driven by the need to navigate and maneuver accurately during autonomous (also to make teleoperated drive easier).

The encoder is on its own axle, driven by round belt by a 3-D printed pulley piggy-backed under main driven pulley. A second printed pulley is mounted on the encoder shaft and driven by the round belt. The printed encoder mount houses the bearing races holding the encoder axle.

Proliferation of Printed Parts

2018 swerve module on calibration stand. piggy-back encoder pulley (yellow) and round belt (orange) under the driven pulley is visible.
Our 2017 swerve modules were the first to employ a significant number of 3-D printed parts. The use of printed parts expanded significantly on the 2018 modules, to include:

25mm Bearing Mount (since 2017)

(2) HTD5 32t pulleys with flanges (new)

(2) Encoder round pulleys (new)

(2) CVT tensioning pulleys (since 2017)

CVT tensioning arm (since 2017)

Steering motor lead protecting ring (since 2017)

Steering motor mount to gearbox (new)

Steering motor/gearbox mount to swerve module (new)

Value Engineering 2018.jpg
Angle encoder mount (since 2016)

Motors

A CIM motor provides the drive power for the wheels.

An AndyMark 9015 motor (am-0912) served as steering motor. This motor is mounted on a 17HS19-1684S-PG100 100:1 reduction gearbox from Steppermotorsonline.com. Mounts for the steering motor are 3-D printed.

The belt tensioning servo is a Rev Robotics Smart Robot Servo (REV 41-1097). Its metal gears provide good durability.

Software

Swerve assembly line
The quadrature encoder enabled a change in CVT Shifting logic from motor current to velocity. This provides more accurate and consistent shifting.

DEWBOT's lift raises the robot's center-of-mass and therefore can create problems of instability. To help compensate for this, the robot's maximum velocity is reduced as the lift raises.

Quality Control

The team established a defined manufacturing process for swerve modules in 2018 to assure quality and consistency in the final product. Inspections were introduced at specific points in the process. Team Co-Captain Laura developed this process and oversaw its execution.

Value Engineering

2018 swerve modules showing finished wiring harnesses
Synchronised autonomous at Westtown
As in 2017 (with the introduction of CVT technology), 2018 value engineering focused on performance improvements relating to allowing straight driving with swerve. We were able to realize this benefit through cross-field scale scoring at Westtown. These changes address and correct a key swerve weakness.

While not a purposeful objective, there has been a significant growth of printed parts on the swerve module, facilitating manufacture and reducing costs.

We took a serious effort in 2018 to sensibly organize the swerve wiring harness. While this had always been a consideration, the 2018 swerve wiring harness can be considered to be designed.