Week 4: Skipping & Lift Assists

Week 4: Skipping & Lift Assists

Build a lift that can raise and hold 4lbs of sacks 12 off the ground Learn build techniques for lifts Learn gear ratios Learn how to program more than the joystick

Topic 1: Intro, set up, basic torque management and lift practices Topic 2: "Keep the box strong" how to make a solid tower and why it's important as you lift Topic 3: Lift types: 2 bar, 4 bar, 6 bar, scissor, linear slide, compound linear slides Topic 4: Gear trains & the dreaded skipping and friction Topic 5: Spacing and motor placement - building with repair in mind Topic 6: Lift assist methods - rubber bands, black bands, counter weights Topic 7: Basic Control & Programming. Sensor selection & usage Topic 8: Programming to control the arm to stay - several techniques and topics here.

Arm Styles Single, 2-bar, 4-bar, or 6-bar rotate about a point Linear slides and Scissor Lifts move up/down Gear Ratios Power up to use more torque Power down to go faster Gear Trains Put gears together to multiply the effects

We re going to use Rally software for our Agile Project Management. In particular, we re going to use their free Community Edition One person per team: Sign up for a free account at: https://www.rallydev.com/platformproducts/community-edition-signup The ONE person can then add up to 9 other people s accounts to that subscription Refer to our coach demo at: https://rally1.rallydev.com. o Sign into the demo with the READ-ONLY account: Account: viewer@stemrobotics.org Password: STEMRobotics

Product Level Robot System Level Mechanic al Control Sub-System Level Manipulator Drive Electrical Programmin g Component Level Chassis Power Transmission Hardware Level Gear Box Wheels Chain/Sprock et

Here s the basic process:

Use 2 Vex 393 motors max out at 7 in-lb 12 long arm weighing 1lb A grabber weighing 1.2lb (12 away) 84 tooth gear attached to a 12 tooth gear 7:1 ratio Torque to be lifted: Arm s contribution: 1lb at 6 for center of mass o 6 in-lb Grabber s contribution: 1.2lb at 12 o 14.4 in-lb Sacks contribution: 4lb at 12 o 48 in-lb Add them all together! Total: 68.4 in-lb

Lift Power: 2 motors: 14 in-lb Geared up 7:1 98 in-lb Should be enough, but friction and inefficient power transfer takes away an estimated 30% of our power New estimate: 68.6 in-lb Rather close So what can we do about it???

Power will be transmitted via the easiest path Gears are typically the easiest Except when load gets too big Shafts want to bend apart don t let them Gear pitch is the space between gear teeth Defines the contact are on Vex gears Contact area is effected by the gear width Wider gears gives you less force per area Double up gears for larger loads

Narrow Spaced Tower Wide Spaced Tower Only minimal axle bending under load Can take more load and stalls motor before skipping gears Axles bend like bananas! When under high load the gears skip well before the motor stalls. Team 7682 Educational Video Essay

Keep gears & towers tight side to side Spacers, other gears, washers stop bends Delrin Bearing Blocks Keep Axles Safe Square holes are larger than the axles need Add a gear on top and/or bottom Squeeze the load gears together to eliminate bends More/wider gears allow for effective transfer Think of the forces put on the smallest areas Keep transmission points to a minimum The fewer steps to lift, the less loss you will have Friction is the enemy as much as bending

Add structure to add a Delrin block in the middle of the span If it s 12-36 tooth gears, use metal lock bars Holds axles at fixed distance from each other Do it on both sides of the gear Use Zip Ties to hold axles tight Not the prettiest solution, but works Make sure zip ties are over spacers reducing frction

Springs are typical left assists Vex allows us two main ways to assist your lift Black surgical tubes #32 Rubber Bands Pneumatics could be considered too but they are more like another motor (not for this class)

The stretch of a rubber band gives a force back to the arm to help it lift The tension of the rubber band helps pull the arm

#32 Rubber Band Average Spring constant k 88 N/m Initial length x min 80 mm 3 inches Maximum extended length x max 460 mm 18 inches Maximum length change Δx = x max - x min 380 mm 15 inches Theoretical Maximum Energy 0.5*k*Δx 2 5.02 Joules 44.25 in-lb Maximum Energy In Measured 4.08 Joules 36.1 in-lb Maximum Available Energy Measured 2.02 Joules 17.87 in-lb http://design.caltech.edu/archive/2001/handouts/elasticband32.html

You have to have some tension on your rubber band to have a pulling force Don t let the rubber band get too slack Stretching a band takes more energy than what you receive but why??? How can we depend on these rubber bands? Look at the graph and choose the deflection length you will use Middle line deflection is about 1.2 J or 10.6 in-lb Adding bands gets more force but weaker over time

To get that lift, you need to preload the band On 4 or 6 bars, the vertical distance is your helping load so make sure there s space On single arms, the over the shoulder axle gives the band strength Vertical band elastics is another option Scissors depend upon horizontal stretch to squeeze Load the arm with bands without the motor, balance the arm with just bands