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Servo Selection
4" STROKE 100 LB 24 VDC WARNER LINEAR ACTUATOR from Surplus center
- Voltage = 24 V
- Max current = 1.5 A
- No load current = 0.4 A
- Stroke length = 101.6 mm
- Speed = 15 mm/s
- Moving force = 445 N
- Holding force = 1300 N
- Maximum duty cycle = 25%
- Price = $50 + $60 shipping
We only require 1 inch of travel but this one has been heavily discounted due to being an end of line item making it easily the cheapest option that will perform the job we require
Servos of sufficient size and power are all well outside our price range ( usually $500 - $1000). Instead we will use a gear motor with an attached encoder. The following motor will be used IG52-04 24VDC from Super Droid Robots. This site charges 65 USD for shipping however the motor + gearbox + encoder is cheaper then almost any other gearbox I could find rated for 10 Nm.
- Voltage = 24 V
- Max current = 2.85 A
- No load current = 0.7 A
- Gearbox ratio = 1: 353
- Speed after gearbox = 10 RPM
- Continuous torque = 10 Nm
- Price = $190 + $85 shipping
A potential problem with this option is it has a very high stall torque of greater then 100 Nm however the gearbox is only rated for momentary torques of 30 Nm. This means that the motor must never be allowed to reach the end of the steering wheels travel or it will produce this high torque that may damage the steering mechanism, the gearbox or both.
The gear motor and linear actuator will be driven by a DRV8432 that was used in the Motor driver boards. This can provide 15A continuous current or 24A peak. The motor should never require more then 12A continuous so this should be more then adequate.
##Servo selection options
###Motor sizing
When stationary on concrete the wheel requires 70 N of force to turn when it is applied 100mm from the steering columns axis.
Thus minimum torque to turn the wheel = 7 Nm continuous torque To be on the safe side and ensuring that the system would work in all conditions a minimum torque of 10Nm was chosen for the possible steering solutions
The best 4 possible steering solutions
###Dynamixel EX-106+ Smart Serial Servo $499 USD Pros-
- Accurate
- Simple to use
- Continuous rotation
- 10.7 Nm holding torque
- 90 RPM
Cons-
- Very expensive
- Requires 18V supply (7A minimum)
- May be difficult to interface with
###Torxis i00600 12V High Torque Servo Motor $289 USD Pros-
- Accurate
- Simple to use
- 10.7 Nm holding torque
Cons-
- Only 90 degrees of travel
- Requires 12V supply (3A minimum)
###IG52-04 24VDC 010 RPM Gear Motor with Encoder $155 USD Pros-
- Easily the cheapest motor + gearbox + encoder I could find
- 10.0 Nm continuous torque, 30Nm peak torque (limited by gearbox)
- Continuous rotation
- Accurate
- Designed for use with 2 lead acid batteries
- Several positive customer reviews
Cons-
- Slow (12 RPM)
- Requires motor driver circuit
- More complex control then servo
###PDX256 - 256:1 Gearmotor $99 USD Pros-
- Very cheap powerful motor
- 94 RPM
- Stall torque = 100 Nm no continuous torque given
- Continuous rotation
Cons-
- No encoder or position sensor included would need external pot or encoder
- Requires motor driver circuit
- More complex control then servos
- 2 customer reviews both mention overheating issues
Linear actuator sizing Through rough testing it was determined that a person can apply roughly 300 N of force to the brake pedal. The pedal had roughly a 2 to 1 leaver before connecting to the hydraulic system. Thus actuator force required = 600 N Actuator travel required = minimum of 25 mm.
The brake system was described as fragile by David Healy so the ability to damage the system was a real concern when deciding on actuators.
###Actuator options
###Firgelli auto 2" Stroke 150 lbs Force Linear Actuators with Potentiometer Feedback $140 USD Pros-
- Inbuilt pot for position feedback
- 50 mm stroke is around what is wanted
- 660 N force is roughly what is desired
Cons-
- Slow 12 mm/s
- Requires motor driver circuit
- Requires 12 V supply
- Maximum duty cycle = 20%
###Linear motion $300 NZD Pros-
- Inbuilt pot for position feedback
- 30 mm stroke, ideal
- 600 N force is roughly what is desired
- 20 mm/s fairly fast
- New Zealand supplier
Cons-
- 1 month lead time
- 12 V supply
- Expensive
###4" STROKE 100 LB 24 VDC WARNER LINEAR ACTUATOR $50 Pros-
- Very very cheap
- Inbuilt pot for position feedback
- Can run straight off two lead acid batteries
Cons-
- End of line item
- Expensive shipping (0 USD)
- 100 mm of travel far exceeds requirements.