# Ways of Driving a High Power Stepper Motor at 24V, from a 12V Battery.

Discussion in 'The Projects Forum' started by sweaterlife, Mar 24, 2010.

1. ### sweaterlife Thread Starter New Member

Mar 24, 2010
2
0
Hi,

I'm an electronic engineering student currently working on a project for my university's student racing team.

Then goal is to provide an electrically actuated gear shifter to operate the cars gear box, i.e. a paddle shift gearbox.
The gearbox on the car is a sequential shift gearbox taken from a honda motorbike.

We require 10Nm of torque to execute a shift, by turning the gear selector shaft where the bikers foot would usually kick a lever. We had considered using a solenoid, however this is a common solution, so we are attempting to solve this problem with a stepper motor. This will offer greater positional control and efficiencies.

In order to deliver the required torque, we must drive our stepper motor at24V, 6A per phase. This equates to 24V, 12A which is a huge power demand of 288W. We have already built a H-Bridge circuit capable of operating under these conditions, and constructed a microcontroller system to drive it, however we are struggling to produce the required voltage and current from the cars on board 12V battery.

Attempts have been made to create a switching power supply (classic PWM with in line inductor style) however the size of inductor required in order to prevent saturation is not practical, and also there is no reason to supply a constant 12A output current as a single shift operation will take 200ms. We also estimate that the driver will not shift more frequently than once every second or so, so we have a duty cycle of 20%.

This has led us towards the possibility of charging a capacitor bank to drive the motor. Are there any other suggestions as to how we might create a 24V 12A supply from a 12V lead acid battery.
N.B. The car also has an alternator which will be recharging the main battery during operation.

Power calculations reveal that we require somewhere in the region of 60-70Joules to execute a shift, which is equivalent to using a 216mF Capacitor @. This would be the size of a couple cans of coke and also impractical.

does anyone have experience with supercapacitors (double layer capacitors) on the forum?
Is it a feasable solution to charge a series bank of supercapacitors up to 12V off the car battery (when not executing a shift), then drop them in series with the battery when required using heavy mosfet switching? This series combination would give 12V and both elements should be capable of delivering 12A.
Three 5.5V, 1.5nF supercaps would give a series capacitance of 500mF, which should allow us to store double the energy required.

Is there a fundamental reason not to use a bank of capacitors in series with a lead acid battery?

This is a first post, but I've tried to give as much information as possible.

Thanks in advance for any help/advice!

Cheers, Paul

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Do you know who Rube Goldberg was?

I suggest that you perform a Google search on his name.

It is challenging enough to simply design a solenoid of adequate size and strength.

You're attempting to engineer a complex monstrosity to perform a relatively simple task. By the time you get done with it, you'll have added 250lbs to the weight of the vehicle, taken up every spare nook and cranny available, spent five grand, and have a mean time between failures of about 5 shifts.

Your challenge should be to make something that is as small, lightweight and reliable as possible within economic feasibility.

The net effect of having a series-dependent non-redundant complex assembly is that the MTBF's of the components used decrease the overall expected MTBF of the assembly. The more complex it is, the more likely it is to fail.

Keep it simple.