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

[Ubergeek63]

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.

Fine...here is my plug for it...

https://www.electro-tech-online.com/custompdfs/2010/03/237593.pdf 4.4V 2A 2 NM stepper at 2Kg (while you say yours is 3Kg the data sheet says 4Kg)

a 7:1 ratio gives 14NM at the shaft

a 24V source with current source electronics limiting it to 2A/phase you should be able to go 360 deg in 133mS or your 36 deg in 93mS. running at 200mS leaves plenty of head room for supply droop.

LiFePO4 18650 Battery: 12.8V 800 mAh (4x18500 4S Flat, 5.0A rate) with PCB $25 800mAHr LiFePO4 rechargeable weighs in at 0.2Kg and should be good for 3600 shifts ... theoretically ... fine, sue me, call it 1000 shifts, it will still last 1.4 Hrs continuously shifting every 5 seconds!

Dan

 

[mneary]

You can get sealed lead-acid batteries as small as 1.2Ah and 0.5kg. Keep it charged in the main 12V circuit, then use a DPDT relay to put it in series when you need 24V.

 

[Warpspeed]

Personally, I would much rather use engine oil pressure to operate some type of hydraulic actuator to change gears.
Then you only require relatively small solenoid valves to control and divert the oil flow.

It would solve a whole heap of problems.

 

[roltex_rohit123]

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

First of all please excuse me because i'm not an engineering student, so my idea could be a crazy one. I think your problem is with the weight and charge Right? you cannot use SLA batteries due to their weight. also super capicators would not provide with very much power. Why dont you use Li-ion battery bank. I know one cell 18650. it is 6.6 cm tall and almost 2050mah. costing $4 each. they deliver 4.18 volts. you can use 10-15 cells in the space of a sla. Also you could buy a Li-ion bigger battery. It is quite light and delivers more current than any other batteries. but Li-ion cells are hard to charege. take a look at the charging algorithm if you intend to use them. Also I can help you with the Li-ion charging circuit designing (I'm working with that). hope this idea wasn't crazy..

 

[tcmtech]

Personally, I would much rather use engine oil pressure to operate some type of hydraulic actuator to change gears.
Then you only require relatively small solenoid valves to control and divert the oil flow.

It would solve a whole heap of problems.

I was thinking of a similar method or just use a air solenoids and engine vacuum or a small CO2 tank for a pressure source. Both concepts are simple powerful and easy to control.

As far as the precision aspect, shifting a motorcycle transmission does not require it. The shift mechanism just relies on getting past the trip point of the internal cam or rack mechanisms that change the gears up or down. The shifter mechanism itself has built in stops that prevent it from going to far in ether direction once the shift has been completed as well. Just using a high torque electric motor directly attached to the shifter mechanism and applying power to it momentarily to make it turn one way or another until it hits the shifter stops is more than enough. The internal springs on the shifting mechanism are more than strong enough to make the motor return to center once the power is turned off.

Your approach you have now is complicated, difficult to design, implement, will cost more to build in materials time and money, and is likely going to have a high chance of failure due to the overall complexity and multiple mismatches in applied components usage. Its basically over though thought unnecessarily complicated, and over engineered for the aplication. Simple effective and reliable are what you need not what you are proposing.

 

[Ubergeek63]

First of all please excuse me because i'm not an engineering student, so my idea could be a crazy one. I think your problem is with the weight and charge Right? you cannot use SLA batteries due to their weight. also super capicators would not provide with very much power. Why dont you use Li-ion battery bank. I know one cell 18650. it is 6.6 cm tall and almost 2050mah. costing $4 each. they deliver 4.18 volts. you can use 10-15 cells in the space of a sla. Also you could buy a Li-ion bigger battery. It is quite light and delivers more current than any other batteries. but Li-ion cells are hard to charege. take a look at the charging algorithm if you intend to use them. Also I can help you with the Li-ion charging circuit designing (I'm working with that). hope this idea wasn't crazy..

I mentioned lithium cells, and gave a link to a pack ... LiFePO4s are safe and easy to charge.

I checked how much power he needed and recommended a 12V 0.8AHr pack. based on a completely different approach.

I suspect a Li-ion would not deliver the 12A that his current motor requires.

Dan

 

[Warpspeed]

I was thinking of a similar method or just use a air solenoids and engine vacuum or a small CO2 tank for a pressure source. Both concepts are simple powerful and easy to control.

Air solenoids and an air actuator were my very first thought too, this technique is commonly used in some drag racing classes.
But then I realized engine oil pressure was already there ready to be used, and the less extra "stuff" you have hanging off your bike, the better it will be.

It has all been done before.

Engine vacuum is now used all the time for the power brake booster in every car on the road. Automatic transmissions all use hydraulic oil pressure to move the internal shift parts.
The latest ones are now fully microprocessor controlled, but still use internal hydraulics to provide the muscle to do the actual shifting.

The very latest generation of tiptronic sequential three shaft gearboxes all use oil pressure to shift conventional shift forks, sliders, and meshing dog rings.

If electric motors were a practical alternative to hydraulics, they would be being used, but the total power consumption and heat dissipation just makes direct electrical actuation impractical.

If you absolutely must use an electric motor, use a small one, and use it to drive an off the shelf hydraulic servo as a force multiplier.

 

[sweaterlife]

Thanks for all the suggestions,

We were given a pneumatic ram and solenoid valves as a possible means of completing this problem, however it was a common solution, and our current set up is no bigger/ heavier than those valves and also the weight/danger of a pressurised gas on board the car.

Our motor will consume approx 300W for a single shift operation, however even the use of a simple solenoid actuator such as this one is 320W, and the pneumatic valves to handle the pressure required also consume power in a similar order of magnitude.

We are now aware that this project has been over-engineered, however our academic project supervisors cannot know that we have put in 3 months of work for nothing, so our final solution must be a system actuated with a stepper motor, and weigh less than 5kg.
(of course we now know that it is possible to do the whole thing with a solenoid in 1.5kg but we can't redesign with 2 weeks left!)
Push Button Electronic Gearchange, Electric Shifter, Quickshifter, Flatshifter, Disabled Bike Adaptations build a neat custom solenoid to perform our exact function.
It is also the existance of systems such as this which led us down our somewhat 'experimental' route...as at least we have proved that a motor is not a viable option.

After trying to design a capacitor bank boost system today, it turns out that that would be either too expensive or too slow (Series combinations of super caps lead to high ESR which increases time constant etc)

We have ordered parts to build a bigger conductor which will not saturate in our switch mode booster, however this will be too heavy when included with the rest of the circuitry required.


So our 'final' plan is to use this SLA battery in series with the car's high capacity battery (which has an alternator running also):
Yuasa | Batteries | Batteries and Chargers | Lead Acid Batteries | Yuasa NP Range |NP1.2-12RS

In series with the car's battery system this will give approx 26V. It's low Ah capacity (chosen to minimise weight) means its voltage will drop to approx 11V under the 13A load. After a shift has been completed, we will use a MOSFET switching circuit to recharge this 12V SLA accross the battery and alternator.

the SLA is 0.65kg, and our motor is 3.95kg
Our coupling mechanism has been built from aluminium and weighs approx 100g
This leaves 300g for the remainder of our electronics!

Could anyone recommend a better battery technology? I think only a SLA will give us the 13A 200ms pulse we require, but if anyone knows of a lighter and capable technology it would be great to hear from you?

Thanks, Paul

 

[Warpspeed]

Paul, student projects are very different to how things are out there in the real engineering and business world.

Real engineers HAVE to come up with a very cost effective solution that works, and they are free to explore all kinds of nutty lateral thinking ideas at engineering planning meetings.
Paid outside consultants and specialists can be called in to advise, and so on.

A student project has far more constraints placed upon it, and in the end you will be judged on the quality of your attempt, the thought processes and reasoning that go into it, and how well documented the project is.

It does not have to be a complete finished commercial product, or even work at all.
But obviously that would be better.

As an electronic student project, just give it your best shot, even though the whole scheme is probably not practical.
Just document all the design avenues you have explored, and the strengths and weaknesses of each approach, and set out your reasoning. If your final report makes very interesting reading, then the project has served it's purpose.

If you cannot find a truly satisfactory solution to this problem, do not be too concerned about that.
The fact that there are no direct electrically actuated automotive gearboxes in production anywhere, and never have been, should tell you something.

No doubt the big car companies have looked at all this, and said too hard, or impractical.
They have far more resources than you have, so do not be discouraged.

 

[Hero999]

So our 'final' plan is to use this SLA battery in series with the car's high capacity battery (which has an alternator running also):
Yuasa | Batteries | Batteries and Chargers | Lead Acid Batteries | Yuasa NP Range |NP1.2-12RS

In series with the car's battery system this will give approx 26V. It's low Ah capacity (chosen to minimise weight) means its voltage will drop to approx 11V under the 13A load. After a shift has been completed, we will use a MOSFET switching circuit to recharge this 12V SLA accross the battery and alternator.

I'd be interested to see how you're going to use MOSFETs to switch the battery in series and parallel.

Beware that:

MOSFETs have an internal diode so to prevent back-flow you need to use two connected back-to-back.

MOSFETs normally have a maximum gate voltage of 20V (although some with a lower threshold are only rated to 10V) so you'll need to protect the gate from higher voltages.

the SLA is 0.65kg, and our motor is 3.95kg
Our coupling mechanism has been built from aluminium and weighs approx 100g
This leaves 300g for the remainder of our electronics!

Could anyone recommend a better battery technology? I think only a SLA will give us the 13A 200ms pulse we require, but if anyone knows of a lighter and capable technology it would be great to hear from you?

Thanks, Paul

I think an SLA is the best chemistry, Li-ion is low current and has fussy charging requirements, NiMH is higher current but even more difficult to charge than Li-ion.

 

[Ubergeek63]

In series with the car's battery system this will give approx 26V. It's low Ah capacity (chosen to minimise weight) means its voltage will drop to approx 11V under the 13A load. After a shift has been completed, we will use a MOSFET switching circuit to recharge this 12V SLA accross the battery and alternator.

the SLA is 0.65kg, and our motor is 3.95kg
Our coupling mechanism has been built from aluminium and weighs approx 100g
This leaves 300g for the remainder of our electronics!

Could anyone recommend a better battery technology? I think only a SLA will give us the 13A 200ms pulse we require, but if anyone knows of a lighter and capable technology it would be great to hear from you?

Thanks, Paul

Customize LiFePO4 18650 Battery: 12V 2400 mAh ( 28.8Wh, 4S2P Flat Pack, 14A rate)

$60, 340gm, 15A continuous discharge. I pointed you to this chemistry before, as well as to a solution that cut the weight in HALF:

https://www.electro-tech-online.com/custompdfs/2010/03/237593-1.pdf 4.4V 2A 2 NM stepper at 2Kg (while you say yours is 3Kg the data sheet says 4Kg)

https://www.batteryspace.com/lifepo418650battery128v800mah4x185004sflat50aratewithpcb.aspx

a 7:1 ratio gives 14NM at the shaft

a 24V source with current source electronics limiting it to 2A/phase you should be able to go 360 deg in 133mS or your 36 deg in 93mS. running at 200mS leaves plenty of head room for supply droop.

LiFePO4 18650 Battery: 12.8V 800 mAh (4x18500 4S Flat, 5.0A rate) with PCB $25 800mAHr LiFePO4 rechargeable weighs in at 0.2Kg and should be good for 3600 shifts ... theoretically ... fine, sue me, call it 1000 shifts, it will still last 1.4 Hrs continuously shifting every 5 seconds!

That makes your battery/motor solution 2.2Kg instead of 4.7Kg. It all stems from the fact that the "voltage" spec of a stepper motor is a courtesy at best. Steppers are current operated devices and the inclusion of the voltage spec is a misleading lie, there is no voltage rating. The voltage is only what you can apply continuously at 0RPM without it burning itself up.

PWM controllers bypass this allowing high step rates while limiting lower ones to the CURRENT rating of the motor. Speed limitations are due to the inductance not allowing the current to climb to the rated current at the step rate, which in turn reduces the torque.

In PWM controllers the higher available voltage charges the inductor faster and then push constant current into the motor. Where holding torque is not required some even throttle it back further or shut down completely when not active... which is what you are proposing to do.

Dan