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Chevrolet Spark EV: speedy, smooth and quiet; the importance of motor control

2014-Chevrolet-SparkEV-049-medium
The Spark EV. Click to enlarge.

Chevrolet is hosting a series of media drives this week for the production version of its new Spark EV (earlier post). Featuring a GM-developed 105 kW electric motor that pushes out 400 lb-ft (542 N·m) of torque and a 21 kWh pack that supports an estimated 82-mile (132 km) range, the Spark EV is a speedy (0-60 mph in 7.6 seconds); efficient (combined city/highway 119 MPGe); affordable (as low as $17,495 in California net after Federal and state rebates); fun-to-drive electric vehicle targeted for the urban environment.

There are multiple interesting aspects to the vehicle, including GM’s rationale for offering fast charging as an option (it’s not to go across country), but one of the most manifest when driving the car (aside from the “Wheee!” of those 400 lb-ft in action) is how smooth and quiet the drive is. Much of this derives from the efforts GM has put over the years into motor control and active damping for hybrids and EVs.

Drive quality was a top priority during the development of the electric motor for the Spark EV, GM’s engineers on site all emphasized. While details on how the damping system works are proprietary and GM is not discussing specifics, a general outline of some of the control problems with electric drive can provide some background.

Electric motors have two primary parts: a stator and a rotor. The stator comprises a steel core that supports the motor winding, while intensifying and directing the magnetic fields between winding and rotor; GM uses bar wound configurations in its permanent magnet motors. The rotor—placed inside the stator—is built around a central hub that links to a shaft to transfer torque.

In a permanent magnet motor such as the Spark EV motor, the rotor comprises thin steel laminates that stack together into sections to form the rotor core. Each of these laminates is loaded with magnets that are placed in a carefully engineered pattern within the steel.

To generate torque from the motor, a controlled electric current is injected into the copper bar windings in the stator, creating a moving electromagnetic field. This pulls on an opposing magnetic field in the rotor, which creates the twisting force that is torque. In GM’s patented rotor design, each section is skewed to distribute efficiently the rotor’s magnetic field, while reducing noise. (Internal GM tools allowed the engineers to model how the motor could make noise and we were able to manage noise at the source.)

Optimizing the control of the current flow to the motor is the basis for efficiency, drive quality and noise. Basically, the controller requests a specific amount of voltage and current appropriate for driver demand and rotor position (commanded torque). However, voltage and current actually present may not be what is commanded, and the rotor position may not be what is expected. That results in actual torque.

The problem is non-trivial; traction motor control systems must deal with a wide range of requests based on speed and traffic conditions, route, weather, roadway conditions, mass load, etc. As a result, the problem has generated volumes of patents and research papers exploring different methods and algorithms.

The charts below show two scenarios for a generic traction motor. The top two charts depict a correctly functioning motor; the blue line is commanded torque, the green line actual torque. The bottom two charts depict a traction motor with a problem.

Known good
Known bad
Generic example of traction motor control (not from a GM vehicle). Blue is commanded torque; green is executed torque. The top two charts show a motor with no issue; the bottom two show a problem. The ideal for motor control is to match executed torque to commanded torque seamlessly (i.e., smooth). Click to enlarge.

In addition to basic motor speed control to deliver smoothly what the driver requests under all the varying conditions of a drive, there is damping control (active damping) intended to reduce transient driveline oscillations before they can reach the drive wheels of the vehicle, or to reduce the impact of an external perturbation.

As an example, take a situation in which you accelerate, and there’s a bit of unevenness due to a wheel slipping on a bit of gravel. If the sensors and processing of the input are sufficiently quick, active damping control of the motor can cancel out those effects in near real-time.

In one of GM’s more recent patent applications on motor control (2012), the inventors assert that speed control and damping control should be integrated, as decoupled damping and speed controls can produce discontinuities in the applied motor torque. In other words, damping and speed control torques should be designed together so that the damping torque does not “fight” a separate speed control—for example, when damping torque is in one direction and speed control torque is in the other direction.

In the approach described in this patent application, GM proposes calculating a speed control torque signal for the traction motor using a motor speed torque (MST) control block of the controller, and calculating the motor damping torque signal using a motor damping torque (MDT) control block of the controller.

The speed control torque signal and the motor damping torque signal can then be combined to generate a total motor control torque for the traction motor. The total motor control torque may be processed through a vehicle driveline model to generate an estimated damper torque, estimated axle torque for the axle, and an estimated wheel speed for the drive wheels. In the method described, the estimated damper torque, the estimated axle torque, and the estimated wheel speed back can then be fed back to the MDT control block as inputs to the MDT control block.

The above is by way of illustration; we have no idea whether or not the company is using any or all of this approach in the Spark EV.

The firsts of the Spark EV. As noted earlier from the preview in November (earlier post) the Spark EV is offering a few firsts. It will be the first vehicle on the market to use the recently approved SAE combo charger for DC Fast Charging (J1772). (Available as a ~$900 option later this year.) Also, its liquid cooled battery pack, developed with tailored A123 Systems Li-ion iron phosphate cells and GM controls, can handle multiple DC fast charges daily without impact on battery life. A fast charge can bring a pack depleted to the lower range of its state of charge window up to 80% capacity in approximately 20 minutes, with lower charge levels in correspondingly smaller amounts of time.

GM is eyeing the fast charge option as one way to address a potential roadblock to urban EV adoption: the lack of a home charging option for many urban dwellers—i.e., no garage or car port in apartments, etc. The company really doesn’t expect Spark EV drives to stutter across the country in 80-odd mile increments punctuated by 20-minute recharges, suggested Britta Gross, Director of Advanced Vehicle Commercialization Policy, during a briefing at the Spark EV drive in Portland, Oregon.

Currently, DOE data suggests that 90% of charging is done at home. However, the ability for a public fast recharge in cities could enable those urban dwellers without a viable home charging option to buy the Spark EV, she said.

Propulsion system. The Spark EV propulsion system comprises the motor, the gearbox, and the power electronics. GM placed a great deal of emphasis on reuse of components, said Stephen Poulos, Global Chief Engineer for eAssist and Battery Electric Propulsion at the briefing. The high-performance motor is shared with another—unnamed—electrified product, and the gearbox is right out of the production 6-speed automatic transmission.

When we are developing a new propulsion system, we have a lot of requirements to meet: safety requirements, performance requirements, all kinds of control interfaces, we have electrical requirements. For this particular product, the big priorities were, one, we needed to go fast and limit the new investment, exploit what we had. Reuse as much as possible. There is a lot of reuse in what we put together here.

Second, we needed to be very efficient. For the same reason that we are using a smaller vehicle that doesn’t require as much battery as a larger car, if we can have a very efficient drive system, we can use the energy of the battery more efficiently. We don’t need as much battery. There was a lot of pressure to keep this as efficient as possible.

The third priority was fun to drive. We wanted to be this car to be a blast to drive.

—Stephen Poulos

Driving. The Spark EV offers a P R N D L indicator; “L” (low) is for maximum regeneration. The Spark EV also offers a Sport mode that delivers improved pedal feel to the driver (without actually increasing torque). After experimenting with the different settings, we found it most comfortable to drive in Sport mode in L.

In the city, especially on the flat, having the vehicle in L basically means never having to touch the brake pedal. The regen is aggressive but smooth, and is more than capable of bring the vehicle to a stop in slow city traffic without brake intervention. Driving on hills, L does a splendid job of re-topping the battery. In California, one of the key initial markets, the Spark EV would be a slam dunk for the hills of San Francisco.

At the same time, the electric acceleration is a quiet delight. Even at the higher end of the speed range on the highway, there is ample power for overtaking. The Spark EV’s maximum speed is limited to 90 mph.

2014-Chevrolet-SparkEV-067-medium
The Spark EV instrument cluster. The range indicator is on the left, with the likely range displayed in the blue circle, along with a graphic indicator of level of battery charge as well as max and min range possibilities given the driver’s recent behavior. As you drive, the indicator will display little bubbles (not shown in the picture) moving up along on the min/max curves to show that you are trending toward one extreme or the other. The goal of this, said GM, is to provide a great deal of confidence with respect to range.

The indicator on the right is a direct readout of power output, or input (in the case of regen). The green “Ready” icon and light at the bottom of the display is simply an indicator that the car is powered up and good to go.

Assistance and information. The Spark EV is far from a spartan car. It is very comfortable to sit in and to drive (especially with the 2LT upper trim level and its steering wheel), but it also is a bit pared down in certain areas in an effort to keep both weight and cost down.

As one example, there is no array of advanced driver assistance functions such as collision warning and avoidance, parking assistance, and so on. Those all add weight and cost.

However, the Spark EV is fitted with the standard Chevrolet MyLink Infotainment system, including a seven-inch color touch screen that shows critical vehicle EV functions. OnStar with three years of Directions & Connections plan is included. Other functions (such as the Bringo navigation app) are made available via a Bluetooth link to your smartphone. (Bringo also features charging spot locations.)

Resources

Comments

Kit P

“(as low as $17,495 in California net after Federal and state rebates) ”

This EV for a young family that do not pay lots of taxes:

MSRP = $27,500

The same 4 car passenger car without 500 lb of batteries:

MSRP = $12,200

We saw one of these going 20 mph over the speed limit in heavy traffic while raining on a recent business trip. It was not getting better mileage than our Corolla. The mean distance between wrecks was 25 miles demonstrating that slowing down should be done not just to save fuel.

I do not object to tax dollars being used to improve society but there is no benefit to society from the Spark EV.

kelly

"..there is no benefit to society from the Spark EV."

Everyone knows burning fossil fuel is bad for health and climate. Thousands of examples exist. EVs are the best ICE vehicle alternative, just as we don't buy gasoline refrigerators or natural gas air conditioners.

The ongoing US healthcare, oil wars, oil wars interest payments, climate damage, increased extreme weather(droughts/floods/'Sandy' storms/..) may already make a Corolla TOTAL cost several times it's oil company subsidized sticker price - and we can't even calculate all the damage yet.

danschl

two things about kellys comments

each time i check there are another billion people in the world

60 years old is the new 40 and more and more people live to be 100

mahonj

The reason we buy electric fridges and air conditioners is that they do not move, and can be plugged in all the time.

This is not the case with EVs, they need large batteries so they can move around.

EVs mainly displace pollution - from the tailpipe to the power station.
If the electricity is from coal, you are not much better off. If it is from hydro / nuclear, you are better off.
If it is from nat gas, you are somewhat better off.
If it is from Wind or solar, you can pick any number you like.
Obviously, most grids are a combination of the above power sources, so the final pollution number is harder to calculate and variable.

If you were able to charge at 11-2pm in Germany, summertime, you could say it was low carbon power, if you charged 11 - 5 am in Germany, it would mostly be coal.

Davemart

A family on relatively low income can just lease the Spark for $99 deposit and $199/month.
At average annual mileages they might save around $100/month on fuel costs.

What interests me is their rationale for fast charging, as even in the US around 50% of cars are parked by the roadside.
In the cities of Europe and Asia garage parking is the exception rather than the rule, so if the EV revolution is to catch on there then this is vital.

Brotherkenny4

The young family that does not pay a lot in taxes however does pay a good portion of the taxes they do pay to defend the profits of fossil energy companies. Eventually they will also have the opportunity to send their children to die in the deserts for the profits of the oil companies.

The point of the tax incentives is to increase the numbers of early adopters such that the costs associated with the development of these preferable vehicles will be offset faster and allow the cost to produce to be lowered so that the price will be in line with what a young family can afford.

In 2010 the price of batteries was at $1000/kWh, and it is at about $500/kWh just 2.5 years later. In a couple more years where will it be? Estimates made by DOE suggest $350/kWh is probable even without breakthroughs in technology and simply based on increased production capacity. A breakthrough like Envia is likely to make, will halve that cost. That would be $175 per kWh. That has to be really scary to the fossil industry.

I have run the cost for myself, if I can get 4000 deep cycles in a battery that cost $175/kWh, I can install solar and go off grid even without further decrease in the cost of solar.

By the way, I like this car, and if they sell it nationally I am highly likely to buy one.

citizen

My comments are about the commenters.

My hat is off to Chevrolet for coming out with a second electric vehicle. Five years ago we were all wishing for an affordable electric vehicle. Now that we have several from which to choose, I am intrigued by the whining.

So I wonder aloud to the very frequent commenters: Do you not realize how difficult it is to create new technology? Do you expect things to be perfect the first time? Are you actually creating something or just complaining and commenting on other people's efforts?

Davemart

@citizen:
My comment is about your comment.
Most of the posters here have been positive, with one exception.
So what are you beefing about?

Kit P

Since my US tax dollars are being spent to subsidize BEV let me check to see how valid the benefits others claim are:

Clean air: Our air is already clean and pollution levels are below the threshold of health concerns. Problem already solved.

GHG: BEV in the US are powered with coal not a very good way to reduce ghg.\

War: I am more than a little skeptical of these claims. The men and women of our all voluntary military are not children. If you are concerned with protecting children I would recommend a good car seat and just driving less.

The BEV would appear to be more of a political agenda. I know of no one who wants war or pollutions. How we avoid those things is an interesting debate. BEV are a waste of tax dollars.

It has also been suggested that people can save money by spending $12,000 more on batteries so they can use cheap coal generated electricity. A BEV would meet the needs of our second car that I use for work and errands. That fuel cost is $40/month. You cannot save $100 if you do not spend that much to start with.

Nick Lyons

@Kit P:

Clean air: Our air is already clean and pollution levels are below the threshold of health concerns. Problem already solved.

Would that this were so. Air in CA central valley, e.g., exceeds clean air standards on a regular basis. You obviously don't live in Fresno or Bakersfield. Some of this pollution blows in from the heavily populated coastal cities.

GHG: BEV in the US are powered with coal not a very good way to reduce ghg.

Not true in CA, which has almost no coal in the mix, or the Northwest, which relies heavily on hydro. According to EPA, in 2011 coal was 42%, gas was 25%, nuclear was 19%, hydro was 8%, wind was 3% for all of USA. As the carbon footprint of the grid declines, switching from gasoline/diesel to electricity for transport will increasingly reduce GHG emissions.

Davemart

@Kit:
You are not very good at joined up thinking, are you?
Aside from your ludicrous claims on air pollution, you have switched decks by first arguing that a relatively low income family would not benefit from buying a Spark, to the entirely different subject of your low mileage second car.

I based the $100 per month saving on fuel on something around the average annual mileage of 12,000 pa in the US, with perhaps 30 miles per gallon since you don't get very good mileage in city driving which is what these cars are good at.

Its clear that as long as the range is adequate on a leased basis the low income family would be far better off in the electric vehicle, and that they would also from the test reports be driving a far nicer vehicle than very low end petrol cars, in fact one with nearly unrivalled smoothness.

As for the rest of your whinging on about taxes and so on, I couldn't care less, it is your inability to perform basic arithmetic which makes your comments here even more worthless than usual.

Bottom line: $199 pm for a car with tiny fuel bills is a good deal, and your blather does not change that.

Kit P

“Do you not realize how difficult it is to create new technology?”

Yes, I am a mechanical and environmental engineer in the power industry with more than 40 years experience.

“Do you expect things to be perfect the first time?”

Perfect is the enemy of good. Since I work in the power industry I would love to take market share from oil drillers and give it to the coal miners. On a personal basis, not using a lot energy makes more sense that buying new toys from the auto industry.

“Are you actually creating something or just complaining and commenting on other people's efforts?”

I create things as part of a team.

Now citizen tell me why you rate the use of the moniker ‘citizen’. Answer your own questions. Some think asking questions is an honorable pursuit. After reading GM marketing BS, I looked up what GM wants for different versions of the Spark and concluded that my tax $$$ are going to rich people.

Citizen may agree or disagree but citizen but he did not support his position, namely “My hat is off to Chevrolet for coming out with a second electric vehicle.”

Neither of which are affordable unless you are a rich fart with nothing better to do whith your money.

Kit P

“You obviously don't live in Fresno or Bakersfield.”

Used to live there and what you said used to be true but the air quality is much better. During the Bush admin ‘clean air standards’ were made stricter so that exceeding those standards no longer implies a health impact.

http://airnow.gov/

“Not true in CA, which has almost no coal in the mix”

California imports coal power from other states. It is true.

“or the Northwest”

Used to live there and I know where the coal plants and see the mile long coal trains go by on a daily basis. Check the line for thermal in the link below, if it is above 1500 MW, power is coming from coal.
http://transmission.bpa.gov/business/operations/wind/baltwg.aspx

Nick suggests that improving the carbon foot print of grid is also improving BEV. This is not true until such a time that renewable energy and nuclear replace fossil. Smoke and mirrors my friend is not the same as doing something.

Kit P

"You are not very good at joined up thinking, are you?"

That is a Brit term for what? My job requires us to queation everything. Group thinking is a big trap.

"Aside from your ludicrous claims on air pollution.."

I certainly know what my local air quality and have provided links to support the facts. It could be that Brits do not know the USA from China.

"mileage of 12,000 "

Some days people drive 200 miles and some days they drive some days zero. A BEV is useless for the 200 miles. Average does not count.

If most days

If most days you drive 20 miles, you are only going to spend $40 per month.

BEV and saving money are mutually exclusive. If you want to save money drive less. If you want to drive a lot, a BEV will run out of juice.

Trevor Carlson

Kit P, good posts defending your perspective.

"Everyone" does not feel the need to think critically and anyone that does (think critically) makes them uncomfortable. When "everyone" is uncomfortable they're prone to attack your character or vilify your intentions.

I respect how you generally don't go down to their level but respond on a point by point basis.

Tra2S

Kit P: Since my US tax dollars are being spent to subsidize BEV...

No they are not.

Biff

"EVs mainly displace pollution - from the tailpipe to the power station.
If the electricity is from coal, you are not much better off."

mahonj, I disagree. From the studies I've seen on GCC EVs on coal are still better unless the coal is really filthy such as in Victoria, Australia. I'm in Sydney and in NSW we have 8 coal-fired power stations and 4.89M registered vehicles. Judging by the numerous smoky exhausts I see every time I drive (thanks to our woeful emissions laws) I would rather remove vehicle point source pollution and strictly monitor those 8 power stations. Should be much easier to control emissions from them.

Davemart

12,000 miles a year is around 30 miles a day, so there is no contradiction at all in not making long journeys but doing enough annual mileage to make savings.

We don't need to guess that for electric cars, as we have the data.
The Nissan Leaf travels on average 37 miles per day:
http://www.autoguide.com/auto-news/2011/10/nissan-leaf-drivers-average-37-miles-per-day-added-range-not-needed-says-product-boss.html

I suggest you write individually to the numerous Nissan Leaf owners who are saving money, and explain that you know their bank accounts better than they do.

Many have kept records of all expenditure, and had records for their previous gasoline cars.

Since Nissan can track mileage driven, these figures will be accurate.

So the average annual mileage would be 13,500 miles a year.

SIliconValleyDude

Come on guys, let's end the pissing contest on this blog. What is the argument about anyway?

I own two gas hogs: an SUV and a sports car with large engine. These get less than 20mpg. The cost of feeding the petro beasts keeps rising every year. My next cars will be (1) an electric vehicle that plugs in for local commuting and shopping and (2) a diesel powered SUV for longer trips and hauling stuff (when it finally comes out in an affordable product closer to 40mpg).

Meanwhile, thanks everyone else who can drive Priuses, electric cars, public transportation, and everything else that saves (my) gas! Someday I'll join you but for now my cars are old, paid for, practical, running and they are fun to drive and buying new cars will cost $$$.

I don't understand the mentality of the petro hogs (I am one) who don't appreciate others' environmental and conservation concerns. If you were in an ice cream shop and wanted to eat a triple scoop of chocolate chip ice cream, would you want everyone else in line to order the same, and would you encourage those at the back of the line to cut in front to do that too? No. But that's what the world oil market is today. There is only a finite "carton" of oil... it's NOT renewable, we just drill and get at more of a finite, declining resource every year until it's gone. Meanwhile, everyone else on the planet is buying cars, especially China and India, and adding to the demand, while we gripe about the price of gas and make up conspiracy theories about why oil costs more every year.

So when you go to fill up your Chevy Suburban petro beast, be thankful for those driving a Prius or a Chevy Spark. They are not the enemy! They are leaving more for the rest of us who are still addicted, and helping to pay for and advance the technology that we will all be using someday.

Nick Lyons

@Kit P:

Pollution in San Joaquin Valley: In 2012 Bakersfield had 87 days of 'good' air quality and 17 rated 'unhealthy'. In 2005 Bakersfield had 89 rated good and 39 rated 'unhealthy'. There may be some progress--fewer really bad days--but air in the southern San Joaquin Valley is still a problem. See http://www.epa.gov/airdata/ad_rep_aqi.html

Coal % of CA generation: 8% as of 2011, including power imports. See http://energyalmanac.ca.gov/electricity/total_system_power.html And CA is working to eliminate imported coal generation, so the 2013 percentage is probably lower. As for the Northwest, it is true that coal is 14% of Washington state's production, e.g., but that is a long way from saying at BEV in Seattle is powered by coal, especially considering that 73% of it's electrons would come from carbon-free hydro (see http://www.commerce.wa.gov/Programs/Energy/Office/Utilities/Pages/FuelMix.aspx

Kit P

@Biff

Are you ready to play?

I try to restrict my comments to the US because that is where I live and what I know. The three most important thing for buying a house and LCA is location, location, location. We have a project in Australia near a coal plant that is not going well because of corrosion from SOx. Not something we have worried about in the US for twenty years.

“Judging by the numerous smoky exhausts I see every time I drive ”

The way you judge air pollution is by measurement.

http://www.epa.vic.gov.au/our-work/monitoring-the-environment/air-quality-bulletins/hourly-air-quality-interactive-map

In the US, regulating point sources and pollution controls on cars have worked very well.

Kit P


@Nick

Where do live? Do you live in fairy tale line.

“San Joaquin Valley ”

Last time I visited air quality was unhealthy. I think was the wild fires. My point in this case is you have to know the root cause of the problem if you want to solve it.

“working to eliminate imported coal generation ”

Smoke and mirrors!

“but that is a long way from saying at BEV in Seattle is powered by coal, ”

No the coal plant is just down the road. Periodically, the governor says they are going to close it. In the real world as I showed load demand is met with fossil fuel. Sure you play lets pretend my power comes from wind mills but that is kind of silly. Go ahead and keep double counting.

How do I know about the BS in some of these places? It is years of listening to BS and watching coal trains.

Biff

Kit, you're not saying anything I don't know or didn't say myself. I'm well aware of where to find air monitoring stats in my own country. And no, seeing a lot of smoky exhausts is not a quantitative measure. But seeing them, marrying them to our very lax emissions/rego check laws, and knowing what goes on OS leads me to infer we have a problem. But then I already knew that. Australia is the land of least resistance - if it's easier, do it. We only have 23 million people in a continent the size of the lower 48 so until recently it was no worries! about pollution. Just pump it into the air or the sea and she'll be right, mate.

My basic point is that monitoring, regulating and reducing emissions in the state of NSW is going to be easier and simpler on 8 coal power stations than nearly 5 million vehicles.

Kit P

@SiliconValleyDude

Do mean the Santa Clara Valley? You may not remember when it was a beautiful place with lots of vineyards and orchards? Well that was 1960.

“appreciate others' environmental and conservation concerns”

Of course they really do not do that. They never bothered to learn how. Listen long enough and you find about how much energy they actually use.

“They are not the enemy! ”

I suppose they have never mounted a campaign to put you our of work. They will not be happy until every nuke and coal plant is closed. California has no coal and few there have ever seen a coal power plant yet they want to close them in West Virginia.

“advance the technology ”

Why do you want to do that SiliconValleyDude? That is the argument. The arguments for BEV are very week. A strong argument looks like this:

“for now my cars are old, paid for, practical, running and they are fun to drive and buying new cars will cost $$$. ”

Nick Lyons

@Kit P: I provided references for my stats--check 'em out. Root cause of pollution in San Joaquin valley is documented there. It is not due primarily to fires. NOx emissions are the biggest problem, and almost all of this comes from internal combustion engines, heavy duty diesel trucks being the biggest contributor. My only related point is that air pollution is not a problem that has been 'solved'--there is lots of work to do.

I don't think I said anything about wind mills except that they provide 3% of CA grid mix, according to the reference cited. You seem to assume that I'm a big wind energy fan, which is not so. Next-gen nuclear is the way forward, IMHO.

FWIW, I am fortunate to live in coastal CA, where the air quality is very good most of the time.

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