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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

@Biff

"easier and simple"

My 25 year old truck has pollution controls and the old coal plant has been retrofitted with pollution controls. I do not see the problem with doing both.

It works but BEV have not proven practical.

Kit P

@Nick

"It is not due primarily to fires."

Of course it is to get to unhealthy levels. I have given you the link and let me know when there is not a fire associated with high levels.

You are more aware of the energy situation in California than most residents but there is a drum beat of misinformation.

yoatmon

@ Nick Lyons:
The rest of humanity is indeed fortunate to have the opportunity to place their faith and trust and future into the hands of a man with your abilities. If you're so convinced that nuclear power is our salvation, I'm confident that you have the perfect solution for dealing safely with nuclear wastes for the next milleniums. If you don't have such a solution, my advice to you would be not to act as a proponent for the nuclear industrie, because - IMHO - they are even worse than big oil.

kelly

@danschl, "two things about kellys comments

each time i check there are another billion people in the world" - Not sure what you mean, but I maybe I'll date less.

Concerning this "EVs move pollution to power plant smoke stacks" nonsense.

WORST CASE: Per scale, a coal power plant is twice as energy efficient(40%) as ICE cars, but coal is twice as polluting = ~ a pollution break-even with ICE.

BUT, an EV motor is ~90% energy efficient, ICE (at varying RPM) is ~20% efficient.

Therefore, EVs will travel ~4 times as far(or pollute one fourth as much) IN WORST COAL POWER PLANT electric generation case.

ICE efficiency just can't compare, besides, there is also the wasted HUGE ICE HEAT(~80%) burned into our atmosphere.

Of course, if EV electric fuel comes from PV solar, concentrated solar, wind, or hydro power - there is virtually NO transportation pollution or global warming.

Statistically, the increasingly annual "once in a century": storms, floods, hurricanes, droughts, sea level rise,.. could be reduced, which would reduce EVERYONE's home, business, etc. insurance premiums and retail product costs.

Kit P

“dealing safely with nuclear wastes for the next milleniums. ”

At least in the US it has never been a problem. We have a 100% safety record when it comes to handling spent fuel. When you have been doing something for many years on a daily basis without an OMG moment, you would think the detractors could at least find a paper cut to show how dangerous it is.

“EV motor is ~90% energy efficient ”

Of course this in not true for EV motors in real life.

“if EV electric fuel comes from ”

But it does not, it comes from coal.

“sea level rise ”

There has not been a significant increase in sea level but there has been a significant increase in expensive homes near the ocean.

If you are looking for ineffective solutions to AGW start with BEV and reject very effective solutions like nuclear power.

kelly

@Kit P, you have been sent the facts and links to the facts. EV motors are actually approaching 95% efficiency and 39% of CA EVs are fueled from PV solar alone.

http://solarchargeddriving.com/news/solar-charged-driving/1055-survey-39-of-ev-owners-have-solar-pv.html

Your lies to readers are aging.

Let's just pretend you commented sufficiently and your utility sponsors will continue paying for their views.

ai_vin

Electric vehicles convert about 59–62% of the electrical energy from the grid to power at the wheels—conventional gasoline vehicles only convert about 17–21% of the energy stored in gasoline to power at the wheels.
http://www.fueleconomy.gov/feg/evtech.shtml

Engineer-Poet

yoatmon, the nuclear waste problem was solved long ago.

  1. Dissolve spent fuel in molten salts.
  2. Extract the uranium by fluorine volatization (reacts to form UF6, which bubbles off).  This uranium can be handled as any other UF6, such as is processed in centrifuge enrichment.
  3. Electrolytically remove the plutonium as an amalgam with cadmium.  Boil off the cadmium externally, leaving plutonium.  Use as driver elements in fast-spectrum reactors, along with the other transuranics.
  4. Package the rest for industrial use (e.g. Sr-90, Cs-137 for heat sources or radiation sterilization) or disposal.  The half-lives of the rest are either less than 30 years or over 100,000 years, so the radiological threat after a few hundred years is minuscule.

Waste middens from ancient Rome are still toxic and stinking, so nuclear waste is arguably safer to leave for the future than modern "sanitary" landfills.

ai_vin

75.5% of total installed electrical generation capacity (2010 est.) of the US is fossil fueled, however that's for all fossil fuels (coal, oil, NG) and many are not run at full capacity all the time. The percentage actually generated by coal is ~44% for the whole of the US with the highest being ~80% in the Midwest;
http://www.epa.gov/cleanenergy/documents/egridzips/eGRID2012V1_0_year09_SummaryTables.pdf

ai_vin

the nuclear waste problem was solved long ago.

In theory, yes. But how much of the spent fuel generated in the US is actually processed in this way?

I'm not trying to be argumentative here E-P, I really do want to know what our outlook for this is.

kelly

@ai vin, "The Spark EV efficiency from DC current to delivered Wheel torque is 85% averaged over the city driving schedule and 92% when averaged over the highway schedule."

http://insideevs.com/gm-general-says-spark-evs-400lb-ft-of-torque-no-misprint/

"Although some energy is lost to heat in the battery pack, the extreme efficiency of the Tesla EV’s powertrain (the motor and the drive inverter) allow for near 90-percent efficiency."

http://www.rapidtrucksblog.com/a-simple-guide-to-tesla-motors-energy-efficiency/

kelly

@ai vin, http://insideevs.com/gm-general-says-spark-evs-400lb-ft-of-torque-no-misprint/

"The Spark EV efficiency from DC current to delivered Wheel torque is 85% averaged over the city driving schedule and 92% when averaged over the highway schedule."

Engineer-Poet
But how much of the spent fuel generated in the US is actually processed in this way?

None of it, because reprocessing was banned by government fiat.  The same government pledged to take spent fuel for disposal... and has reneged on that promise.

The pyroprocessing technology, developed by the USA, is being transferred to S. Korea.  They are not nuke-phobes, and have a thriving nuclear export industry (selling plants to the Middle East).

ai_vin

Kelly, I believe the 59–62% number is an average for all EVs. The Spark may indeed deliver 85-92% but the GM Exec goes on to confirm my belief by saying "This is the highest in the industry, and that is one of the reasons why the Spark EV sets the benchmark for most efficient car."

Of course, now that 90% has been done it's only a matter of time until it becomes as common to EVs as the four-minute mile is to running.

kelly

@av, the Tesla motor, WITH inverter, is rated at 90% efficency.

Kit P

@E-P

“nuke-phobes ”

The way we do things in the US nuclear industry is not controlled by nuke-phobes. Presently it is cheaper in the US it is cheaper to mine uranium and use it in fuel once leaving behind 95% of the energy.

The spent fuel is water cooled for about 5 years. The decay heat is low enough that it can be air cooled in dry cask storage. Just letting it sit there is the perfect solution. It is safe.

The nuke-phobes will say what about terrorists stealing it and making a dirty bomb. Never going to happen except on TV shows. TV screen writers must employ a lot nuke-phobes and it does make for good drama.

Kit P

“convert about 59–62% of the electrical energy from the grid ”

Assuming the power is coming from an average coal plant that is used for load following @ 40%, BEV and ICE would have about the same ghg emissions.

The solution is not going to come from engineers, changes have to come from society. For example, improving efficiencies is not all that easy in the real world but a 100% improvement in the amount of fuel used can be achieved just by carpooling with one passenger, 200% with the third person riding in the car.

It is a matter of how we use our resources. Put a PV panel on the roof it will produce half the power of putting on a carefully designed utility scale system. Once you connect the PV to the grid, it is reducing the ghg emissions of the grid. Taking credit for the BEV reducing ghg at the dame time is just a fantasy.

I am very skeptical of those who want to reduce ghg but want to do it by engaging in a fantasy.

Kit P

"an average for all EVs"
@ai vin

It is what it is. Your number looks like it includes all the energy losses that go along with an BEV.

But it is never going to matter what they are for the Spark. You, kelly, and everyone else is not going to foot the $15k bill for a BEV.

Energy scam artist are very clever a using words like 'rated' to tell the lie while leaving out the full breaker to wheels energy losses.

Jeff

I like the price, but I sure would be more comfortable about buying a pure EV if it had a minimum of 150 miles of range. I'd be happy to pay another $4,000-5000 to get it. The fastest battery charging technology you can get is a priority for me too.

John L.

Please tell me that GM won't EV1 the Spark EV. I will try it if I can BUY it -- and not be forced to lease it.

I'm a Californian. I already have solar power on my roof. I fact, I have a little room to expand my PV array. My family's primary car is a Prius, but we also have a 1990's-era Honda Civic that we will soon need to replace. The Spark could be perfect.

Engineer-Poet
The way we do things in the US nuclear industry is not controlled by nuke-phobes.

The NRC mandated much more restrictive (and expensive) security at nuclear plants, forcing things like the closure of visitor centers.  These expenses create little or no reduction in risk to the public, which forces me to conclude that the NRC is a nuke-phobic organization.

Kit P

Of course maybe some important event occurred in the US that is a logical reason for 'much more restrictive security'.

Could it be multiple airplanes being hijacked and flown into iconic buildings with massive fires changed how we thing about security?

kelly

"Could it be multiple airplanes being hijacked and flown into iconic buildings with massive fires changed how we thing about security?"

ANOTHER excellent reason to distribute power via rooftop PV solar, like John L.

"Assuming the power is coming from an average coal plant that is used for load following @ 40%, BEV and ICE would have about the same ghg emissions." is a stone lie when EVs need a fraction as much electricity.

Over and over, electric motors are many times MORE EFFICIENT(~4X). This is mainly why all the ~>100 MPGe EV ratings VS ~25 mpg ICE CAFE results.

Electric efficiency is not that hard to understand, especially when it instantly makes possible each of these comments from around the world.

Engineer-Poet

Tell me again how adding more layers of site security and guards to nuclear plants prevents terrorists from flying airplanes into them.

Tell me again how putting more guards on spent-fuel pools radioactive enough to kill anyone who tries e.g. carrying off a spent rod gives any further protection against theft.

Kit P

"Tell me again how putting more guards ..."

I did not tell E-P that in the first. E-P makes assumptions that pander to his agenda without checking to see what we are actually doing.

For example, last I had responsibility for a spent fuel pool; we could hook a fire hose to connection on the outside of the fuel building to supply makeup water in an extreme emergency. After 9/11, having connections on both sides of the building might be a good idea.

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