by Donald GeorgiWhen Segway LLC (www.Segway.com) received 3 reports of Transporter riders falling because of a lack of available battery power, a voluntary recall in cooperation with the U.S. Consumer Product Safety Commission on September 26, 2003 was initiated. Mimicking the venerable ‘Chicken Little,’ various news sources blamed the battery in their headlines to an unaware public. Just six days after the recall, the headline at Fredricksburg.com (see www.freelancestar.com/News) read “Batteries may cause scooter accidents.” Two days earlier the San Jose Mercury News had touted a story with the Title “Two-wheeler tips over when battery depleted.” This second title is not as incriminating as the first headline, but it definitely implied that battery problems cause failure.
The Human Transporter created by Dean Kamen of Segway LLC is a revolutionary device for personal commuting. With the simple shifting of body weight, the rider controls starting, stopping and speed from energy stored in two rechargeable Nickel-metal hydride batteries. (Photo is courtesy of Segway LLC.)
Batteries are the catchalls for a myriad of problems whether real or perceived. Any motorist who has been driving for a few years will remember blaming starting battery problems not on the lack of battery status information, but on the battery itself. Nuisance battery failures may extend to a very untimely dropout of a cell phone during a big contract negotiation or a marriage proposal. Whatever the situation, the battery always catches the blame. Why don’t we blame the gasoline itself when our fuel tank runs dry on the freeway?
Three models of the Transporter are affected by the recall: (from left to right, the ‘i’ series, the ‘p’ series and the ‘e’ series. Approximately 6,000 Transporters have been built with only the three incidents which have triggered the recall. Owners can call the Segway LLC toll free number 877-889-9020 to initiate receipt of the software upgrade which will alert the rider sooner to excessive power demands or low battery condition. The ‘e’ and ‘i’ series are powered by 60 cell Nickel-metal hydride batteries to provide 6, 8 and 12 m.p.h. speed with 5-15 mile range. In the ‘p’ series, each battery consists of 48 cells, to provide a lower peak speed of 10 mph and range of 4-12 miles. The batteries are not involved in the recall. (Photo is courtesy of Segway LLC.) +
In the case of this transporter, less power available at the end of discharge can cause instability , but only because the Transporter is called upon to deliver performance beyond the present maximum power available from the battery. Although the handlebar’s panel indicator shows a red battery low indication with an audible alert and stick shake, there have been three situations where people continue to ride, leading to a fall. While not a condition which can be overlooked, the numbers are not suggestive of a big problem. There are 6,000 Transporters in the field and if we think of an average distance of 10 miles per day, they would cover a total of 22 million miles a year. That would be about 7 million miles per incident, which is not a negitive characteristic. Compare that to the fact that U.S motorists appear to be comfortable with the 40,000 highway deaths each year which equate to about one per 1.3 million miles traveled. The auto ‘stats’ don’t even get into the nonlethal numbers.
The Human interface Control Panel is straightforward with only a key lock on the left which selects one of the three speed ranges (0- 6, 0-8 or 0-12.5 m.p.h.) On the right is the battery condition gauge which displays battery charge quantity on a circular bar around the periphery and a face with a colored background displaying a green background when in the operating range and red when the system is either asking for excessive power demands or implementing a safety shutdown. Operator commands are inputted as forward or backward lean of the body on the platform. Turns are initiated from the handlebars. When a low battery alert or an excessive power request is sensed by the software, there is an additional audible chirping and a tactile shaking of the handlebar to tell the operator to reduce power requests.
One might suggest that the display include a state of charge indicator. Such an indication would be difficult to present since the operating profile of the Transporter has a range of power levels. While traveling at 5 m.p.h. on a flat surface the ‘fuel gauge’ might indicate a 1/4 full, tank, but this number could rapidly change as the Transporter gains speed as a hill is encountered and thus instantaneously change to a capacity of less than 1/16 full at the higher demand. To date the simpler indicators are being used and may be best for a rider who is supposed to be attentive to the driving situation. (Photo is courtesy of Segway LLC.) +
Under constant power conditions, a Nickel-metal hydride battery exhibits reasonably flat discharge conditions,1 but the Transporter, which may be cruising at a medium power discharge rate near the end of charge, is not a constant power load to the battery. If the operator suddenly asks for a surge of power, one could think of the battery performance shifting to a higher load discharge curve where the terminal Voltage drops off more sharply. Even if the current is held constant (which it probably isn’t), the total power delivered is reduced.
Another way of thinking of the situation is to consider that the battery internal resistance builds with use. That internal resistance absorbs some of the stored battery energy flowing to the load. We normally think of a weaker battery as being indicated by dimmer lights or a slower starter motor, but when the Transporter system stability requires a certain amount of power for the control gyros, too little available power can disable platform balance.
Exploded view of the Segway Transporter. At the center bottom can be seen the two Nickel-metal hydride battery modules each consisting of 63 cells. To provide proper operation, the batteries send current to two - 2 horsepower brushless drive motors coupled to the drive wheels through a gearbox. (Graphic is courtesy of Segway LLC.) +
Segway designers identify the point at which the rider should discontinue use, but the three reported events highlighted that the region of low power should be extended. That is the basis for the software change implemented in the recall. The original battery condition ‘gauge’ on the handlebars continues to function as before, and the red warning background color of the ‘gauge’ also continues to alert to the excessive power demand. Similarly, the low power audible alarm and stick (handlebar) shake(vibrating) tactile indicators still function. With the new alerts, the rider is notified earlier in the discharge cycle before the battery energy gets so low that platform stability is lost.
With the updated software, a rider who asks for increased speed while encountering a hill will receive the four warnings, indicating that a less aggressive speed should be desired sooner. In other words, there is nothing new in providing operator safety and the batteries have not been changed; only the warningcondition is noted earlier. The condition does not necessarily signal that rider must stop using the vehicle yet, but that less power should be requested. Earlier in the discharge cycle, there could come a point where temporary excessive power demand could initiate the warning, thus allowing the operator to use the Transporter at a lower speed for a considerable amount of time. There is a point, however, where because of the dwindling stored energy in the batteries, the Transporter initiates a safety shutdown, rendering the two wheeled “magic carpet’ unusable. Then the vehicle must be recharged.
For the rider who does not heed the warning alerts, the outcome could still be loss of stability, but then there is only so much an automatic system can do to protect a user who demands operation beyond the system’s capabilities. The shaking handlebar is especially helpful, because as the still-living pilot2 of a small aircraft without stall warning instruments knows, the buffeting which precedes stall is a life saving indicator to take rotely memorized corrective action. In a noisy area, or where the Segway rider is preoccupied, the additional tactile stick shake allows for ‘second nature response’ to automatically back off, as a part of the ‘feel’ of driving.
What are some of the future alternatives to extend operation in future designs? Already there is consideration of using Lithium-ion batteries for greater total energy and range, but these batteries also have a power dropoff as the cycle nears end of charge, so a warning region still is required. One might argue that the battery should provide a uniform power output over the complete operating profile. This could be implemented with a hybrid combination by adding a supercapacitor or peak current battery to be called upon only when the rider demands acceleration or hill climbing late in the cycle. Although depleted when called on for short power demands, the secondary power source could be refilled from the main batteries.
Whatever the technological future for the Transporter, Segway LLC appears committed to providing as safe and reliable device as possible. In order for this new transportation device to make inroads into people’s thinking, continual focus on safety, convenience, and reliability must be pursued.
1. Handbook of Batteries, Third Edition, paragraph 29.4.8
2. Author’s personal experience