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Balancing act
An exclusive look at the engineering behind the Segway HT, with some never-before-published technical details
By John Lewis, Northeast Technical Editor, Karen Auguston Field, Chief Editor -- Design News, March 25, 2002
Manchester, NH — Manchester, NH ¡ªDean Kamen's much publicized Segway Human Transporter (HT) may or may not ever power a revolution in personal transportation (20% of Design News readers surveyed think it will). But even if it is only modestly successful, it will almost certainly make a new wheelchair technology more affordable.
And it could lead to advancements in robotics technology, including the development of highly maneuverable, autonomous machines with the ability to navigate environments normally reserved only for people.
Both the Segway HT and iBOTTM Mobility System, DEKA Research and Development's stair-climbing powerchair that is currently going through the FDA approval process, incorporate similar dynamic stabilization technologies. They rely on costly gyros, sensors, motors and other critical components¡ªperhaps too expensive for a powerchair with relatively low volume sales. But if Segway HT catches on, it will generate economies of scale that will spill over to other applications.
No tipping. Dynamic stabilization technology gives these machines the ability to self-balance, freeing them from the constraints of static physics that would ordinarily dictate a design with a low center of gravity and large, stable base to avoid tipping. No simple feat, given that they rely on the way in which humans respond to instability to maintain equilibrium. In fact, the first application of this novel technology was specifically intended to replace the human capabilities of standing, balancing, and walking for individuals who no longer had use of their legs.
While on the face of it an inherent lack of stability may sound like a bad thing, it isn't if the goal is to have high maneuverability¡ªas is the case with the unicycle, which Kamen rode around his college campus, and many modern military aircraft. An absolute necessity, however, is a highly sophisticated control system that can distinguish between a fall and a move and direct the machine to respond accordingly. "What you are actually trying to do is make the Segway HT fall forward, and then instead of falling forward it moves underneath you," says Doug Field, chief engineer and vice president of product development at Segway LLC.
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The Segway HT avoids tipping over by applying the appropriate motor torque. As shown in the free body diagram, this restorative torque is equal to F (sin0)x where F=mg, 0 is the tilt angle, and x is the distance from the center of axis of rotation of the motor to the center of gravity. F moves the device forward. |
In principle, the rider makes the Segway HT move simply by shifting his/her weight. Lean forward and the machine goes forward. Lean back and the machine decelerates to a controlled stop. Continue leaning back, and the machine goes in reverse. The harder you lean, the faster you move. Steering is a bit more conventional. In order to turn, twist a handle grip clockwise or counterclockwise, which changes the speed of one wheel relative to the other.
The Segway HT responds as if it were an extension of the rider's body, driving the wheels as needed to stay upright while moving forward, backward, or standing still. Ironically, it does not do so by detecting the rider's weight, but is controlled by angular position and angular rate of change data (as well as a plethora of other information such as wheel position and steering inputs). Essentially, the machine's goal is to avoid tipping over by applying an appropriate restorative torque. It does so through the interaction of three main subsystems:
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While the rider is doing something simple, behind the scenes a sophisticated servo control system is working to maintain metastable equilibrium. Based on steering inputs, inertial data, and motor speed and position data, the controller determines how much energy to supply from the batteries to the motor. The torque output from the motor drives the wheels. |
Sensors and input subsystem. Combines information from five inertial rate sensors (gyros); optical foot pad sensors; two tilt sensors; motor encoders; and steering sensors. Its function is to provide information on machine status and operating conditions to the controls subsystem, including data on motor and wheel speed, and tilt angle and its rate of change.
Controls subsystem. Consists of two controller boards, each with a DSP that runs closed-loop motor control and balance computations; a user interface controller board; two motors (one for each wheel); power modules for commutating the motors; and batteries. Its function is to process input data and determine how much energy to put into the motors and batteries.
Propulsion subsystem. Consists of couplings; two-stage helical gearboxes; wheels; and pneumatic tires. Its function is to take the torque output from the motor and convert it to propulsion.
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The Segway HT relies on a novel concept called "headroom management." The basic premise is to preserve an extra margin of torque and speed in the motors, so that the machine has the capability to respond to a transient, such as hitting a bump. |
How it works. To determine angular position and angular rate of change, the machine employs five inertial rate sensors that provide yaw, pitch, and roll data in three axes. Unlike mechanical gyros, which depend on rotation and the conservation of angular momentum, these tiny, solid-state gyros incorporate a vibrating ring that is excited using the piezoelectric effect. When the Segway HT tilts, this ring is rotated about that axis, causing a change in vibration proportional to the degree of tilt.
When either the angular position or angular rate of change reaches a predetermined value set by the system designers, the controller outputs a current signal to the drive to apply the appropriate torque. The motors are controlled through all four quadrants, meaning that speed and torque can be variously applied in the same or opposite directions. An added advantage is that when torque and speed oppose one another, the motor operates as a generator, providing braking torque to the wheels and energy to the batteries. Ordinary friction brakes would not work on the Segway HT, of course, because the wheels must be free to balance the machine. If not, you would fall flat on your face.
To ensure that the tilt angle, ¦È, never exceeds a critical number beyond which the motor can no longer catch up, engineers employed a scheme that Field calls "headroom management" (see diagram). It involves operating inside the performance envelope of the motor.
Response time is critical for recovery. Field would not divulge exactly what the system bandwidth is, but he says that hundreds of calculations take place per second. The velocity bandwidth of the motor control system is measured in Hz. While that's not atypical for commutating a brushless dc motor, Field stresses that the unique aspect of the design is the way in which the entire control system collects and processes data. To verify that system response time is adequate under the most challenging conditions, engineers built proving grounds filled with obstacles and hazards. They even hired an extreme sports team made up of BMX bike jumpers and skateboarders¡ªobvious candidates to push the envelope.
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Given the complexity involved with self-balancing and the fact that there is redundancy in the batteries, motors, and electrical system, the Segway HT has relatively few parts and is surprisingly compact. Overall width is a mere 25 inches on the largest model (21 inches on a smaller version), exactly the size of a typical adult's shoulder span. |
Because of the overriding need to maintain balance at all times (even while standing in place the motor torque may be cycling between positive and negative), there is redundancy throughout the design, including two batteries, two control boards, and two motors driving each wheel. Although at first glance it looks like just one motor per wheel, closer inspection reveals two separate connections on the back of each housing. First developed in a preliminary form for the iBOTTM mobility system, the motor features a patented, hemispherically-wound stator with redundant windings so that each motor is wired electrically as two separate motors with separate electrical paths for excitation. When one fails, the other takes over.
The winding technology also reduces the motor size (2.6 inch diameter ¡Á 3.5 inch length) by a factor of two¡ªpumping out 40% more torque per unit volume than comparable-size motors. Peak torque is 36.0 in-lb.
The fact that the motor mounts directly inside a die-cast chassis with integrated motor housings helps to maximize motor performance. Typically, heat limits a motor's ability to put out power and torque. Acting as a giant heat sink around the motor, the cast housing boosts torque about 10 to 15%, say the engineers.
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