GOLD DRUM HIGH VOLTAGE (HV) Super compact Top Efficiency EtherCAT/CANopen servo drive (up to 65kW)
Advanced driver assistance systems (ADAS), including lane-departure warning systems, anti-collision systems, and adaptive cruise controls, all make up a quickly expanding section of the automotive market.
AB Dynamics developed the Guided Soft Target (GST) to assist with the testing of vehicle systems without damaging the cars used. Consisting of a disposable vehicle mockup (the Soft Car 360) mounted on an autonomous robotic platform, the GST needs to propel the soft shell at highway speeds while keeping a low enough profile to ensure a direct collision with a test vehicle will not cause damage. To do this, AB enlisted the help of Elmo Motion Control's high-power, compact servo drives.
The Soft Car 360 comprises of a foam-core structure surrounded by a printed vinyl skin (as shown in Figure 1). A metal mesh is included throughout the skin. Due to this, the Soft Car 360 contains visual and radar signatures which mimic those of a genuine vehicle.
Figure 1: The Guided Soft Target (GST) consists of a foam-coreframe (top) covered by a vinyl skin (the Soft Car 360); anautonomous robotic platform (bottom) carries the mockup throughtest protocols at highway speeds and accelerations. (Courtesyof AB Dynamics)
Throughout testing, the Soft Car 360 is carried by the GST platform down the test track which follows a preprogrammed sequence of movements. If the test vehicle comes into collision with the GST, only the mockup is damaged, which can be rebuilt simply and efficiently (as shown in Figure 2).
Figure 2: When the test vehicle collides with the GST, the foamof the Soft Car 360 flies apart harmlessly, leaving the testvehicle undamaged. (Courtesy of AB Dynamics)
The GST not only has to appear like a vehicle, it also needs to move like one, which is where the autonomous robotic platform comes into play. The platform needs to be capable of high speeds while delivering precise, faultless steering that allows it to repeat test sequences with accuracy.
To reduce the part count, complexity, and costs, ABD decided on a three-wheel design comprising of two passive steering wheels to the front and a single active drive wheel in the back. The 2.8 m x 1.5 m mobile platform is only 12.5 cm in height with slanted sides. The low profile benefits the application but significantly impacted the challenge of motion-system design.
The small space beneath the platform needed to accommodate wheels, motors, gearboxes, and drives. Each component had to be as compact as possible. This created a particular challenge for the wheels and the drives.
Boosting Motor Speeds
The drive-wheel assembly comprises of a single DC servo motor controlled by two Gold Drum High Voltage servo drives from Elmo Motion Control. Each Gold Drum drive provides 100 A of output in a package that is only 75 millimeters tall, allowing the unit to be compatible for the application in both a figurative and literal sense.
If the wheel diameter is smaller, it needs to turn faster to reach highway speeds. Elmo drives utilize a method known as phase advance to allow motors to safely operate at speeds many times faster than originally rated.
Phase advance is commonly applied to drives for AC induction motors. The implementation of phase advance in servo drives is far less frequent, mostly because of its challenging nature. In a permanent-magnet servo motor, current in the stator windings creates a magnetic flux that connects with the field of the rotor magnets and makes the rotor turn.
The windings are energized in sequence to keep motion stable. A slight lag, or rise time, is needed for the field to reach full strength. Almost in unison, the magnets moving through the field generates a back EMF that contradicts the force created by the motor.
While operating in phase-advance mode, the drive energizes each winding quicker than usual. This advances stator field, placing it in phase with the back EMF. The change reduces the drag of the back EMF.
Consequently, Elmo drives can improve motor RPM by a factor of two or three over the rated amount. In the case of the GST, the raised RPMs allowed the AB Dynamics team to reach the high-speed operation that they required.
The adverse effect when using phase-advance mode is a slight decline in efficiency. The drives are already 99.6% efficient, therefore this decline does not significantly affect the application.
GOLD TROMBONE Compact, Direct to Mains, Network-based Servo Drive (Up to 16 A/800 V and up to 22 A/400 V). Up to 10 kW of Qualitative Power.
High Power Density
While the Soft Car 360 is highly lightweight, its large cross-section makes air resistance a component. High torque from the motor is required to oppose the resistance created. This necessitates a higher voltage drive which generally means a larger drive is required.
The latter option wasn’t a possibility for the AB Dynamics team. “Drive size is an issue, as the components we use determine how small we can make the platform,” stated Hubbard. Elmo drives deliver higher power densities, allowing the drives to fit in the limited free space underneath the platform while still creating the necessary voltage.
This combination was well suited to the AB Dynamics team. “We are continually looking for ways to improve the GST’s acceleration and top speed to better simulate driving scenarios,” explained Hubbard. “Elmo is the only company that can offer sufficient power density to deliver the speeds we require.”
High torque was also a necessity for the steering wheels. The wheels need to be able to quickly and correctly modify the angle when the vehicle has stopped and friction is at its most extreme. As mentioned before, the AB Dynamics design team were looking for the highest possible power density in the servo drive.
As before, they found the solution with Elmo Motion Control. The steering wheels are positioned by a single DC servo motor coupled to the wheels by a rack and pinion assembly. A single Elmo Gold Trombone servo drive is utilized to power the steering motor.
The drive generates up to 10 kW of qualitative power with an efficiency of 99%. It delivers from 16 A or 800 V up to 22 A or 400 V.
Systems that Last
Time at test tracks is at a premium. Organizations wish to make the most out of every session. Due to this, they will perform test after test in quick succession. The GST must operate accurately each time. The GST is powered by a 240 VDC bus that is generated by lithium ion phosphate batteries.
The high-efficiency Elmo servo drives allow the target vehicle to be operational throughout the day on a single charge. To guarantee reliability, vehicle testing must replicate the conditions of use.
As such, outdoor testing at high duty cycles and in various weather conditions must be carried out. Therefore, the GST robotic platform is often subjected to high shock and vibration, extreme temperatures, and contamination..
Contamination entails not only moisture and dust, but also corrosive materials, for example salt and snow melt. Elmo servo drives are rated to function at temperatures ranging from 0 °C to 40 °C.
They are designed to resist up to 15 G of shock and 5 G of vibration. Under a substantial amount of testing, the Elmo drives remained functional through prolonged exposure to track conditions with ease.
“We have adopted Elmo motor drives across our track testing products and they have proved to be extremely reliable, which is important given that 98% of our sales are to overseas customers,” stated Hubbard.
An updated version of the GST that delivers even better performance is being worked on by the company. AB Dynamics is partnering once again with Elmo Motion Control to help to achieve its goals and provide an even better service for its customers.
“We’re currently working on a new product that is only feasible due to the availability of Elmo’s Twitter drive, which has unparalleled power density,” Hubbard added. By better serving its customers, AB Dynamics is assisting in the development of an Advanced Driver Assistance System (ADAS) created to make the roads even safer.
This information has been sourced, reviewed and adapted from materials provided by Elmo Motion Control Ltd.
For more information on this source, please visit Elmo Motion Control Ltd.