Using NI Single-Board RIO as an OEM Motor Controller for Medical Muscle Testing and Training Equipment

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"The NI platform saved us at least one year of programming time and one year of hardware development time. By using the NI Single-Board RIO platform, we have the flexibility to choose different devices when machine requirements change and we can add on additional components."

- Anders Hedeager Pedersen, ePower technology ApS

The Challenge:
Developing a controller for the E-core Transverse Flux Machine (ETFM), a five-phased version of a patented electric motor, to run with exact, smooth loads in medical and fitness muscle testing and training equipment.

The Solution:
Using an NI PCIe-7841R device to develop the control system and implement the first prototype before porting the NI LabVIEW software code to the NI Single-Board RIO OEM boards used in the production model.

Author(s):
Anders Hedeager Pedersen - ePower technology ApS

ePower Technology is an R&D engineering company that develops concepts for industrial partners based on the patented ETFM motor. We develop, produce, and sell the ETFM technology to industrial partners. We initially developed the ETFM concept for the rehab and fitness industries, but our next R&D project is for an industrial partner within the energy industry.

The ETFM 

The ETFM, shown in Figure 1, is a variation of the classical switched Reluctance machine (SRM) that was originally invented and patented by ePower Technology shareholder Aalborg University. The ETFM technology has unique mechanical motor construction features such as a large diameter, short length, modular construction, fault detection, and tolerance. The ETFM design also has some advantages inherited  from the classical SRM such as high starting torque, wide speed range, and high efficiency.

ePower Technology and Aalborg University have collaborated on the ETFM since 2003. The first engagement involved work on the eTenzor indoor training bike (Figure 2) , and the second involved a new strength training concept.

From Bike to Strength Training

ePower Technology is currently working on using the ETFM technology to provide some unique features in a new strength training and testing equipment concept called SYGNUM energy, which ePower Technology is developing for an industrial partner in Germany. Over the last 20 years, several companies have developed motor-based fitness equipment, but they have had limited success and offer only simple features. Research labs and rehabilitation clinics for professional athletes have had very good results using motor-based training and test equipment during the years But it have been with very expensive and and complex system only suitable research use

The purpose of SYGNUM energy training equipment is to bring the training principles from professional athletes to normal fitness center users. Compared to normal fitness equipment with weight stacks, the SYGNUM energy ETFM motor-controlled equipment has the following unique features:
• Individual load curve in isotonic (torque feedback) training with different load in concentric and eccentric training
• Isokinetic (constant velocity) test and training gives dynamic maximal test and very good results in rehabilitation training
• Isometric (constant position) with measured force and static test and training in all positions of the joint

High Load, Low Noise, and a Smooth Feeling

When developing the SYGNUM energy strength training system, it became clear that the audible commutation noise, and thereby the torque ripple of the ETFM, was inherited from the SRM. This was a problem because strength training equipment needs very fast, smooth load regulations down to zero speed, compared to the eTenzor bike where the load is constant and the speed is high. This required changing the motor configuration from three to five phases and changing to a more sophisticated controller.

Development on the PCI Express FPGA Board and Implementation With NI Single-Board RIO

We started developing the SYGNUM energy strength training project just after restructuring the company and reducing the R&D department to one person, so economical resources were limited.
But, along with National Instruments Alliance Partner DELTA, we rapidly developed a working prototype of the control system on the NI PCIe-7841R board in just two weeks. This helped us quickly develop and test a number of different motor control strategies during the next three months.

It took two developers working for one year to implement the first simple ETFM motor controller with dynamic signal processing (DSP) on the bike. Once we decided the NI R Series board was the best control strategy for this system, it took just one month to port the code to the NI sbRIO-9611 devices. We showed six working prototypes to the public at the FIBO fitness exhibition in Essen, Germany, just four months after we started using LabVIEW (Figure 3). Nobody in the company had prior experience working with LabVIEW.

We still use the R Series field-programmable gate array (FPGA) board for development and we use the NI Single-Board RIO OEM devices for the final production model.

Using the FPGA for Precise Control, Real-Time Targets for High-Level Control, and the PC for the System Setup
The main reason we chose the FPGA board was to control the ETFM motor current with exact precision at high speeds and to get a fault tolerant system that does not hang in endless loops. With LabVIEW graphical programming, it is very intuitive to see how the FPGA functions and it is straightforward to program it. The FPGA also controls the torque feedback from the strain gages, the velocity feedback from the encoder, the seat adjustment with actuators, and the overall security of the system.

Even though LabVIEW has made it easier to work with an FPGA, it still takes a long time to compile the code in the LabVIEW FPGA Module, as with every other FPGA. We frequently simulate the code before compiling it, but there are still high compilation times when optimizing the code with the hardware. The parts of the control that can run with a lower frequency of <500 Hz are handled by the real-time target. This is mainly the start and stop position, calculating the user-specific load profile, collecting data, and communicating with the touch screen display. Finally, we use a PC VI  through networked variables to set up all the control parameters and read some debug values when the system is running.
Overall, it is a very powerful setup that gives us a very exact and fast response from the FPGA, sophisticated control with the real-time target, and the opportunity to optimize the control online when the system is in use (Figure 4).  

Reaching Our Goals

We changed our entire platform and created six working prototypes in just four months. In one year, we had six different production model benches. Within a year and a half, we expect to install our system in the first fitness center. We achieved all of this with only one person in the R&D department handling the hardware and part of the mechanical development. When working with motor-based training equipment, it is very important to design a feeling that is easily adaptable for the client. This means the control should be exact so the correct test results are reached and the feeling must be user friendly. The feedback from the customers and test users is that the feeling is very easy to adapt to compared to other motor-based test and training equipment.

Cost and Time Savings

The first SYGNUM energy test and training machines are not a volume product; therefore, the development cost is an important part of the economy of the whole project. The initial cost is higher to use the NI Single-Board RIO OEM devices, but purchasing a finished device gives us more flexibility because we do not have to handle production or outdated components. As for development time, the NI platform saved us at least one year of programming time and one year of hardware development time. By using the NI Single-Board RIO platform, we have the flexibility to choose different devices when machine requirements change and we can add on additional  components.

Future Opportunities for ETFM Control Using NI OEM Boards 

We built the power electronic board as a generic daughter board for the NI Single-Board RIO that, in addition to the ETFM, can control all SRM, AC, and brushless DC motors. Furthermore, there are some generic interfaces that can be used for encoders, actuators, or SPI devices, and a dedicated strain gage amplifier input. Therefore, we can easily change the application for the ETFM and with the power of LabVIEW programming we can rapidly have a running device that is dedicated to future customers needs.

Author Information:
Anders Hedeager Pedersen
ePower technology ApS
Diplomvej 373
Lyngby 2800 Kgs
Denmark

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