When German wheelchair market leader Alber (alber.de) set out to develop a high-performance product, it recognized the need for a high-efficiency motor to reduce power consumption and downsize battery weight.
To enable Alber to achieve its objectives, supplier Avago Technlogies (avagotech.com) developed an eight-pole pair, brushless DC (BLDC) motor and addressed the many challenges unique to the Viamobil V25 wheelchair.
Multiple encoder technologies in one
To ensure that the motor starts under full load conditions, an absolute electronic rotor position sensor (RPS) is required. Normally this can be done with Hall elements and a magnet, but in Viamobil’s case the magnetic field of the electric brake can disturb the feedback function, and the accuracy of such a RPS does not allow power efficiency of 89%.
A standard optical encoder is too large to integrate into this motor with its small form factor. Existing kit or module level encoders are not able to provide the requested absolute position for all 16 poles. Customization of an encoder engine is too expensive.
Therefore, the challenge for Avago as the encoder supplier was to combine the following encoder technologies into one application:
• Incremental encoder
• Block commutation encoder
• Pseudo absolute coded index
• Incremental absolute encoder mode.
A standard BLDC encoder provides the six different channels required: Incremental A, B and Index, as well as the commutation channels U, V and W. The fundamental idea is to use the U, V and W channels as a 3-bit Gray code absolute encoder. This will allow having eight absolute coded sectors on the code wheel. With this basic information, the motor can be started. Not in the high efficiency mode, but it will turn.
With the index marker as LSB, resolution can be increased by one additional bit. LSB is PA-coded. With four bits now available, they can correspond to the 16 pole positions on the rotor.
This solution can be easily integrated into the motor and will use motor’s bearings to guide a code wheel in the correct position. The use of the incremental encoder and BLDC encoder is common, but to have a PA-coded index is a new technique.
How PA coding works
An incremental encoder provides two incremental channels A and B. In addition to this an index channel sends every revolution an index marker (Fig.1).
To build a pseudo absolute encoder, more index markers with unique distances are required. To simplify this explanation only two more indexes are added to it (Fig. 2).
To get the correct position a counter is triggered by the first index. All increments are summarized up until the next index stops the counter. To illustrate, assume all counters and registers are set to zero. The first index—let’s say it’s I2—will trigger the process. The next index could be I1 or I3 (Fig.3).
Let’s assume it’s I3, after the next index the counter contains a value of 32. This corresponds to I3 (Table 1).
The absolute position of “48” will be loaded to the absolute position register. From now on this value will be incremented or decremented with the incremental information of the encoder. The system is now in the absolute mode (Fig. 4).
The Index marker can be used further on to verify content of the absolute position register.
A full sequence of this coding is shown in (Fig. 5).
Controller and software are critical
To control a motor with this type of encoder, a very precise motor controller and software combination is required. This is especially true for a motor used in a wheelchair, as it is an absolute must to have smooth torque and acceleration control.
For the Viamobil V25, Avago’s AEDT-9340 6-Channel incremental encoder is used with a customized code wheel. This encoder kit is integrated into the motor to keep the form factor small.
The code wheel of the encoder is guided by the existing motor bearings, saving cost, space, and weight. A plastic cover protects the encoder against contamination.
The first test results of engineering samples were successful enough to build a prototype. This prototype operates at 36V with a 6.6Ah battery and reaches a distance of 20 kilometers. While this optical encoder solution has a higher price compared to a Hall-effect solution, the electrical performance and the plug-and-play assembly makes the end product more cost-effective.