With the term SSI we refer to a wide varietly of sensors using the SSI inerface to transim their data. These sensors are usually absolute and can be single-turn, multi-turn, having a different frame length and operate at different frequencies. Also the hardware layer can vary. Due to these peculiarities, the datasheet of each sensor must be read carefully before deciding if the sensor is supported or not. This article will explain the supported characteristics of an SSI sensor for the F3 and G4 products.
Both F3 and G4 products
For both products, the hardware layer must be RS-422. SSI sensors using RS-422 will have the following connections:
- Clock +
- Clock -
- Data +
- Data -
If one sensor does not have all the mentioned connections, then it is not supported.
F3 products support only single turn sensors up to 16 bits. There are beta firmware releases that can support higher resolution sensor but you should consult the Roboteq support team before trying one.
The datasheet of the sensor will give the data frame structure. When looking at the frame, we don't want to see any bits that do not provide angle information (such as status bits) comming prior to the data bits. The controller will drive the sensor by using the clock signal and will read the data up to the number of expected bits. If there are irrelevant data comming prior to the angle bits, the angle readings will be affected.
In the below example, the status data come first and they will mess with the angle data:
On the other hand, if the angle bits come first, then the controller will read only these, and stop providing the clock pusle after the configured bits have be read. In the below example with a 16 bits SSI sensor, only clock pulses up to 16 will be provided.
The controller will provide the clock pusle once in every firmware interrupt that happens on the PWM frequency, that is 16 KHz (62.5 us). The clock frequency needs to be fast enough so the whole angle frame will be read before the next PWM iterrupt occurs.
In the below example, a 8 bit angle frame can be read before the next PWM interrupt occurs.
In the next example, the clock frequency is two times lower, so the frame cannot be read on time
Since PWM frequency is 16 KHz, we want the SSI sensors supported frequencies to be higher than 16 KHz * number of bits. Also this frequency must be on the range of the supported frequencies by Roboteq controllers, that for F3 products are 280 ~ 2250 KHz.
G4 products support both signle-turn and multi-turn encoders with resolution up to 47 bits. One thing to take into consideration is that the Feedback parameter is S32, so if the snesor resolution is bigger than than, whole range cannot be used. In that case, the multi-turn or single-turn resolution should be decreased by configuring the controller to read less bits from the angle data.
In the G4 implementation, the user can indicate where the multiturn and single turn angle data are located inside the frame. So literally all the data frames are supported, as long as their total length do not exceed 47 bits.
Here the same limitations apply as with F3 products. Again the PWM frequency is 16 KHz but this time the supported clock frequency range has been increased and is 680 ~ 10800 KHz.
The user must confirm that the SSI supported frequnecy is on that range and also that is satisfies the following equation:
SSI freq > 16 KHz * number of bits