US Digital MA3

[Team1710]

I was wondering which type of interface would be best with the Us Digital MA3?
Us Digital MA3
It is offered in 10 bit analog, 10 bit pwm, and 12 bit pwm.

[AdamHeard]

I believe analog is the only interface of those compatible with our control system.

That's what we've used as well, and it works just fine.

[Jared Russell]

Actually, you could use any of the options with our control system (by setting up a timer to measure the PWM period, for example). Still, analog is what I've seen most teams use.

[Mr. Lim]

Neat device! Out of curiousity, what type of applications (FRC or non-FRC related) would you choose a device like this over a high-precision pot?

[Jared Russell]

Originally Posted by Mr. Lim

Neat device! Out of curiousity, what type of applications (FRC or non-FRC related) would you choose a device like this over a high-precision pot?

Steering feedback for crab/swerve drives is the most common application for these in FIRST. They are nice because most single turn pots have a 30+ degree "deadband" when they roll over. These devices have a full 360 degrees of measurement.

[Team1710]

Yeah we are looking into using these for a swerve drive.

[Gdeaver]

Magnetic absolute position sensors can have many advantages over mechanical pots or optical code wheel sensors. The mentioned sensor should work very well in the analog version. To point out another option, AVAGO makes a magnetic absolute position sensor that would work also.
The Mouser part number is 630-AEAT-6010-A06. This device has a serial output. Labview does have a SPI VI that probably could be configured to read this device. You can clock the data out up to 1 MHZ. There are also non contact magnetic absolute rotary position sensors. Cherry makes a 180 and 360 degree unit. The price is not to bad. Austria micro makes sensor SOC devices that would be good if a team wanted to make there own. Future and Newark sell these and they have links to the data sheets.

[Paul Copioli]

This year we used these for all of our position feedback needs. They are far superior to pots. We used the analog versions. We used the ball bearing versions for our 4 encoders on the drive train and the bushing versions for our turret and shooter hood. They are absolute position encoders which means they can be made to work over more than a 360 degree range, but are absolute (meaning, same functionality as what a pot gives you) over a 360 degree range.

The singe biggest advantage: no more blowing past the pot hard stops. Their size is awesome too.

[Team1710]

We are actually not going to use this model. Instead, we are using the MAE3 Absolute Magnetic Kit Encoder.

[Abrakadabra]

Originally Posted by Paul Copioli

This year we used these for all of our position feedback needs. They are far superior to pots. We used the analog versions. We used the ball bearing versions for our 4 encoders on the drive train and the bushing versions for our turret and shooter hood. They are absolute position encoders which means they can be made to work over more than a 360 degree range, but are absolute (meaning, same functionality as what a pot gives you) over a 360 degree range.

The singe biggest advantage: no more blowing past the pot hard stops. Their size is awesome too.

Paul -

Because they are magnetic, how much do you need to worry about proximity to motors with these? The datasheet talks about not mounting two of them closer than one inch without shielding in order to avoid crosstalk, but I was wondering if you had any experience with larger mag field issues?

The magnetics are certainly a bargain compared to the optical equivalent!

Thanks.

[Gdeaver]

That is probable better since it's a non-contact type. Don't have to worry about bearings and misalignment. Misalignment does cause some inaccuracy. If your going to use these for wheel position and intend to have full 360 degree range of motion one should look at the data sheet. There is some degree of non-linearity at the 0 volt and 5 volt output. This is because the DAC has problems at both rails. Some magnetic sensors eliminate this problem by narrowing the out put band. Many put out a voltage from 0.5 to 4.5 volts there by eliminating the DAC problem. However, this does cause some loss of precision in the A TO D measurement. Reading the value by serial output eliminates the D to A and A to D problem. Like the AVAGO unit I mentioned. I wouldn't be surprised if this unit used the Austriamicro AS5043 chip. It's off season. A nice little project would be to buy some magnetic absolute position sensors and some magnetic encoder chips, make some PC boards and play. 6mm by 2mm neodymium diametrically magnetized magnets are also needed. It would be a useful exercise to be able to read these by the c-rio SPI interface. Today SPI tomorrow CAN. In the future the programing will probably involve passing and receiving serial packets.

[AdamHeard]

Originally Posted by Abrakadabra

Paul -

Because they are magnetic, how much do you need to worry about proximity to motors with these? The datasheet talks about not mounting two of them closer than one inch without shielding in order to avoid crosstalk, but I was wondering if you had any experience with larger mag field issues?

The magnetics are certainly a bargain compared to the optical equivalent!

Thanks.

We had them mounted within 2" of globe motors on our prototype crab without issue. They weren't near enough to CIMs to let you know how that went.

[Gdeaver]

The magnetic position sensors depend on measuring the orientation of a diametrically magnetized neodymium magnet. The on chip sensor bridges need to be in saturation. The magnet typically needs to be with in 1 to 2 mm of the chip surface. I doubt most motors we use would have a strong enough field to influence the sensor and there are many ECM motors using these chips in place of optical encoders. However having the sensor wires close to a motor could cause problems with brush noise.

[Qbranch]

Wow... I wish I would have known about these in 2008. The only interfaces available then were the analog, and a really high frequency (~5KHz) pwm output option.

Having super high resolution like this is nice when you want to start controlling velocity and acceleration in your positioning application... otherwise you have to slow down your control loop rate (ouch) or start timing encoder pulses (double ouch) to get your velocity+acceleration resolution back.

The only thing I'll mention about (at least the previous) analog encoders for US Digital is their relatively high output impedance (the output is 'weak' as far as drive current goes) so we found their output signals more susceptible to noise than you may be accustomed to. If it's a huge problem, you can always run the signal as a twisted pair (one ground wire attached at the encoder, the other the signal wire) into an op amp configured as a unity gain differential amplifier to reject common mode noise (i.e. motors).

-q

[Dave Flowerday]

For reference, here's a previous thread about the MA3. As I mentioned in that thread, the MA3 has cost us several matches during the 2 seasons we used them, and have been banished from designs from that point on. However, plenty of other people report success with them. Take it for whatever it's worth.