Quick question to the designer/engineers of the HB25 from Parallax..

deno

First...picture this...Robo Caddy is following me along the fairway, and I start going down hill.· Due to the car battery and two sets of clubs (wife and mine) Robo Caddy is sort of heavy.· When I start to walk down hill, Robo Caddy gains speed and sort of looses control and "rounds up".

Anyway, with the addition of a tilt switch, this slope can be detected..and I can alter the PWM to the HB25.··If the tilt switch tells the stamp to· pulse a "neutral" command, then Robo Caddy will still "coast" downhill loosing control.·

The big question.....Can I damage the HB25 if when the tilt sensor detects a downhill slope of lets say 10 degrees, the stamp orders up a small amount of "reverse" PWM, not much, just enough to slow down Robo Caddy to the bottom of the hill where the tilt sensor will indicate "level" and the HB25 will resume normal forward PWM operation.· Keep in mind Robo Caddy will still have momentum going down the hill and will no longer require "forward" PWM, so just enough "reverse" will be applied by the HB25 to "slow" the decent.· By the word damage, I mean, the HB25 will be applying "reverse" voltage to the DC motors, while the DC motors themselves will be developing there own "forward" voltage feeding back into the HB25.· Will the two "counter" voltages damage the HB25? Especially if the counter EMF is higher then the "reverse" PWM.

I have built a control or power relay to apply power to the HB25, done remotely with a FM handheld remote, and when I disable this relay and it goes back to it's normal position, a dead short is applied to the dc motors on Robo Caddy, which does produce a braking effect for complete stops and parking on a hill.··Not good for going downhill.

Getting back to the above big question...Parallax engineers...what do you think?· I would like to add that the HB25 is a great motor controller device.·


Deno

Ken Gracey

Deno, I think this depends on the current draws of your motors. What are the motor ratings? What kind of battery will you provide to power the motors (voltage, amp-hours of capacity)? If the battery won't provide the current maybe this is a non-issue (but, of course, your battery may be too small).

The consequences of "blowing" an HB-25 are not big - you will simply need to load up on some fuses from the nearby auto parts store. Also, get the 30A fuses even though we ship with 25A fuses if you desire. This will give you a few extra amps. My experience with HB-25s is here: http://www.parallax.com/hybrid. Of all the disasters I created with this project all of my original HB-25s still work perfectly though I have replaced fuses. The HB-25 is a very solid product.

Ken Gracey

deno

Ken, I was just wondering what YOU do for brakes on your hybrid design.· What happens to your hybrid when you are going down hill with it?· Does it loose control?·

I did enjoy your videos...

Deno

Bruce Bates

Deno -

Shorting the windings on the motor will cause it to brake. Just remember to turn off power to the HB-25 before connecting the (+) and (-) connections together. A DPST relay with 1 NO and 1 NC contact could effect that safely, to prevent damage. One pole makes, shorting the windings while the other pole breaks, opening the circuit to the HB-25.

Regards,

Bruce Bates

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Involvement and committment can be best understood by looking at a plate of ham and eggs. The chicken was involved, but the pig was committed. ANON

Post Edited (Bruce Bates) : 8/24/2008 3:45:43 AM GMT

Beau Schwabe

deno,

Here is a link that might be helpful...

http://forums.parallax.com/showthread.php?p=739652

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Chris Savage

Have you tried breaking by using pulse values just on the other side of 1.5ms from the direction you’re going? This would put the motor in reverse, but just enough to put pressure against the motors…I was actually going to experiment with this on another robot once I get some encoders so I can see when it is going to fast…The idea is that as the robot gets faster than you want, you back off until you’re at stop, then start slowly reversing until it slows. Should work actually…especially if you have encoders.

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Chris Savage
Parallax Engineering

Beau Schwabe

deno,

Just make sure your motors are not rated for more than 12.5 Amps (Half of the controllers capability) when using the technique that Chris mentions. When you brake the motors like this, the amount of current required to brake contributes to their stall current rating.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.