Using Spikes to emulate Victors

[jgannon]

I am working on a robotics project with FIRST components, and I'm trying to keep costs as low as possible so I can still afford things like tuition and ramen.

I need my drivetrain motor to have some degree of variable speed, but Victors are very expensive. My low-cost solution is to use a Spike, and turn it on and off quickly to vary the speed. The way I see it is that I can get five different speeds (0-4) by having the Spike turned on for x out of every four program cycles, and then off for the remainder. Four program cycles is a very short duration of time, about 1/10 of a second, so I'm thinking that the output would just seem like a lower voltage to the motor.

Has anyone done anything like this? Is there any reason why I wouldn't be able to do this? I see that the switching time for the Spike is 5ms, which is far less than the 26.4ms program loop, so that shouldn't be a problem. Any advice would be appreciated.

[dez250]

Is the only reason you are thinking of using spikes or victors because of your familiarity to them. While i would suggest them normally because of the quality they are, there are many other speed controllers out there. Many of which are a low cost product. I would have to check around but ill link up a few later tonight.

[jgannon]

Originally Posted by dez250

Is the only reason you are thinking of using spikes or victors because of your familiarity to them. While i would suggest them normally because of the quality they are, there are many other speed controllers out there. Many of which are a low cost product. I would have to check around but ill link up a few later tonight.

I'm looking at Spikes and Victors because I want something that I know will work 100% out of the box with the EDU-RC.

[sciguy125]

I'm sure that it would work, but I don't think that it's a good idea. The Spikes are mechanical. Generally, small mechanical things don't like to move in and out that fast. At least not for very long. The FIRST systems use hobby PWM to control the Victors, so any R/C speed controller will work. I think those guys are in the $50 range. $15 if you have a servo and only need 3 speeds.

[eugenebrooks]

Originally Posted by sciguy125

I'm sure that it would work, but I don't think that it's a good idea. The Spikes are mechanical. Generally, small mechanical things don't like to move in and out that fast. At least not for very long. The FIRST systems use hobby PWM to control the Victors, so any R/C speed controller will work. I think those guys are in the $50 range. $15 if you have a servo and only need 3 speeds.

See the specifications for the spike at the link below.
http://www.ifirobotics.com/spike.shtml#Specifications
In particular, it is rated at only 6 operations per minute
for rated life at rated load. Not a good idea, indeed...

[Al Skierkiewicz]

Originally Posted by jgannon

My low-cost solution is to use a Spike, and turn it on and off quickly to vary the speed.
Has anyone done anything like this? Is there any reason why I wouldn't be able to do this? I see that the switching time for the Spike is 5ms, which is far less than the 26.4ms program loop, so that shouldn't be a problem. Any advice would be appreciated.

Don't use the Spikes in this application. They are just little mechanical relays and a rapid switch with an inductive load will kill them very quickly. The disconnect currents will eat the relay contacts due to arcing and the poor little relays will over heat from friction and constant switching. They are only designed to handle motors with a small current demand and that is why they are fused at 20 amps.

[Gdeaver]

RC car electronic speed controllers are much cheaper. They don't have the amp capacity of the first stuff but can take quite a bit. Tower hobbies is a place to start. If your project is a small robot , Servos can be modified for constant rotation. You can get some powerful servos. This would eliminate the speed controller. Don't forget VEX - Not a bad bang for the buck.

[KenWittlief]

nobody has pointed this out, you have the right idea.

The Victor speed controllers work by switching their outputs fully on and fully off, varying the duty cycle of the pulse to control the speed.

what you proposed will work, but like others have already stated, the Spike will wear out quickly.

Great idea, great insight, but the spikes are not up to it.

[jgannon]

Thanks to everyone for your input. I'm really, really glad that I asked ahead of time... that would have been quite the waste of a Spike.

Originally Posted by eugenebrooks

See the specifications for the spike at the link below.
http://www.ifirobotics.com/spike.shtml#Specifications
In particular, it is rated at only 6 operations per minute
for rated life at rated load. Not a good idea, indeed...

Despite looking at the specifications list many times, I never noticed the part about the limit of six operations per minute. You're definitely right that this is not a good idea.

Originally Posted by Gdeaver

RC car electronic speed controllers are much cheaper. They don't have the amp capacity of the first stuff but can take quite a bit. Tower hobbies is a place to start. If your project is a small robot , Servos can be modified for constant rotation. You can get some powerful servos. This would eliminate the speed controller. Don't forget VEX - Not a bad bang for the buck.

Hm... Tower has some interesting stuff. When I'm looking at other speed controllers, what do I need to look for to ensure that what I'm getting will be compatible with the IFI RC?

[Greg Needel]

look for speed controllers that utilize a pwm signal and you will be guaranteed it will work. pwm is a standard in the RC industry.

[billbo911]

Originally Posted by Greg Needel

look for speed controllers that utilize a pwm signal and you will be guaranteed it will work. pwm is a standard in the RC industry.

You will also want to look at the voltage range it will operate at. Most R/C speed controllers are designed for 7.2 to 9.6 VDC, but some will work at a higher voltage. Also, make certain that the max continuous and peek current ratings are compatible with what the motor you will be driving and what it is capable of drawing.