Our intake lift is powered by two 393’s and goes up fine. The problem is it then drops when power is off. Going to try and lighten the load with aluminum structure material but am looking for other options. Is there a way to keep a certain amount of power going to the motors at all time to hold lift up?
You could set all the motors to a small PWM value such that the motors them selves don’t move, but so that they receive power - somewhere around 10 - 15 should do it.
You should still lighten up the lift, but that’s how to do it if you’re still having problems.
You could also add some rubber bands to the lift.
What is your gear ratio?
Putting a small PWM value as a constant is a fair idea, however the motors are much more likely to burn out.
Consider putting rubber bands or surgical tubing to hold the arm up, and help the 393 motors lift the weight better, especially with balls in an intake.
Our gear ratio is a 12 to 60 gear ratio using the high strength gears. The 12 tooth gear is connected to the motor, while the 60 tooth is directly connected to the arm. If you guys need a picture, I can post one.
Apply a constant 16 power to the arm.
motor[m_arm] = (vexRT[Btn5U]-vexRT[Btn5D])*127+16;
If you look at Owens robot in this video, you can see the rubber bands on the back of the lift which hold up the lift when its raised. We also used this on our Sack Attack robot so when we raised it lift it flew up and stayed up while having around 20 sacks.
The lift then ran a PID which kept it in any position it stopped in.
When the motor is stalled, the current is proportional to the pwm input. Keep the control value below 20 and you will not burn out the motors, however, this current will be heating the PTC and may cause it to trip a little sooner when the motors are used to drive the arm again.
A 2 minute match won’t be nearly enough time to cause the arm to stall though. I’ve run my robot for over 10 minutes before the pwm has caused any problems.
Do some torque calculations. Weigh the grabber mechanism and then the arm. The length of the arm times the weight of the grabber is the torque required to lift that guy. Half the length of the arm times the weight of the arm is the arm’s contribution. Add those together for the torque needed when horizontal and you have all of gravity’s effect. Reducing weight far away on the grabber has a bigger effect on the torque required to lift.
Rubber bands will subtract the amount of torque needed by about 2-3 in/lb when really stretched (see post this past summer about rubber bands).
If you can, use enough rubber bands/elastic to hold the arm up when the motors are disconnected. This way the motors are driving the arm and not contributing a lot to the lifting. But this can cause prblems like a big old catapult (unless that is what you’re going for) or shredding of the bands agains metal corners.
Finding that hold strength will vary based upon how many bands you have and the weight of the arm.
PID control would keep it in place no matter what but send current to the motors during the match and drain battery/trip the PTC sooner.
In sack attack, we had a lift going at 25 for 4 motors. Motors got really hot, but it wasn’t enough to trip the cortex.
In toss up, we use about 16 (if I remember correctly), But I prefer stalling the lift as much as possible with rubber bands. I’m sure with more sophisticated programming you can use potentiometers.
elastic bands are a good suggestion.
also swapping aluminium for steel is good, any thoughts on switching from a 1:5 to a 1:7 ?
^ what i suggest.
the lowest gear box we go for is a 1:7, i have a 1:9 compound gear box on my arm for this year, as it aids with hanging