joystick controlled powerwheel/toy car

[xray028]

I am recently working on a project where I will be modifying a John Deere Gator powerwheel/toy car for a disabled child. The powerwheel will be contolled using a joystick from a wheelchair. I want the joystick to control all movements of the powerwheel (left, right, forward, reverse). There is a 12V DC motor located on each back tire for forward and reverse. Another motor will be attached to the steering rod to control steering. I want to know if the PLC will be able to control all three motors. I need the PLC able to tell the steering motor to only turn a certain degree in both directions. I also need the PLC to be able to control the two rear motors for forward and reverse. If anyone knows that a PLC will be able to give me this control please respond. Also, if anyone has any kind of information in dealing with this type of control please respond.

Thank you,

Ray Knight
Mechanical Engineer

 

[rsdoran]

Personally this is one application where I wouldnt use a plc. Robots and remote control technology (for the hobbyist) has been around along time. I would look at sites like this:http://www.robotics.com/robots.html

Also info on PIC microcontrollers, PICBasic, STAMP controllers like this: http://www.rentron.com/

Looking at some of the above can show you inexpensive ways to control the motor and steering, add remote control thru the joystick or use a remote control unit with joystick control that many inexpensive RC cars have today. Control packages can be much smaller and use existing 6, 9 or 12volt battery power.

 

[xray028]

I never really thought of using remote control. It could be an option, but what I really am trying to do is get a controller that already exists for the functions I need. The joystick has four wires controlling +Voltage, -Voltage, steering, and direction. I am going to use the +Voltage and -Voltage for the two rear motors to control forward and reverse. As for controlling the steering, this is where I need some type of controller to perform the motors only turning a total of 55 degrees.

There will also be a kill switch that will be attached to a harness. Once the harness becomes unattached, the system shuts down. I have a relay that was donated with the socket, just have to figure out how everything will go together. I am not the best when it comes to electrical stuff, but I am willing to learn.

The powerwheel is being modified as well with a roll bar, frame extension, and adjustability for growth. The child's parents would like him to be able to use this powerwheel for up to 7 years. Remember that the kid spends all day in a wheelchair and having this powerwheel I believe will give him another option of transportation and having fun.

 

[rsdoran]

I want to make sure I understand the item and the controls needed..for me and any others that may offer suggestions. Please do not take offense if I seem blunt in my evaluation and subsequent comments.

I take it the car is actually capable of the child riding in it and is disabled to a point where he/she can use a joystick for speed/direction control but a steering wheel and pedal are not an option?

This will not be hard to accomplish depending on what your limitations are. My first question would be should the car have a preset speed to run in either forward or reverse? If that is the case it will simplify many things.

Lets assume you have a PLC that meets the voltage requirements needed for the car (I am leaving that issue for you to contend with). Controlling the speed of any DC motor can be done by regulating voltage/current in many ways, the simplest by just using resistors in series to limit current. This is another area that may take a little experimentation to obtain a speed you are satisfied with. Its also possible to look at the gearing of the car to make it what you want, for that matter this unit may run at an acceptable speed for what you want.

Now come the questions. Does the joystick have just switches or are your planning on implementing a joystick with switches and potentiometer control? The latter would offer more ability to control the steering. I would have to do some looking but they make joysticks that offer switches that are enabled above a 0-180 degree plane and another below that plane. These would be enacted with any movement above or below the center point of the plane. A potentiometer would be included that when moved left or right would send an analog signal to determine degree of turn.

Its possible I havent stated this properly so I going to give it a very layman/brief overview.
Joystick forward would activate a digital input to enable forward motion.
Joystick pulled back would enable reverse position.
...With a DC motor this can be done easily by using relays/contactors if needed (or no motor controller is used) to switch rotation of the motor. Its unlikely a plc will be able to handle the currents involved.
Joystick left (with potentiometer centered) would provide an analog signal to the plc..example 0-10v, centered at 5v, 0-4.9 would actuated left turn...5.1 to 10 would actuate right turn.
Under the conditions stated preset the speed of the device.
NOTE: Using the right joystick the forward and reverse switches would be enabled with any movement forward or reverse and right/left movement would actuate the pot to determine degree of direction.

I am not the best (or even close to good) here but have you obtained a PLC yet? I would gladly assist in this endeavor in any way I could. At this time I do not have a plc that works with 12vdc but do have the software for many brands. Please let me (us) know what plc, joystick etc you will be using and I (we) will gladly assist.

Let me know what I can do to help.

 

[Eric Nelson]

First off, Kudos to xray for taking on this project. It's a great idea!...

I checked around a little bit, and found that something quite similar was done as a senior project at UNC. Download THIS PDF to read the article. They only had joystick control for forward/reverse (no steering), but the article may help with your project.

Controlling the steering may prove to be difficult, as it need to be proportional (not full left/full right). As Ron said, you may need to get into analog stuff, which may put you over-budget really quick...

Another approach may be to built a simple comparator circuit to compare joystick position to a potentiometer mounted to the steering shaft. The control system would run a motor CW/CCW to move the front wheels left or right as needed. I suck at explaining stuff, so maybe someone else wants to expand on this "Master/Follower" idea? Terry could probably give a great analogy. Time for him to come out of hiding anyway...

If controlling the steering becomes too big of an issue, you may want to consider something with only 2 wheels. Much easier to adapt as you can get left or right by controlling motor direction alone. Something like Fisher-Price's "Wild Thing" looks more suitable for your project (looks pretty fun too!). Check out the second Customer Review in THIS REVIEW from Amazon.com. Sounds like they have at least one happy user. By the way, this toy has already been "modified" by some bored college kids. You can see their "CurlyCart" project at THIS LINK.

Please keep us posted on your progress, and best of luck with it!...

beerchug

-Eric

 

[okiebob]

Don't forget the sped adjustments

Good job for tackling this one. My Pop's just came home 04/09/03 from the hospital, after being there since 11/19/02 and is now confined to a wheelchair. I was working on it yesterday and noticed it had two speed ranges for use. One was a range from 0-12 maybe 14 and the other was a range of 0-24. This was handled throught a toggle and two relays to select range the the joystick to advance within the range. The low speed is VERY handy in tight spots, not so quick on the take off and easier to turn in a confined area (Living room). Just a thought, let us know how this works out.

 

[Tom Jenkins]

The short answer is that you can get the PLC to do what you want.

If you want proportional steering you will need to get into analog data. It will require a potentiometer on the joystick and a feedback potentiometer on the steering gear. You can set it up for a ±10 VDC analog input. You would use a couple of compares to check the joystick pot tothe feedback pot. If they are not equal your PLC discrete output would power the steering motor in the correct direction, and stop when the two pots were equal. You probably need to put some deadband (a ±tolerance) in there too or the steering will constantly hunt.

It is much simpler, and perhaps adequate for your purposes, to have a simple left/right switch on the joystick and a limit switch on the steering gear to stop the motor at a specific point.

I actually designed steering gear and control for a mobile nuclear camera (X-Ray type system for hospital use) many years ago, but the budget wasn't restricted. That makes lots of things possible that you may not be able to implement.

Post your code, and we will be glad to help more.

 

[xray028]

Well guys I can't express how much I thank all of you for the feedback.

rsdoran-->A steering wheel and pedal is not an option. Now to answer your questions: The car presently runs at 4.5mph with no load, and with max load (130 lbs) it can run 4.2mph, for max velocity. I have been donated a John Deere Gator for this project and two wheelchairs. I have contrusted one operating wheelchair out of the two for analysis purposes. The joystick is made from Invacare w/ model #1038638, it is called a compact/dual joystick. One idea is to have a joystick with switches and implement the potentiometer for steering. I need the car to operate at 2.5mph in reverse and 4.5mph in forward.

I have not obtained a PLC or actually bought anything as of yet. Right now I am just considering suggestions and going to use one that gives me the best and simpliest control system. Currently I have $750 for this project, this money goes for all modifications. I have set aside $400 for the controls. The project has a deadline of December 2003.

Eric--> I have seen what the UNC students created. I am using a more complex joystick then they used. As of contolling the steering, two wheel control is not an option. I want to be able to control the two front wheels. And with the disease the child has, he needs to use as little energy as possible.

Tom--> I am thinking of using the deadband. What I really wanted though, was something that would already have the deadband with it and possibly a pot. I am willing to consider other joysticks, being the one I have was donated and cost about $700.

If anyone is interested, they can reach me at [email protected] or [email protected]

I will try to give all of you more information later. I hope this information helps.

 

[Tom Jenkins]

The hardware you need is essentially that used by valve suppliers for controlling quarter turn valve position from an analog signal. They include the comparator circuit, a couple of relays, deadband adjustment, the whole 9 yards. M-Systems among others has this kind of thing. Unless you can sweet talk them into donating it, though, it will break your budget.

The deadband and comparisons are easy to implement in your PLC. Once you decide on external hardware and the PLC platform you will use we can help with the logic. The nice thing about PLC logic is, for your purposes, it is free. You can get PLCs with the analog I/O and programming software for well within your budget, and if you talk to some local distributors they may be able to donate what you need as well - maybe last year's demo!

 

[xray028]

Perhaps someone could tell me more about the 2 potentiometers and deadband with the PLC.

I have been getting a lot of contacts from vendors on my email and telephone, but all were messages. I have not been able to talk to any of them being that I have been really busy, although I have intentions on getting back to them. The reason I am telling you this is that I had found a website for PC/104, could anyone tell me what type of controllers these are and if they could apply to my project. These are the controllers that all the vendors are trying to set me up with. I have recieved about 15 different emails with different websites and trying to understand what the hell these controllers can do.

I guess I would like to know how the whole controlling process works. Don't get me wrong, I can do analysis from the mechanical side of things and finding transfer functions, but I have no clue how to set up a controller and these potentiometers and deadband stuff. I guess I need to read a "book for dummies" on control applications.

-ray knight

 

[Tom Jenkins]

If you go through the tutorial here, and get Phil's book, you will get the fundamentals.

A PC 104 card won't really solve your problem better than a small PLC. You still need to get Inputs and Outputs and program the logic, and in many ways ladder logic is simpler to implement for a one time project.

The fundamental logic is this:

Read the signal from the potentiometer on the joystick.
Compare it to the pot on the steering gear. REmeber you need comparisons for each direction.
If they differ by more than your deadband turn on the output to move the steering motor in the direction required.
Continuously compare the two pots. When the feedback pot signal on the steering gear is within your deadband of the joystick potentiometer, turn off the output to the steering motor.
Repeat the above continuously.

The mechanics of the comparison and the outputs will vary depending on the PLC you finally select. Get your hardware selected, and then we can help with specific code.

 

[Leonardo2003]

May we know what's the progress of this projects?

Thanks.

 

[glenncovington]

Have you considered going low tech. Does the Gator use two 6 volt batteries in series? Most of these play cars use switches (that could be replaced by relays of the right size) to apply either 6 volt for low speed and 12 volt for high speed to the drive motors. Another relay can be used to reverse polarity to set direction of the motor. The joystick would just make a switch to set forward direction (at 12V) or reverse (at 6V).

As far as steering could you just use a relay and timer to bump the steering motor in the right direction. Each time the joystick is held to go left the wheels would turn just a little bit (adjusted by timer) and then to go more left the joystick would be released and pushed left again (or the circuit could have a second timer to allow it to regenerate itself if the joystick is continued to be held in one direction) This would give a jog, jog, motion, but in a toy car at 4.5 mph I think this would be quite acceptable. Make sure there are limit switches on the end of travel to disable travel in that direction.

Since this will all be DC amperages will be high and will go higher as the batteries drain down with voltage drop so size all wiring and contactors accordingly.

I know this is not the ultimate smooth control, but it should meet the needs of the toy car at a low expense.

Good Luck.

 

[xray028]

Thanks for the suggestions. As of now, i took a break from the project. I will try to look into what you have said glenn. If i were to get a PLC, i believe the cost would put me close too or over my budget. Maybe relays are the way to go.

The project has now started back on pace, but slow pace for the summer. I believe most of the mechanical stuff will be done either by the end of the summer. hopefully i will be able to finish this electrical stuff by the end of september. i would like to have it running in middle october for some practice trials.

I will try to keep everyone posted on the status of this project.

-ray