Servo Mechanism to initiate movement of an electric motor trolley

[namith11]

hey,

So I am working on this project where I am trying to come up with a motor set up to drive two wheels of a trolley. The entire trolley weighs 650 kgs and i require a motor with a torque of about 400Nm. I want to set it up in such a way that i want to be able to apply very little force to push the trolley after which the motor takes over n it moves forward by itself. When i apply a little force to pull it backwards the motor takes over n it moves in the backward direction. I don't know how to go about this. I will have to use some kind of servo assist mechanism but I am really not sure. Also steering will be completely manual. I need only forward and backward motion. Somebody please help me!

 

[SirAskalot]

Are you looking for advice on selecting a motor (drive system), or the control part of the system, i.e. the sensing of motion/force in a direction?

 

[Okefenokee]

Is the problem that you are having difficulty getting the main motor to start moving the trolley from a dead stop?

 

[namith11]

@Okefenokee: I want to know how i can get it moving in the forward direction with a slight push and in the backward direction with a slight pull.
@ SirAskalot: I initially wanted to go with a PMDC(Permanent Magnet) Motor that will run both wheels. I want to know if it possible to come up with a control system that will suit my requirement. Also do i need to replace the PMDC Motor with a Servo Motor? I have little knowledge about control systems and hence i have no idea where to start.

 

[namith11]

@Okefenokee: I want to know how i can get it moving in the forward direction with a slight push and in the backward direction with a slight pull.
@ SirAskalot: I initially wanted to go with a PMDC(Permanent Magnet) Motor that will run both wheels. I want to know if it possible to come up with a control system that will suit my requirement. Also do i need to replace the PMDC Motor with a Servo Motor? I have little knowledge about control systems and hence i have no idea where to start.

 

[namith11]

Here are the specifications of the motor if it helps :)
Power: 2HP
Torque: 400 Nm
Voltage: 12V/24V
Current: 120 A
Number of Drive Wheels: 2
Diameter of drive wheels: .435m
Type of motor: PMDC
Gear Reduction set up: Worm drive arrangement
Total weight being driven: 700kgs
Speed at motor: 1500rpm
Speed at wheels: 50rpm
Gear reduction ratio: 25:1

 

[namith11]

Here are the specifications of the motor if it helps :)
Power: 2HP
Torque: 400 Nm
Voltage: 12V/24V
Current: 120 A
Number of Drive Wheels: 2
Diameter of drive wheels: .435m
Type of motor: PMDC
Gear Reduction set up: Worm drive arrangement
Total weight being driven: 700kgs
Speed at motor: 1500rpm
Speed at wheels: 50rpm
Gear reduction ratio: 25:1

 

[SirAskalot]

Choice of motor also depends on your power source (i.e. battery, fuel cell, generator), load torque (you have said 400 Nm, based on what?), power ( what speed do you want?, air drag coefficient etc.) and speed/gear ratio.

A DC motor might be the simplest motor to control, so it would be a good place to start.

As for the control of speed and direction; you don't want a switch (i.e. a "gas pedal") to make it move? Or is it critical that it moves according to the pushing/pulling on the trolly? How can you make it stop?

Sensing a force (push/pull) might be easiest to do in the "handle bar" with the use of a switch. It could be done with the use of the motor or other sensors but its more difficult.

Sensing a motion in the trolly is easiest to do with a encoder or speed sensor.

With the input signal from one of the above choices some logic must be used to start/stop the drive unit. Simplest logic is a on/off switch between a battery and the motor. A more advanced choice is use of a micro controller or eqvivalent to initiate the starting and stopping.

I´m not going to design the system for you. So you have to do some reading up on motor, drives and control to get it going. And might I advice to start at a smaller scale, with a smaller prototype. 700kg, 120 A are serious stuff!

 

[namith11]

The set up will be powered by dry cell batteries. I calculated the torque using a motor sizing tool online based on my weight, average speed etc. I require an average speed of 1.2 m/s which is approx 50rpm based on my wheel dia. Speed at the motor is 1500rpm so i will be using a gear reduction set up to achieve this. Gear ratio will be 25:1
Also i do not require speed control and steering will be manual. So there will be this switch on the handle i have to press down and push/pull for direction and when i release the switch it will stop. There is enough friction in my motor to prevent it from rolling down a 15 degree slope when the motor is off so i don't need any kind of braking system.
I do not know much about the controls so i will read up on encoders and speed sensors. How hard is it to design a circuit for the same?
http://www.usa.canon.com/cusa/healthcare/products/digital_radiography/mobile_portable_solutions/radpro_40kw_digital_mobile_x_ray_system#Specifications

Check out the above link. This set up is to run a Mobile Digital Radiography machine like the one the website mentioned above.