Bearing Selection

Single-row deep groove ball bearing:
Both radial and axial loads, low torque. Suitable for applications requiring high speeds and low power loss.

Magneto Bearing:
Outer ring can be removed for the ease of mounting. Suitable for small applications, pressed brass cages are generally used.

Single-Row Angular Contact Ball Bearing:
Take both axial and radial loads in single direction. The larger the contact angle, the higher the axial load capacity. However, smaller contact angles are preferred for high speed operation.

Four-Piont Contact Vall Bearing:
Separable inner and outer ring. Can take axial loads in both directions only. Equivalent to face-to-face or back-to -back angular contact bearings.


Duplex Bearing:
A combination of two radial bearings. Can be combined face-to-face, back-to -back or facing the same direction(DT). DT is used when there is strong axial load.


Self-Aligning Ball Bearing:
Correction of minor angular misalignment of the shaft and housing caused by machining or mounting error.

Thrust Ball Bearing:
Washer-like bearings, axial load only.

Load Rating:
The rate of radial bearing is defined as the constant central radial load applied on bearing with stationary outer rings that the inner rings can endure for a rating life of one million revolutions.

The actual load on the bearing would be greater than those calculated, thus a load factor is required to be taken in consideration. For smooth operations such as electric motors, the factor is typically 1 to 1.2, however, considering the shock and vibration the ambulance might brought, this value could boost up to 1.5 to 3.  When a 10N axial force is applied, the bearing will withstand roughly 15N. If the ball bearings are on an angle, the effective load centre will be shifted, thus the overall load will change according to the angle.

AccelStepper Library

I have found this stepper motor library that allows the use of a driver. I spent Monday trying to compose a set of codes to control the motor using the position as a parameter, however it has been unsuccessful until now. I changed my scope on Tuesday and changed the parameter to speed, which I am now able to control the speed of the motor with the force input. Whereas it still has some problems that I could not run the motor in a speed that is fast enough.

Thu 18 April

Today I started adding codes to my software to control the speed which the motor turns . I started by trying to manipulate the frequency of the PWM produced by the analogWrite command, however this is not recommended and 490Hz is the highest I could go. After searching around on the website, I have found a library aiding the control on speed and acceleration. It is based on the Stepper Motor library in the Arduino program. It outputs 4 separate signals to the four terminals of the motor eliminating the need of the driver. I looked into its source files and trying to seek a solution to write my own commands with the driver which I will do tomorrow.

Microstepping

The relationship between step sizes and back-driving force is not steady, where the smaller step sizes like 1/32 and 1/16 as well as the larger steps (full and half) withstand less force the the mid-ranged step sizes.  There is a 'threshold' of force that the motor can withstand before it gets very easy to push backwards. An interesting fact occurs when the delay goes below 0.1 millisecond for step sizes larger than 1/8, the motor will not rotate straight away, it requires a little pull in the positive direction. I searched online trying to find a solution to improve the performance, however there seems to be no one trying to use a stepper motor in my application. I am now looking into the programming side seeking for solutions. I have successfully programmed the controller so the direction of motion of the motor can be controlled by the load.

Thu 11 April

I have amended the code for controlling the brightness of the LED with the force measured by the load cell for it to be true when the force is above a threshold. This could be implemented on the motor to control the direction of motion according to the load.

In order to minimise the bias voltage, I have calculate the theoretical value of the resistors using the current flowing into both the inverting and non-inverting terminals of the op-amp. Due to the limited amount of current, no difference can be found. According to the theory of op-amps, when current and voltage of both terminals equal, minimum offset of input bias is obtained to R1//Rf = R2//Rg. I have changed a number of different combinations and ended up lowering the offset by 200. I have found another trail and error method on the internet which I will try tomorrow.

The motor driver and the male connection wires will arrive tomorrow.

Attempted to program the controller

My first attempt of the day was to control the stepper motor with the controller. The thought was to supply a square wave to the 'STEP' terminal of the motor driver. I searched the command manual through but was unable to get a solution. I then started trying to use the slot J1 for the servo motor as an input to the driver which I thought should be an Analogue square wave. The wider the pulse the faster the servo turns, whereas the principle of a stepper motor is merely the opposite, the narrower the pulse, the faster the frequency and motor speed.

I moved onto trying to read the voltage from the sensor. I have firstly configured the I/O ports of the extension board, then read the analogue input from port 1 with the code MG @AN[1]. I have used the +12V power supply on the extension board to power the load cell, but the reading is steady despite of the force I apply. I tried to read some other ports and see if I can find out what has gone wrong, however all other ports read 0. I then reset the controller wishing it would help. Unfortunately, after resetting, the total number of available ports had decreased, and port configuration using the command CO had no effect.

Thr 04 April

I have managed to get the amplifier to work, a freeload voltage of 1.418V is detected with a 10V input. George had advice me a instrumentation amplifier INA125 that has a higher tolerance of noise, which may be useful. The output is ready to be connected to the microcontroller and to be used as a parameter to control the stepper motor.

The motor is connected to the driver, that needs to be connected to the microcontroller in the next stage. It requires several digital inputs, one will turn the motor in every rising edge, another selects the direction the motor is truing.

I am still navigating my way round the language by reading the command manual as well as examples on the Galil wbsite.

Update

I have connect the driver to the motor without the controller and producing rising edge by switching on the power manually. I am unable to get the motor to turn as I suspect the driver needs to be welded, which will ask George to supervise me tomorrow to weld them and try it out.

I have also connected the load cell with the op-amp under a inverting configuration. I chose the gain to be 100, which will bring the voltage output of the load cell from 1~10mV to 0.1~1 V. The output I got out this afternoon only varies from 8 to a lit less than 10V which makes me think it is not correct (with a supply voltage of 10V).