When you are plotting step responses on the oscilloscope, you want to have the response curve fill up as much of the screen as possible. This gives you the best resolution for measuring time and voltage quantities on the scope.
When you start the scope up, the time scale and voltage scale are more than likely not what you need for your measurements. Typically the time sweep is too fast, and the voltage scale is set too small to measure the quantities needed in the lab. You will first adjust these scales to make sure the screen will capture the output well. Then you will set up the scope triggering to make sure the step event will capture the output response.
The scales for time and voltage on the scope are shown above the graphing screen. Under the default set-up, the voltage per division (per grid block) is 100 ms. The time per division is 100 ms. These scales are too fine for measurements of mechanical systems. If you use them you are likely to find that you cannot see the response curve. Because the scales are so fine, you are only looking at a small part of the time/voltage space. Most of what happens with the step response happens outside of that space.
A good strategy to use to get the scales set right is to set both the voltage and the time scales as coarse as possible. Set the voltage scale to 5 volts/division and the time scale to 5 seconds/division. Now you are looking at a wide part of the time/voltage space. If you still do not see a sweep, your voltage cursor is probably too high or too low. Look for a little 1 to the right side of the screen grid with an arrow pointing up or down, off the screen (this assumes input is entering the scope through channel 1.) Use the small Position knob above the scope input to move the sweep on the vertical scale so it appears on the screen. Make sure the scope trigger mode is set to Auto Level. Set the triggering position to the left side of the screen (MAIN-DELAYED/TIME REF/LFT). When you see the screen trace appear on the left side of the screen, perform your step. At first your response curve will occupy only a small part of the screen. Increase the fineness of your voltage resolution and your time resolution. Keep doing sample steps, checking the fit of the response curve on the screen. Besides changing the scaling, you might also want to move the curve up or down with the Position knob. The scaling is equivalent to zooming in a CAD program. The Position knob is equivalent to panning.
By trial and error, you should arrive at a point where the plot on the screen uses as much of the screen as possible. Now it is time to set the triggering on the scope to capture the step event properly. Under Trigger push SOURCE. Immediately after pushing this, turn the Level knob. You will see a level line appear. Set this about 1 voltage division above the output voltage you are measuring for the step when the step value is 0. (This assumes the step produces an upward shift in the output voltage that you are measuring.) What this does is set a reference position on the scope screen. It is the intersection of the triggering level (horizontal line) and the vertical line between the two solid triangles that appear along the top and bottom of the screen graph. Your response curve will be drawn on the screen so that it passes very near this point.
Now set your trigger mode. Hit MODE and set the mode to Single. Now if you instigate your step, the scope should capture it as planned. If you want to adjust the horizontal location of the curve, you can do this with the Delay knob. Move the solid triangles to the left or right with this knob. Now do a new test by pushing RUN. The new curve should shift by the amount you moved the solid triangles.
Once you have a good response curve on the oscilloscope, you can use the time and voltage cursors to get the time constant, peak time, steady state gain, overshoot, etc. There is a neat trick you can do with the voltage cursors. If you set the V1 cursor to the pre-step voltage level and the V2 cursor to the post-step, steady-state voltage level, you can then put the voltage in %. Then hit the Set 100% soft key. Now if you dial V2 up or down, the voltage reads out in %. This is useful on a first order system for finding the time constant. Set V2 down to 63.2%. Then use the time cursor to get the time constant. Set t1 to where the response just leaves the 0 level. Set t2 to the intersection of the response curve and the V2 (63.2%) line. Dt will be the motor's time constant. With an underdamped second order system, you can use V2 to find out the system overshoot.